cuttings descriptions-clastic

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Cutting description Guide-Clastic 1

Cuttings Descriptions Clastic Description order – memorise this!! 1. Rock type (% and modifier, if required) 2. Colour or colour range 3. Hardness 4. Fracture and texture (Break) 5. Grain size: Range and Dominant size 6. Sorting 7. Angularity or Roundness 8. Sphericity 9. Matrix 10. Cementation: Degree, Percentage of each cement and composition 11. Accessories and Fossils: Type and Percentage of rock 12. Effective Visual porosity, type(s) and amount 13. Hydrocarbon indications – shows description (separate module) Rock Name Arenaceous Siliclastics

Arenaceous rocks may be clastic but generally they are resistate (i.e. without clay), comprising predominantly quartz, minor feldspar and other detrital accessories (rock fragments).

Little useful information can be obtained about the quartz mineralogy at the wellsite although the physical condition of the grains may tell you some information. Like?

The type, condition and abundance of minerals other than quartz will be of help in interpreting the environment and rate of sedimentation and may help in isolating the source and history of the sediment.

It will also help the identification of the sediment for later correlation. Identification of rock mineralogy may also be important in selecting matrix properties for the interpretation of porosity and other wireline logs.

A guide to proper naming of the rocks is shown in the next slide.

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Lithology Definition – after Folk, 1974 Examples 80% Q, 16% F, 4% R = Sub-Arkose Sandstone 74% Q, 7% F, 21% R = Litharenite Sandstone 50% Q, 40% F 10% R = Arkose Sandstone 50% Q, 24% F, 26% R = Feldspathic Litharenite

By using this naming method, it is immediately obvious to the reader what type of arenaceous rock is being described.

The FOLK method is primarily useful when describing sidewall cores (SWC and RCOR – rotary side wall cores) and conventional core chips as you can see the original rock textures which has not been totally destroyed by the drilling action of PDC bits.

However, you CAN use this as part of a drilled cutting description i.e. Litharenite or ‘Quartzite’ Sandstones, these are quite easy to identify.

If used, be careful to be correct (the WSG may well be asked to explain his findings in a conference call with town).

As stated in the first slides - It is best practice when unsure of naming a rock to follow the rock name with a ? if not sure i.e. Lithic Arkose?: pinkish grey, etc.

Argillaceous Rocks – Reference text

Argillaceous rocks and much of the matrix and secondary mineralisation in rudaceous (coarse grained) and arenaceous rocks a production of hydrolysis, e.g. clay minerals, hydrous micas, hydroxides and some oxides. It is important to realise the subtle though significant difference between hydrolysate sediments and the other so called “chemical” sediments.

Hydrolysate minerals result from the chemical weathering of the parent minerals at the point of weathering and throughout the period of transport and sedimentation.

True chemical sediments are produced by crystallisation or precipitation at the place of sedimentation and may show no direct relationship to the parent, or parents, or the means of weathering and transport.

The five most significant minerals present in argillaceous rocks are the sheet silicates: illite, montmorillonite, vermiculite, kaolinite (all clay minerals) and chlorite. (Note: each of these mineral names encompasses a range of varying composition, i.e. a group of minerals related by a common structure.

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For your reference - The term “smectite” is commonly used to describe the montmorillonite group, sometimes to include vermiculite.

Clay minerals are usually the products of weathering and hydrothermal alteration of parent rocks, the latter probably being of lesser and possibly not quantitative importance.

Acidic rocks, deficient in calcium, magnesium and sodium tend to yield kaolinite, whereas Alkaline rocks generally yield montmorillonite.

Illite may result from either rock type when potassium and aluminium concentrations are high.

Chlorite is often detrital in sediments but may form from the degradation of ferromagnesian minerals.

Vermiculite may result from the degradation of micas and is also present in a mixed-layered form with detrital or secondary chlorite.

In addition to the sheet silicates, fractions of accessories include unaltered parent minerals and resistant material, e.g. Quartz.

Reworked, previously compacted and re-weathered clay minerals may also be present. The presence or absence of these in quantity gives clues to energy and activity of the

environments of weathering, transport and sedimentation. Since the physic-chemical weathering process is continuous, conditions within the

environments of weathering, transport and sedimentation have as large, if not larger effect on the mineral product as the parent.

Lithology Definition - General WSG Field Examples 20% clay, 80% sand = Argillaceous Sandstone 49% clay, 51% sand = Argillaceous Sandstone 19% clay 81% sand = Sandstone 20% silt, 30% clay, 50% sand = Argillaceous Silty Sandstone 10% silt, 30% clay, 60% sand = Argillaceous Sandstone

If a rock has 20 – 50% of a minor constituent then the name of the lithology

MUST have a modifier.

Sand / Silt / Clay

Siltstone Claystone

20-80

20-80

20-80

80-20

80-20

80-20

50-50

50-50

50-50

Sand

y Si

ltsto

ne

Silty

Sand

ston

e

ArgillaceousSiltstone

SiltyClaystone

Sandy

Claystone

Argillaceous

Sandstone

Sandstone

Sand / Silt / Clay

Siltstone Claystone

20-80

20-80

20-80

80-20

80-20

80-20

50-50

50-50

50-50

Sand

y Si

ltsto

ne

Silty

Sand

ston

e

ArgillaceousSiltstone

SiltyClaystone

Sandy

Claystone

Argillaceous

Sandstone

Sandstone

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Lithology Percentages No easy way to do this. Practice and experience helps. TIP: Geoprolog have a good chapter in there Field Handbook that discussed percentages and the apparent differences of light on dark cuttings and vise versa. Colour

GSA Rock Colour Chart Published by the Geological Society of America, this chart contains 115 colour chips for

identifying the range of rock colours. The chart is based on the Munsell colour system. The Munsell system consists of three independent dimensions which can be represented

cylindrically in three dimensions as an irregular colour solid: hue, measured by degrees around horizontal circles; chroma, measured radially outward from the neutral (grey) vertical axis; and value, measured vertically from 0 (black) to 10 (white).

Colour estimations should NOT be made without the aid of the colour chart.

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Ascertaining accurate colours is a critical part of the cuttings description e.g. slight colour changes can reflect facies, depositional environment and mineralogical changes and can vital in aiding correlation with offset wells.

VERY IMPORTANT: DESCRIBE THE COLOUR AND EVERYTHING ELSE WHEN THE CUTTINGS ARE WET, AND STRESS THE PREDOMINANT COLOUR! How is this done correctly?

Firstly select a suitable cutting of the LITHOLOGY you wish to described, OR a number of cuttings if they are small and have a tendency to stick together (or there is a big colour range between cuttings).

The cutting/s should be placed on the colour chart square eyeball the cutting/s first (in visible light) to ROUGHLY determine which page of the colour chart you will need, and roughly which colour square your Lithology lies in the range of the cutting i.e. colour chips in the range of olive grey to greenish grey.

Then, place the colour chart WITH the cutting placed on top of the colour chip square under the binocular microscope. The WSG must then look down the microscope to ascertain the colour using the microscopes light source.

Using this method you can easily move the cutting onto different colour squares. The cutting lies on top of the colour square so it is a direct comparison and it is EASY to see.

Use this method to determine colour

Some other useful descriptive terms for colour, the WSG can use before the colour in the description; varicoloured, banded, iridescent, speckled, spotted, scattered, disseminated, variegated, mottled.

Its more accurate that just ‘dim mudlogging unit lighting,’ it produces consistency and it is easier to determine the colour down a microscope AND even IF the light source strength (too high/too low) changes then the colour squares appearance will ALSO change.

As the cutting is directly next to the colour square then you ALWAYS get and accurate color/colour range.

Also if ALL WSG use this method, when you look at an offset well – the colours described should be the same!

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Commonly used to indicate a fine grained, well lithified tight rock (usually limestone) with sub-conchoidal fracture.

Dense

Moderately hard, but breaks easily with firm pressure. Generally applies to shale with platey fracture, coal or certain limestones.

Brittle

Can not be scratched with a knife blade, usually siliceous in nature.Very Hard

Solidly cemented or lithified. Does not break under slight pressure, but can be scratched with knife blade.

Hard

Grains can be detached using knife. Small chips can easily be broken by hand.

Moderately Hard

Compact, breaks under slight pressure.FirmPliant clays that show putty-like deformationPlastic

Clays, marls and silts which can be deformed by slight pressureSoftCoherent, but crumbling under slight pressure.Friable

Particles are discrete and non-coherent, unsonsolidated sands.Loose/Uncon-solidated

Commonly used to indicate a fine grained, well lithified tight rock (usually limestone) with sub-conchoidal fracture.

Dense

Moderately hard, but breaks easily with firm pressure. Generally applies to shale with platey fracture, coal or certain limestones.

Brittle

Can not be scratched with a knife blade, usually siliceous in nature.Very Hard

Solidly cemented or lithified. Does not break under slight pressure, but can be scratched with knife blade.

Hard

Grains can be detached using knife. Small chips can easily be broken by hand.

Moderately Hard

Compact, breaks under slight pressure.FirmPliant clays that show putty-like deformationPlastic

Clays, marls and silts which can be deformed by slight pressureSoftCoherent, but crumbling under slight pressure.Friable

Particles are discrete and non-coherent, unsonsolidated sands.Loose/Uncon-solidated

TIPS

Try and pick out clean well formed cuttings. If drilling with PDC bits normally there is one flat clean ‘CUT’ surface – use that side. Depending on the mud system that is being used, the mud is liable to stain the cuttings

(particularly if they are at all porous). Take this into consideration and when the cutting/s are placed on the colour square break

it open to find and nice clean surface with NO mud staining. Staining

Staining is important and can originate from a variety of colouring agents: Carbonaceous or Phosphatic material plus Iron Sulphide and Manganese oxide can range

from grey to black or even brown lignite. Glauconite, Ferrous Iron, Serpentine, Chlorite and Epidote are green colouring agents. Red or orange mottling can be derived from surface weathering or subsurface oxidation by

circulating waters. Haematite or Limonite (hydrated ferric oxide) gives red, brown or yellow shades.

Hardness/Induration

This cohesive strength should refer to individual cuttings or chips and not to individual grains.

How is this done correctly? Use the forceps or the steel pointed ‘prodder’ provided by ALL mud logging companies. Pressure should be applied to the cutting/s and the WSG must determine from how much

pressure is applied what the hardness of the rock is. Please NOTE: due to the shearing cutting action of PDC bits the original rock fabric is lost

by this cutting action. This will affect the apparent cutting hardness dramatically. i.e. a well consolidated, very

hard siliceous Sandstone after being drilled by a PDC bit will appear in the cuttings as amorphous soft rock flour OR very fine silt accretions which are friable and soft.

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Used to describe shales in which the fissility is not strongly developed, but exists sufficiently to cause irregular surfaces and edges, like a board broken across the grain.

Splintery

Used to describe shale and marl in which fissility is well developed. The rock breaks in parallel sided thin plates. This is commonly caused by fracture along bedding planes, or along cleavage directions.

Platy/Fissile &

Sub Fissile

The rock fractures into small flakes or chips. Common in some marls and occasionally in metamorphic rocks.

Flaky

Commonly seen in dense rocks such as chert, argillite and flint and or coal. The term refers to the concave and convex surfaces developed on fractures. The fracture of hard limestone produces somewhat less strongly developed curved surfaces and the fracture has been called "sub- conchoidal".

Conchoidal

Used to describe well lithified formations that break chips with angular and surfaces, generally as limestones, and siliceous hard formations.

Angular

Commonly used to describe PDC drilled cutting that are not quite 100% blocky with clean breaks not perfect right angles and not perfectly angular.

Sub blocky

Used to describe claystone, marl and limestone in which fractures are developed at approximately right angles, so that small blocks are formed.

Blocky

Used to describe shales in which the fissility is not strongly developed, but exists sufficiently to cause irregular surfaces and edges, like a board broken across the grain.

Splintery

Used to describe shale and marl in which fissility is well developed. The rock breaks in parallel sided thin plates. This is commonly caused by fracture along bedding planes, or along cleavage directions.

Platy/Fissile &

Sub Fissile

The rock fractures into small flakes or chips. Common in some marls and occasionally in metamorphic rocks.

Flaky

Commonly seen in dense rocks such as chert, argillite and flint and or coal. The term refers to the concave and convex surfaces developed on fractures. The fracture of hard limestone produces somewhat less strongly developed curved surfaces and the fracture has been called "sub- conchoidal".

Conchoidal

Used to describe well lithified formations that break chips with angular and surfaces, generally as limestones, and siliceous hard formations.

Angular

Commonly used to describe PDC drilled cutting that are not quite 100% blocky with clean breaks not perfect right angles and not perfectly angular.

Sub blocky

Used to describe claystone, marl and limestone in which fractures are developed at approximately right angles, so that small blocks are formed.

Blocky

Texture and Fabric

After you have applied pressure with the ‘prodder’ breaking the cutting (if it is not too hard), next you describe the surface fabric, habit and fracture – or the ‘break’ of the cutting.

Texture is defined by the size, shape and arrangement of the component particles of a rock and will have be described under the headings of grain size, shape and sorting. Other textural descriptions fall under the terms fabric, habit and fracture.

The nature of the break is indicative of internal rock stresses and composition e.g. angular break, conchoidal, crumbly, fissile, hackly (rough or jagged), splintery, and earthy.

Fabric - Several descriptive terms are used to describe the type of fabric, commonly as a result of cleavage or bedding, seen in argillaceous and carbonaceous cuttings. These include:

Fracture & Break

Example of blocky break – cuttings breaks in half with slight pressure (moderately hard), approximately right angles, so that small blocks are formed

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Of the nature of earth or soil/unglazed pottery commonly used in conjunction (together) with gritty as a textural term.

Earthy

Surfaces marked with striae; furrowed; striped; streaked common on flat cut surfaces of PDC drilled cuttings.

Striated

As stated (sandstones and limestones).Etched Frosted, Pitted,

As stated.Smooth/rough

Characterized by or consisting of vesiclesVesicular

Surface breaks have a sugar like crystalline appearance (limestones and some siliceous siltstones).

Sucrosic

Composed of parts or elements of different kinds; having widely dissimilar elements or constituents.

HeterogeneousComposed of parts or elements that are all of the same kind.HomogeneousCuttings with no distinct shape.Amorphous

Of the nature of earth or soil/unglazed pottery commonly used in conjunction (together) with gritty as a textural term.

Earthy

Surfaces marked with striae; furrowed; striped; streaked common on flat cut surfaces of PDC drilled cuttings.

Striated

As stated (sandstones and limestones).Etched Frosted, Pitted,

As stated.Smooth/rough

Characterized by or consisting of vesiclesVesicular

Surface breaks have a sugar like crystalline appearance (limestones and some siliceous siltstones).

Sucrosic

Composed of parts or elements of different kinds; having widely dissimilar elements or constituents.

HeterogeneousComposed of parts or elements that are all of the same kind.HomogeneousCuttings with no distinct shape.Amorphous

Surface Texture & Fabric Lustre

Together with surface texture the lustre of clean cuttings or free mineral grains, chipped surfaces can also be used:

Definition: The quality and intensity of light reflected from the surface of a mineral (or in our case drilled cuttings). This property must be observed first-hand and cannot be demonstrated in a photograph.

Metallic - strong reflection, shines like metal, may be very shiny (like a chrome car part) or less shiny (like the surface of a broken piece of iron); Vitreous - glassy, bright (shines like glass); Resinous - a resin-like shine (resembling amber for example); Greasy - a dull sheen, has the appearance of being coated with an oily substance; Pearly - a whitish iridescence (resembling pearl for example); Silky - a sheen like that of a fibrous material, e.g. silk; Adamantine - a brilliant lustre such as that of diamond; Earthy - like the surface of unglazed pottery.

Shale Swelling

After a Claystone cutting has been broken and the fracture/break interpreted, place a small sample in a porcaline spot tray – add water to determine the hygroturgid (swelling nature) of the Clays.

Marked slaking or swelling in water is characteristic of montmorillonites and distinguishes them from kaolinite and illite.

Drilling with OBM. Cuttings may have a film of oil coating the cuttings. In these cases look for clean break surfaces, add some dilute HCL break the oil film.

Using the binocular microscope, watch the clean surfaces for speed of the swelling (hydrating) reaction.

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Swelling Descriptive terms

Non-swelling: does not break up in water even after adding 1% HCl Hygroturgid: swelling in a random manner Hygroclastic: swelling into irregular pieces Hygrofissile: swelling into flakes Cryptofissile: swelling into flakes only after adding 1% HCl

NB: If reaction in distilled water is inhibited by traces of oil add droplet of HCl to break oil film. Udden-Wentworth Scale

The scales used to define grain sizes in sediments and sedimentary rocks are grade scales; that is, they are created by imposing arbitrary subdivisions on a natural continuum. The terminology which is most familiar to us is that of the Wentworth Scale, which includes the major classes: gravel, sand and clay, with their numerous subdivisions. Because the range of grain sizes found in nature is so large, a logarithmic scale, such as the Udden-Wentworth scale shown to the left, is more practical than a linear scale.

The phi scale, devised by Krumbein, is computed by the following equation:

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After FOLK 1974

Medium SiltFine Silt

Very Fine Silt

6.03.90.0039

Coarse Silt5.0310.031Analysed using pipette or hydrometer

Very Fine Sand4.06250.0625230

Fine Sand3.01250.125120

Medium Sand2.02500.2560

Coarse Sand1.05000.535

Very Coarse Sand01.018

Granule-1.02.010

-245

Pebble-416

Cobble-664

Bolder-8256Use Wire Squares

Wentworth Size ClassPhi (φ)MicronsGrain size (mm)U.S. Standard Sieve Mesh Number

After FOLK 1974

Medium SiltFine Silt

Very Fine Silt

6.03.90.0039

Coarse Silt5.0310.031Analysed using pipette or hydrometer

Very Fine Sand4.06250.0625230

Fine Sand3.01250.125120

Medium Sand2.02500.2560

Coarse Sand1.05000.535

Very Coarse Sand01.018

Granule-1.02.010

-245

Pebble-416

Cobble-664

Bolder-8256Use Wire Squares

Wentworth Size ClassPhi (φ)MicronsGrain size (mm)U.S. Standard Sieve Mesh Number

GRAVELSAND

MUD

Grain Size – with shaker screen sizes Always use a grain size comparator. The best type are the translucent plastic comparators as they can be placed on the sample tray. This eliminates the need to retrain your eye when the zoom on the microscope is adjusted.

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Grain Size Comparator

Numerous times (like the colour chart) have I entered the mudlogging unit to find a pristine unused grain size chart – or on some TEPI operations NO grain size chart at all. Discuss.

If you don’t carry your own (I DO) and Geoprolog don’t provide one then have them order some immediately. It is very important.

IF for some ‘crazy’ reason there isn’t a grain size comparator at hand in the mudlogging unit, AND the WSG does not posses his own then…

By using this simple method of using the tip of a propeller pencil (0.5 = medium) you can make a rough estimation of grainsize.

Sorting

Very well 90% of grains in one grain size class. Well 90% of grains in two or grain size classes. Moderate 90% of grains in three grain size classes. Poor 90% of grains in four or more grain size classes. Very Poorly 90% of grains in five or more grain size classes.

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Very Well SortedVery Well Sorted

Well SortedWell Sorted

Moderately SortedModerately Sorted

VF F M C VCVF F M C VC

Grain Size

Dis

tribu

tion

A term sometimes used when one can not decide which to choose.

Subangual-subrounded

Sharp edges and corners, little or no evidence of abrasion.Angular

Somewhat angular, free from sharp edges but not smoothly rounded, showing signs of slight abrasion but retaining originalform. Faces untouched while edges and corners are rounded off to some extent.

Subangular

Partially rounded, showing considerable but not complete abrasion, original form still evident but the edges and corners are rounded to smooth curves. Reduced area of original faces.

Subrounded

Round or curving in shape; original edges and corners have been smoothed of to rather broad curves and whose original faces are almost completely removed by abrasion. Some flat areas may remain.

Rounded

Original faces, edges, and corners have been destroyed by abrasion and whose entire surface consists of broad curves without any flat areas.

Well-rounded

A term sometimes used when one can not decide which to choose.

Subangual-subrounded

Sharp edges and corners, little or no evidence of abrasion.Angular

Somewhat angular, free from sharp edges but not smoothly rounded, showing signs of slight abrasion but retaining originalform. Faces untouched while edges and corners are rounded off to some extent.

Subangular

Partially rounded, showing considerable but not complete abrasion, original form still evident but the edges and corners are rounded to smooth curves. Reduced area of original faces.

Subrounded

Round or curving in shape; original edges and corners have been smoothed of to rather broad curves and whose original faces are almost completely removed by abrasion. Some flat areas may remain.

Rounded

Original faces, edges, and corners have been destroyed by abrasion and whose entire surface consists of broad curves without any flat areas.

Well-rounded

Angularity or Roundness "The degree of abrasion of a clastic particle as shown by the sharpness of its edges and corners can be expressed as the ratio of the average radius of curvature of the several edges or corners of the particle to the radius of curvature of the maximum inscribed sphere (or to one-half the nominal diameter of the particle.)"

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Angularity or Roundness It is important that the description given should be of the original detrital grain. If the grain is affected by authigenic overgrowths, this should be noted and the concepts of angularity abandoned.

Sphericity

Grains can also be described according to their shape, either low, medium or high sphericity.

Alternately they may be described as elongate, sub-elongate, sub-spherical and spherical. When choosing your preference stick to that way of describing – remember CONSISTENCY.

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15-20%Abundant10-15%Common

≤1%Trace1-5%Rare 5-10%Minor

15-20%Abundant10-15%Common

≤1%Trace1-5%Rare 5-10%Minor

Matrix and / or CEMENT

Cement is deposited chemically and matrix mechanically.

Should be described by type (silt, clay, etc) and proportion (%) of overall rock. In cuttings, clay is always described as matrix as it is not possible to determine its mode of

origin by use of a binocular microscope. Matrix

Silt acts as a matrix, speeding cementation by filling interstices, thus decreasing the size of interstitial spaces

Clay is a matrix material, which may cause loss of porosity either by compaction, or by swelling when water is introduced into the formation.

Argillaceous material can be evenly distributed in siliciclastic or carbonate rocks, or have laminated, lenticular, detrital or nodular form.

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% of Pore Space Filled

Adjective

0-30Poorly

30-70Moderately

70-100 Well

% of Pore Space Filled

Adjective

0-30Poorly

30-70Moderately

70-100 Well

Cement

Identified by type and effectiveness of the cement (calcite, quartz, dolomite etc.).

The order of precipitation of cement depends on the type of solution, number of ions in solution and the general geochemical environment.

Several different cements, or generations of cement, may occur in a given rock, separately or overgrown on or replacing one another.

Chemical cement is uncommon in sandstone which has a clay matrix. The commonest cementing materials are silica and calcite. Silica cement is common in nearly all quartz sandstones. This cement generally occurs as

secondary crystal overgrowth deposition. Opal, chalcedony and chert are other forms of siliceous cement. Dolomite and calcite are

deposited as crystals in the interstices and as aggregates in the voids. Dolomite and calcite may be indigenous to the sandstone (the sands having been a

mixture of quartz and dolomite or calcite grains) or the carbonate may have been precipitated as a coating around the sand grains before they were lithified.

Anhydrite and gypsum cements are more commonly associated with dolomite and silica than with calcite.

Additional cementing materials, usually of minor importance, include pyrite (generally as small crystals) siderite, haematite, limonite, zeolites and phosphatic material.

Cement Interpretation TIPS - Calc vs. Silica

Quite often you will not be able to see cutting aggregates to determine what the nature and amount of cementation is. i.e. PDC drilling destroys rock fabric.

When this happens you have to use your well tuned WSG detective skills. To a sample of bit crushed Quartz add HCL acid and look for reaction (calcite/dolomite or

even a proportion of each. If no reaction and drilling of the formation was relatively slow over that depth interval, you

can safely assume there is some siliceous cementation – look closer for any Quartz overgrowths.

Determining Silt detritus Content of CLST & SLST’s

I devised this method as a fairly accurate way to determine silt detritus content of claystones and siltstones. i.e. remember rock naming – over 20% of a constituent requires a modifier (Silty Claystone).

Place a cuttings sample of the lithology in a white porcelain spot tray as in the below picture.

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Add either water of some dilute HCL to the spot tray (if you add acid you can combine the 2 test at one time – saving time).

Crush the cutting/s with the bottom of a test tube or the other side of your ‘prodder’ – like this.

This will give you the first indication of Silt content – i.e. is the cutting gritty against the glass – you will also be able to hear a grinding noise.

Then look down the microscope with the test tube displacing the liquid and you will be able to clearly distinguish any silt / or sand detritus.

Look down the microscope through the test tube glass to look at the silt

content.

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From what you see you can describe the nature of the silt i.e. Quartz, detrital, and even hazard a guess at the minerology, glauconite, apatite, etc.

Also you can pour off the fluid with the clay dissolved in it leaving the detritus in the spot tray as it is more dense.

From the size of the original cuttings vs. what is left you can give a fair estimation (percentage wise) of the SILT/SAND content of the bulk lithology being described.

Using this method – granular break clean claystones that look like siltstones can easily be identified.

Common Accessory Minerals

Identified by Type: carbonaceous, pyritic, feldspathic,

micaceous, fossiliferous, cherty, glauconitic.

Amount – Trace Appearance Scattered, speckled, disseminated, floating.

Additionally colour, hardness, form (prismatic, tabular, globular, euhedral, anhedral, cubic, fibrous, rhombic, etc) can also be described..

Common Accessory Minerals Pyrite

Pale brass yellow Hardness of 6.5. Cubic crystalline structure GR = 0API Can act as a cement or be found as aggregates of crystals or disseminated, common also

replacement mineral. Calcite

Colorless, White, Pink, Yellow, Brown. Hardness of 2.5 GR = 0API Can occur as clear or milky white crystal, veins, fibrous or be amorphous.

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Dolomite

Variable: pinkish, brown, yellow, colourless, white, yellow, black. Hardness of 3.5 - 4 GR = 0 API

Siderite

Yellowish brown colour Hardness of 3.5 – 4.5 GR = 0API Sideritic carbonates usually give a dull orange mineral fluorescence when viewed in UV

light and have a slow rate of effervescence with dilute HCl. Can easily be mistaken for dolomite.

Glauconite

Varying shades of green, blue green, yellow green. Hardness of 2 High in potassium GR = 78.31 API Generally fairly glassy BUT can occur as pellets, or may be very soft and amorphous

(mushy) – not to be confused with chlorite.

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Chlorite – can look very much like Glauconite Varying shades of green, rarely red, yellow and white Hardness of 2-2.5 Vitreous pearly lustre GR = 180-250 API Chlorite is widespread in low grade metamorphic rocks such as slate and schist, in

sedimentary rocks, and as a weathering product of any rocks that are low in silica (especially igneous rocks).

Chlorite and hematite

Othoclase KAlSi3O8

Variable, Pinkish white, off-white, yellow, or shades of red, orange to brown Specific gravity - 2.6 Transparency - Translucent to opaque (rarely transparent) Hardness of 6 Lustre - Vitreous

Cleavage/fracture - Perfect in two directions, seldom twinned High in potassium GR = ~200 API Orthoclase is a member of the feldspar group and is a framework silicate. Orthoclase, also

known as alkali feldspar or K-feldspar, is one end-member of a solid solution between orthoclase and albite.

Orthoclase is found in silica-rich igneous rocks such as granite, and in high grade metamorphic rocks.

Plagioclase CaAl2Si2O8 (anorthite), NaAlSi3O8 (albite) Hardness - 6-6.5 Specific gravity - 2.6-2.8 Transparency - Translucent to opaque (rarely transparent) Colour - Usually white, grey or colourless Lustre - Vitreous Cleavage/fracture - Perfect in two directions,

Crystal habit - Prismatic, tabular GR = ~200 API

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Plagioclase consists of a solid solution between the albite and anorthite end-members, and together with quartz is the most common of the rock forming minerals.

The twinning in plagioclase produces stacks of twin layers that are typically fractions to several millimetres thick. These twinned layers can be seen as striation like grooves on the surface of the crystal and, unlike true striations, these also appear on cleavage surfaces.

Chert (microcrystalline quartz) (SiO2) includes chalcedony, agate, jasper and flint.

Variable colour Hardness of approximately 7 Conchoidal fracture Can be clear to opaque and may be mistaken for dolomite as calcareous inclusions may

occur which will effervesce slowly. Check the hardness to identify if it’s chert. Inform the company immediately on finding chert as it will ‘kill’ a PDC that is rotating at high RPM bit very fast.

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Detrital from sodium rich plutonic rocks. May survive several cycles of weathering and deposition

Red brown yellow-grey green: tetragonal crystal form

7.54.6 to 4.7Zircon

1. Hydrothermal veins2. Detrital from metasomaticphyllites3. Biogenic and diagenic in muds

Brassy yellow: occasionally black metallic lustre: conchoidal-uneven fracture: cubic or pyritohedral crystal form

6 to 6.54.95 to 5.03Pyrite

1. Detrital from acid igneous and associated metamorphic rocks 2. Low grade phyllites and schists

Colourless-pale brown/green: high lustre, strong cleavage: may be difficult to distinguish from Biotite if colour is not discernable

2.5 to 32.77 to 2.88Muscovite

1. Detrital from many small igneous rocks2. Thermally altered sediments

Black-dark grey: opaque brittle: fine -dull metallic lustre: grains lacking structure: strongly magnetic

65.2Magnetite

1. Alteration product of iron-bearing minerals2. Biogenic deposit

Yellow/brown-dark orange/brown: earthy: occasionally vitreous “varnish-1ike”coating: slowly soluble in hydrochloric acid: yellow streak

4 to 5.52.7 to 4.3LimoniteVERY COMMON

OCCURRENCEOBSERVABLE FEATURESHARDNESS (MOH’S)

DENSITY (S.G.)

MINERAL

Detrital from sodium rich plutonic rocks. May survive several cycles of weathering and deposition

Red brown yellow-grey green: tetragonal crystal form

7.54.6 to 4.7Zircon

1. Hydrothermal veins2. Detrital from metasomaticphyllites3. Biogenic and diagenic in muds

Brassy yellow: occasionally black metallic lustre: conchoidal-uneven fracture: cubic or pyritohedral crystal form

6 to 6.54.95 to 5.03Pyrite

1. Detrital from acid igneous and associated metamorphic rocks 2. Low grade phyllites and schists

Colourless-pale brown/green: high lustre, strong cleavage: may be difficult to distinguish from Biotite if colour is not discernable

2.5 to 32.77 to 2.88Muscovite

1. Detrital from many small igneous rocks2. Thermally altered sediments

Black-dark grey: opaque brittle: fine -dull metallic lustre: grains lacking structure: strongly magnetic

65.2Magnetite

1. Alteration product of iron-bearing minerals2. Biogenic deposit

Yellow/brown-dark orange/brown: earthy: occasionally vitreous “varnish-1ike”coating: slowly soluble in hydrochloric acid: yellow streak

4 to 5.52.7 to 4.3LimoniteVERY COMMON


DENSITY (S.G.)

MINERAL

Detrital from many igneous and metamorphic rocks

Black: rarely with red/brown tinge: sub-metallic lustre: embedded masses or irregular-hexagonal plates; difficulty soluble in acid: moderately magnetic: may be distinguished from magnetite by presence of greyish white alteration product, Leucoxene

5 to 64.70 to 4.78Ilmenite


Dark green-black, good cleavage: weak to moderately magnetic

5 to 63.02 to 3.45Hornblende

Detrital from all igneous and metamorphic rocks

Red/brown: dodecahedral crystal form or as spherical masses or grains: weakly magnetic

_3.13. to 3.594Hydro-grossular

Detrital from SerpentinesDark Green_3.9Uvarovite

Detrital from metamorphosed impure calcareous and calcicigneous rocks

Golden yellow-black_3.859Andradite

Detrital from metamorphosed impure calcareous rocks

Pale green-yellow: some times white

_3.594GrossularVERY COMMON


DENSITY (S.G.)

MINERAL


Black: rarely with red/brown tinge: sub-metallic lustre: embedded masses or irregular-hexagonal plates; difficulty soluble in acid: moderately magnetic: may be distinguished from magnetite by presence of greyish white alteration product, Leucoxene

5 to 64.70 to 4.78Ilmenite


Dark green-black, good cleavage: weak to moderately magnetic

5 to 63.02 to 3.45Hornblende

Detrital from all igneous and metamorphic rocks

Red/brown: dodecahedral crystal form or as spherical masses or grains: weakly magnetic

_3.13. to 3.594Hydro-grossular

Detrital from SerpentinesDark Green_3.9Uvarovite

Detrital from metamorphosed impure calcareous and calcicigneous rocks

Golden yellow-black_3.859Andradite

Detrital from metamorphosed impure calcareous rocks

Pale green-yellow: some times white

_3.594GrossularVERY COMMON


DENSITY (S.G.)

MINERAL

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1. Detrital from ultra basic igneous rocks 2. Detrital from medium grade metamorphosed argillites

Grey or green, yellow-brown: similar to Augite but iron-poor

5 to 63.21 to 3.96Enstatite

Detrital from alkaline and silica-poor metamorphic rocks

Dark blue/grey: smoky: adamantine-vitreous lustre: translucent-opaque, grains or shapeless lumps

93.98 to 4.02Corundum

Detrital from basaltic and ultramafic igneous rocks

Red, brown, black, green: high lustre; pithy, rarely of megascopic size

7.5 to 85.09Chromite

Detrital from tin-bearing acid igneous rocks

Red/brown-black: adamantine lustre: slowly dissolved by acids

93.98 to 4.02Cassiterite

1. Detrital from gabbros, dolerites and basalts2. Detrital from metamorphosed Limestones

Dull green-brown/black: presence of opaque black from weathering products will distinguish from hornblende

5 to 62.96 to 3.52Augite

Detrital from metamorphosed argillites

Pink: may be white-rose/red: subtranslucent: brittle splintery

6.5 to 7.53.13 to 3.16Andalusite

Detrital from contact and regional metamorphic rocks

Grey-bright green: opaque-translucent: vitreous lustre: may occur as tibrous growths

5 to 63.02 to 3.44ActinoliteCOMMON


DENSITY (S.G.)

MINERAL

1. Detrital from ultra basic igneous rocks 2. Detrital from medium grade metamorphosed argillites

Grey or green, yellow-brown: similar to Augite but iron-poor

5 to 63.21 to 3.96Enstatite

Detrital from alkaline and silica-poor metamorphic rocks

Dark blue/grey: smoky: adamantine-vitreous lustre: translucent-opaque, grains or shapeless lumps

93.98 to 4.02Corundum

Detrital from basaltic and ultramafic igneous rocks

Red, brown, black, green: high lustre; pithy, rarely of megascopic size

7.5 to 85.09Chromite

Detrital from tin-bearing acid igneous rocks

Red/brown-black: adamantine lustre: slowly dissolved by acids

93.98 to 4.02Cassiterite

1. Detrital from gabbros, dolerites and basalts2. Detrital from metamorphosed Limestones

Dull green-brown/black: presence of opaque black from weathering products will distinguish from hornblende

5 to 62.96 to 3.52Augite

Detrital from metamorphosed argillites

Pink: may be white-rose/red: subtranslucent: brittle splintery

6.5 to 7.53.13 to 3.16Andalusite

Detrital from contact and regional metamorphic rocks

Grey-bright green: opaque-translucent: vitreous lustre: may occur as tibrous growths

5 to 63.02 to 3.44ActinoliteCOMMON


DENSITY (S.G.)

MINERAL

Detrital from metamorphosed sandstones

White-pure blue: vitreous or pearly lustre: bladed crystals or columnar masses

5.5 to 73.53 to 3.65Kyanite

Covered in separate section

3 to 3.52.90 to 3Anhydrite

1. Dehydration of sea water2. Groundwater alteration of calcium carbonate

White or colourless: occasionally with red or blue tinge: white precipitate with barium chloride: distinguished by density and hardness

22.30 to 2.37Gypsum

Detrital from highly deformed meta-sediments e.g. greenschists, meta-greywackes

Lavender-deep blue: similar to Hornblende: distinguished by colour

63.08 to 3.30Glaucophane

Detrital from metamorphosed basic igneous rocks

Olive-yellow green; opaque-translucent: vitreous lustre, bundles of bladed prisms or needles, slow reaction with acid

63.38 to 3.49EpidoteCOMMON


DENSITY (S.G.)

MINERAL

Detrital from metamorphosed sandstones

White-pure blue: vitreous or pearly lustre: bladed crystals or columnar masses

5.5 to 73.53 to 3.65Kyanite

Covered in separate section

3 to 3.52.90 to 3Anhydrite

1. Dehydration of sea water2. Groundwater alteration of calcium carbonate

White or colourless: occasionally with red or blue tinge: white precipitate with barium chloride: distinguished by density and hardness

22.30 to 2.37Gypsum

Detrital from highly deformed meta-sediments e.g. greenschists, meta-greywackes

Lavender-deep blue: similar to Hornblende: distinguished by colour

63.08 to 3.30Glaucophane

Detrital from metamorphosed basic igneous rocks

Olive-yellow green; opaque-translucent: vitreous lustre, bundles of bladed prisms or needles, slow reaction with acid

63.38 to 3.49EpidoteCOMMON


DENSITY (S.G.)

MINERAL

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1. Detrital from granitic rocks 2. Detrital from metasomatisedbasic igneous rocks 3. Secondary mineral growth on detrital grains in sandstones 4. Replacement in Limestones

Black: very rarely green, brown, red: opaque: glassy dull lustre, long thin prisms with curved triangular cross section

73.03 to 3.25Tourmaline

1. Detrital from acid igneous rocks 2. Detrital from metamorphosed bauxite

Colourless, rarely yellow-brown or white: brittle with uneven fracture

83.49 to 3.57Topaz

1. Detrital from intermediate and acid plutonic rocks 2. Detrital from Impure calc-silicate metamorphic rocks 3. Possibly (?) digenetic in sandstones

Colourless, yellow, green brown: rhombic cross section

53.45 to 3.55Titanite

Detrital from medium grade metamorphosed argillites grits and carbonates

Blood red-yellowish brown: stout thick crystal: commonly associated with garnets

7.53.74 to 3.83Staurolite

1. Detrital from granite pegmatitesand quartz veins 2. Detrital from metamorphosed argillites3. Maturation of clays and shales

Red/brown: may be black, violet green: fine needle-like crystals in shale

6 to 6.54.23 to 5.5Rutile

1. Detrital from granitic rocks 2. Detrital from dolomitic marble

Yellow-red/brown: spherical masses or grains

3 55.0 to 5MonaziteCOMMON

OCCURRENCEOBSERVABLE FEATURESHARDNESS (MOH’S)DENSITY (S.G.)MINERAL

1. Detrital from granitic rocks 2. Detrital from metasomatisedbasic igneous rocks 3. Secondary mineral growth on detrital grains in sandstones 4. Replacement in Limestones

Black: very rarely green, brown, red: opaque: glassy dull lustre, long thin prisms with curved triangular cross section

73.03 to 3.25Tourmaline

1. Detrital from acid igneous rocks 2. Detrital from metamorphosed bauxite

Colourless, rarely yellow-brown or white: brittle with uneven fracture

83.49 to 3.57Topaz

1. Detrital from intermediate and acid plutonic rocks 2. Detrital from Impure calc-silicate metamorphic rocks 3. Possibly (?) digenetic in sandstones

Colourless, yellow, green brown: rhombic cross section

53.45 to 3.55Titanite

Detrital from medium grade metamorphosed argillites grits and carbonates

Blood red-yellowish brown: stout thick crystal: commonly associated with garnets

7.53.74 to 3.83Staurolite

1. Detrital from granite pegmatitesand quartz veins 2. Detrital from metamorphosed argillites3. Maturation of clays and shales

Red/brown: may be black, violet green: fine needle-like crystals in shale

6 to 6.54.23 to 5.5Rutile

1. Detrital from granitic rocks 2. Detrital from dolomitic marble

Yellow-red/brown: spherical masses or grains

3 55.0 to 5MonaziteCOMMON

OCCURRENCEOBSERVABLE FEATURESHARDNESS (MOH’S)DENSITY (S.G.)MINERAL

0 - 5%Nil (Tight)

5 - 10%Poor

10 - 15%Fair

15 - 20%Good

20% and greaterExcellent

0 - 5%Nil (Tight)

5 - 10%Poor

10 - 15%Fair

15 - 20%Good

20% and greaterExcellent

Porosity

Porosity estimation is very SUBJECTIVE. Different WSG have different ideas on what is good and what is bad porosity.

Visual porosity is a difficult, but a critically important parameter to evaluate. Generally one cannot see the pore spaces under the binocular microscope, except in

cases of high porosity - the observer must rely on other features for apparent porosity estimations.

NOTE: Porosity does not systematically vary with the size of the particles making up the rock. Rocks with a fine grain size may be more porous than those with coarse grain size, since porosity is defined as the percentage of pore space to the total volume of the rock.

Factors such as sorting, packing/compaction, cementation and other effects determines ultimate effective porosity.

Porosity

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In general, if you can see the porosity, it is very good to excellent. If you cannot see pores, there is a high percentage of matrix, the cuttings are smooth

textured and the interval drilled relatively slowly, then the rock is likely to have poor porosity.

The fair to good grades of porosity lie between these two described cases and experience will guide the observer. A useful technique is to describe cuttings of an offset well and to “calibrate” the descriptions of porosity with the wireline offset or RT LWD data.

Inferred Porosity

Poorly cemented sandstone cuttings will often arrive in the sample tray as loose quartz grains.

The wellsite geologist needs to search for clues as to what the real ‘in-situ’ porosity is. When this is done it is usually referred to as inferred porosity.

The constraints are:

ROP: The faster the ROP, the better the porosity? Hmm, not necessarily with modern PDC bits and deviated holes.

Cement: Observe for cementing minerals such as calcite and silica. Well developed quartz overgrowths or angular ‘broken grains’ will generally indicate harder drilling and greatly reduced porosity, while well rounded grains are generally indications of better porosity. But not if you have a lot of…

Matrix: Observe for “mushy” argillaceous material that may be associated with the sand where argillaceous material is more likely to originate from the matrix of a sand rather than a separate Claystone lithology.

Other minerals: the cleaner the sand the less likely that growth of authigenic matrix such as Illite will develop from the decay of unstable minerals such as feldspar and mica.

Fossil Identification in Cuttings Samples

The destructive action of any drill bit will almost completely destroy the vast majority of any fossils contained in the original rock.

Therefore, most commonly known macrofossils (i.e. those that can be normally seen by the naked eye) such as ammonites, bivalves, gastropods, echinoids, corals etc. will become almost unrecognisable in cuttings samples.

However, fragments of such fossils may be observed and, in some rare cases, extremely small specimens may be preserved whole. In the latter case, this can apply particularly to gastropods and bivalves (in which case they are referred to in literature as "microgastropods" and "microbivalves").

Another group of fossils that can be observed whole in cuttings samples (i.e. unaffected by the drilling process) are microfossils, specifically foraminifera, ostracods, diatoms, radiolaria and sponge spicules.

Other familiar "microfossils" such as palynomorphs (spores, pollen and dinoflagellates) and calcareous nannofossils are likewise preserved whole, but are much too small to be observed even with a higher-powered geological binocular microscope.

Even those microfossils mentioned are quite small with the most common sizes ranging from 0.2mm – 0.5mm, and therefore even they may be difficult to spot using a normal microscope.

The identification of fossils or fossil fragments cuttings sample, even at a relatively non-specific level, can often provide much useful information concerning the depositional environment of the original sediment.

Several drilling factors can affect the likelihood of observing fossils in cuttings samples. The most important factor in this respect is bit selection.

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Cuttings generated by rock bits and most PDC type bits on "traditional" or rotary-steerable assemblies tend to yield relatively good numbers of fossils and fossil debris.

PDC bits when combined with downhole (mud) motors generally yield only moderate fossil recovery. When PDC bits are coupled with a downhole turbine, almost all fossil evidence is destroyed by the high RPMs (and consequent thermal attrition) associated with such assemblies.

Mud type is also a factor in that oil-based-muds may also have a detrimental effect on fossil recovery.

Microgastropods, look like very small versions of their "normal" size counterparts. However, they can also easily be confused with certain types of foraminifera (a microfossil).

IDENTIFICATION - "Microgastropods"

The simplest comparison to make for gastropods is that they look like snails or certain types of sea shells such as whelks or periwinkles. The shell is coiled - either in a high, cone-like appearance similar to a whelk, or in a lower, more globular fashion similar to a periwinkle or land snail.

"Microbivalves"

Microbivalves also look like very small versions of their counterparts - bivalves. As the name suggests, these are comprised of two similar-size half-shells which lock together along a hinge line. They are vaguely similar in appearance to a pair of castanets and tend to be somewhat circular in outline.

Foraminifera

Foraminifera are a very common component of marine sediments and therefore may be expected to be found in most cuttings samples from marine sediments. Foraminifera ("forams") are single-celled animals and have a bewildering variety of different shapes. They can range in size from <0.2mm up to several centimetres, although the vast majority are between 0.2mm - 0.5mm in size.

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Benthonic foraminiferaA sub-group of the calcareous benthonic foraminifera known as the "porcellanous" group because of their walls' resemblance to white porcelain are typically recorded from shallow, warm, tropical waters (see below) The below picture shows several types of benthonic foraminifera .

This large, brown specimen near the bottom right corner is an agglutinated foram and has a

distinctly "grainy" surface texture. The specimen in the top right corner is also

agglutinated.

All the other specimens are calcareous

benthonic forams. They have generally smooth glassy or opaque walls

although the ovoid specimen near the top

middle of the picture has longitudinal striations

on the surface.

Three main groups of Foraminifera (Forams)

PLANCTONIC - those that live by floating in oceanic waters and form their shells by secreting calcium

carbonate

CACLACEOUS BENTHONIC - those

that live on the sea bed and also form their shells by secreting calcium carbonate

AGGLUTINATED - those that also live on the sea

bed but form their shells by sticking

detrital grains (normally sand or silt) onto their

naked bodies.

Generally (but not always) have a golf-ball-like punctated/ reticulated shell wall

Generally (but not always) have a smooth and shiny, or sometimes smooth and dull, surface texture

It is sometimes possible (if conditions are good enough), to determine which of the three groups a specimen belongs to under the normal geological microscope

Generally (but not always) have a sugary-like or "gritty" surface texture

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Planktonic Foraminifera

This SEM (Scanning electron Microscope) illustration below shows several types of planktonic foraminifera. The golf ball-like texture can be seen on most of the specimens though spinose ones (top right) do occur.

Osctarcods

Ostracods are occasionally observed in unprepared cuttings samples but, like bivalves, are comprised of two similar-size half-shells which lock together along a hinge line. However, in many cases the two ostracod half-shells will have become separated. Unlike bivalves, ostracods generally tend to be more elongated in outline and have a vaguely "potato" shaped appearance. The surface may also be variously ornamented with ribs, reticulation and pustules.

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Diatoms

Diatoms are single-celled algae with a siliceous shell and are also only rarely observed in unprepared cuttings. However, they are often preserved as pyrite moulds which causes them to stand out from the background rock cuttings. They are almost invariably either disk-like, often resembling a pill-box or aspirin tablet, or flattened triangular in shape. The photos below are somewhat atypical in that the detailed surface features shown are almost never observed in fossil specimens .

Radiolaria

Radiolaria are, like forams, single-celled animals, but they construct their shells using silica (like Diatoms) rather than calcium carbonate, and also build their shells in a slightly different way. In appearance that tend to resemble planktonic foraminifera in that they also display a golf-ball-like surface texture.

However, being siliceous rather than calcareous, they will not of course react to acid (although replacement by calcite has been known to take place occasionally). Radiolaria tend to be either spherical, lens-like or bell-shaped, although the spherical forms are likely to be more commonly observed. In certain formations such as they, they are often found as pyritised moulds.

CASE STUDY:

Diatomite. What is it?

Russia/WSG roll

Nano-paleontologist

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Most of the ("normal size") shell fragments in this picture are of bivalves

although a gastropod (blueish colour near bottom right corner) can also be

seen.

It is not unusual to find horizons within formations with abundant shell

fragments/shell debris – know as “shell beds.”

Sponge Spicules

Sponges are metazoans (multicellular animals) which inhabit the sea floor. They are built by many thousands of interlocking, siliceous rods called "spicules." They are very delicate and are not commonly seen in cuttings samples. Some spicules can be subspheroid or ovoid in shape and typical of these types is a form called "Rhaxella“ which resembles a very well rounded, frosted quartz grain with a slight dimple on one side giving it the vague appearance of a glassy kidney bean.

Shell Fragments (very common in samples)

General shelly material is often found in cuttings samples although it can be difficult to determine its origins. The most likely origin for most cuttings-size shelly material is probably from bivalves although gastropod and echinoid origins cannot be ruled out without specialist scrutiny.

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Inoceramus Fragments

Inoceramus is a particular kind of Cretaceous bivalve (often achieving very large size of up to 1 metre across), small plate-like fragments of which are commonly recorded from chalks and marls. They tend to have a pale orange or brown colouration and appear somewhat "chunky". The Inoceramus shell is composed of calcite prismatic hexagonal rods and therefore the broken surface of an Inoceramus fragment when viewed side-on may resemble columnar basalt in appearance if not in size.

Also, Inoceramus fragments have frequently been recorded erroneously (wrongly) by some WSG as "vein calcite".

Echinoids

Echinoid (starfish, sea urchins etc.) debris can often be indistinguishable from general shell debris without specialist knowledge.

However, echinoids often possess spines and these can sometimes be identified. The spines can range from long and thin spikes which are often fragile and completely destroyed by drilling, to short stubby spikes which can sometimes be observed.

A typical echinoid spine will often have a more bulbous knob on the end which originally formed the point of attachment to the main body of the animal although in some species the bulbous knob is at the distal end of the spine (see photo).

The spine itself is often striated in appearance rather than being completely smooth. Echinoid spines can be commonly found in some chalks.

These echinoid spines – a common feature within chalk samples – are

characteristically bulbous at the distal end.

These echinoid spines are from large-size specimens but those found within cuttings samples are similar in overall

shape and appearance.

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Ammonites

Ammonite fragments are only very rarely observed in cuttings, and unless certain characteristic features can be observed at that scale (i.e. suture lines) the fragments may easily be mistaken for something else.

This ammonite specimen clearly shows numerous complex, florid suture lines which may be observable in some cuttings-sized fragments.

Charaphytes

Charaphytes are the remains of part of the reproductive mechanisms of a specialist group of freshwater algae.

In appearance they are of similar size to the microfossils (0.2mm – 1mm) and are generally globular or ovoid in shape. Characteristically they have a spiral groove-like structure covering the entire surface.

However, they are only extremely rarely recorded in cuttings samples as they originate from fresh to slightly brackish water settings – an environment which does not "preserve" well in the sedimentary record.

This illustration shows the Charaphyte plant, together with the reproductive cells which are the only parts found as fossils.

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Significance of Fossils in Cuttings - reference

With the exception of charaphytes which have a fresh water origin, the vast majority of fossils described above are recorded from the marine realm. However, it is possible to derive at least some palaeoenvironmental information from any data observed.

Most of the groups live in habitats found at the sea floor (benthonic organisms). The exceptions to this are the planktonic foraminifera, diatoms and radiolaria (planktonic organisms).

These planktonic organisms are, in most cases, restricted to true oceanic environments, or to shelf seas which have good open marine connections to oceans. Planktonic forams and radiolaria are particularly sensitive to reductions in salinity and therefore their presence in a cuttings sample is usually a good indicator of open marine conditions with water depths of no less than 30 metres and little or no fresh water influences.

Planktonic forams and radiolaria are extremely abundant in the surface waters of the open ocean and can form foraminiferal and radiolarian "oozes" as deep oceanic sediments – discussed previously Diatomite/Diatomaceous Ooze.

Benthonic organisms tend to be more sensitive to local environmental conditions and can vary widely from place to place.

The majority of marine benthonic organisms tend to occur on the shelf and upper parts of the continental slope, although benthonic foraminifera (both types) can be found in very deep waters.

Agglutinated forams and radiolaria, since they have no calcium carbonate in their shell structure, can withstand conditions (anoxic or dysaerobic), therefore they can be found down to water depths of 6000m plus.

Agglutinated forams are also often found thriving in marshy or shallow brackish water conditions also so their presence cannot alone be relied upon for exact palaeoenvironmental determination without specialist knowledge.

The presence of types of forams known as the "porcellanous" group can be useful to identify warm (tropical), shallow, clear water environments.

They are common in many limestones. Care should be noted however, as some of the "porcellanous" forms are also recorded from oceanic sediments beneath waters several thousands of metres deep.

Sedimentary context of the cuttings samples, will enable to geologist to differentiate between the two environments.

Calcareous / Domomitic nature of Clastic Rocks

As well as carbonates, Argillaceous rocks should be tested with HCL for calcium carbonate and dolomite composition.

Arenaceous Siliclastics should be tested to determine the cement and matrix composition. I usually place the calcareous comment as the last item on a Claystone and Siltstone

description. Arenaceous Siliclastics descriptions should denote when describing the matrix and/or

cement.

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80 - 10020 - 0Claystone

60 - 8040 - 20Calcareous claystone

40 - 6060 - 40Marl

20 - 4080 - 60Argillaceous calcilutite

0 - 20100 - 80Calcilutite(mudstone)

% Clay Material% Calcareous Material

Rock type

80 - 10020 - 0Claystone

60 - 8040 - 20Calcareous claystone

40 - 6060 - 40Marl

20 - 4080 - 60Argillaceous calcilutite

0 - 20100 - 80Calcilutite(mudstone)

% Clay Material% Calcareous Material

Rock type

NOTE: carbonates will be covered in a separate module

Violent effervescence; frothy audible reactions.Dolomite

No effervescence; no immediate reaction; slow formation of CO2 beads, reaction slowly accelerates until a thin stream of fine beads rises to the surface – heat to increase speed of reaction.

Dolomite

50% HCL reactionRock Type

Mild emission of CO2 beads, specimen may rock up and down, but tends to remain in one place

Calcareous Dolomite

Brisk, quiet effervescence; specimen skids about the bottom of the container, rises slightly off the bottom, continuous flow of CO2 beads through the acid

Dolomitic Limestone

Violent effervescence; frothy audible reactions; specimen bobs about and tends to float to the surface

Limestone


Violent effervescence; frothy audible reactions.Dolomite

No effervescence; no immediate reaction; slow formation of CO2 beads, reaction slowly accelerates until a thin stream of fine beads rises to the surface – heat to increase speed of reaction.

Dolomite


Mild emission of CO2 beads, specimen may rock up and down, but tends to remain in one place

Calcareous Dolomite

Brisk, quiet effervescence; specimen skids about the bottom of the container, rises slightly off the bottom, continuous flow of CO2 beads through the acid

Dolomitic Limestone

Violent effervescence; frothy audible reactions; specimen bobs about and tends to float to the surface

Limestone


Calcareous Rocks Classification HCL test TIPs

To save time describing samples and if dolomite is suspected I tend to forget about the 10% and test directly with the 50%. This will give an immediate vigorous reaction.

As I sated earlier I tend to combine the HCL test with my SILT/SAND test to save time. Some people may not agree with me BUT (like with shows) if a lithology being described

is non calcareous then state so in your description.

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Anthracite

Bituminous coalLignite

Humic Coal

Humic Coal: woody, plant tissue dominant (gas-prone source rock). Further divisible by rank i.e. on the decreasing proportion of volatile constituents (primarily water) ie. peat → lignite → sub-bituminous → bituminous → semi-bituminous → anthracitic (decreasing water).

Distinguished by appearance and texture - laminated, friable in part, jointed, fibrous, bright ‘jet’ like layers, variable lustre, hardness/brittleness.

Sapropelic Coal

Non-woody, comprises spores, algae and macerated plant material (oil-prone source rock). Distinguished by massive unlaminated glassy appearance, conchoidal fracture, firm rather

than hard.

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>5090 - 1201.40 - 1.80Anthracite

>50110 - 1401.24 - 1.50Bituminous Coal

>500.90 - 1.25SapropelicCoal

>50140 - 1800.70 - 1.50Lignite

∅NΔt(μsec/ft)

ρ(g/cm3)COAL TYPE

>5090 - 1201.40 - 1.80Anthracite

>50110 - 1401.24 - 1.50Bituminous Coal

>500.90 - 1.25SapropelicCoal

>50140 - 1800.70 - 1.50Lignite

∅NΔt(μsec/ft)

ρ(g/cm3)COAL TYPE

Be aware of what is being added to the

mud and what it looks like in a sample tray, these are “raw”examples and very often change when added to the mud system! Discuss.

Coal

Check coals for fluorescence, cut and crush cut fluorescence. Coals are clearly definable on wireline logs, particularly density-neutron. Neutron porosity

is high due to the high hydrogen content of coal. Bituminous Rocks

Dark shales and carbonates may contain organic matter in the form of kerogen or bitumen. Dark, bituminous shales have a characteristic chocolate brown streak which is very

distinctive. The reverse side of a porcelain spot dish makes a handy streak plate for testing this.

Mud Additives

A variety are used in drilling operations for various reasons. Reference samples should be kept in the logging unit like the below picture.

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Calcium Carbonate AKA Baracarb

Is used as a fluid loss additive when drilling through reservoirs. Very fine to medium sized clear to translucent calcite crystals. Often mistaken for sand. Add 10% HCl to identify.

If graded calcium carbonate has just been added to the mud system, and is flooding the samples making it hard to identify the presence of sand, do the following:

Take a small amount of sample and place it on a separate sample tray and apply acid to dissolve the calcium carbonate.

Whatever is left is the real formation sand minus any calcite cement of course – be aware of that.

Common Mud Additives

LCM material to control drilling fluid losses: Nut plug: Black very hard, sometimes brown, woody, doesn’t look like any formation –

easy to distinguish. Mica: LCM material. White mica is generally used, often graded into fine, medium and

coarse. Barite: orange brown material used to weight up the mud, often mistaken for silt to very

fine sand, high density. Be careful when drilling with heavy muds (high barite content). Numerous geologist have described barite as Quartz sand!

Ilmenite: Recently barite has been replaced in some counties (for environmental reasons) for ILMENITE. This is a black powder and unlike Barite it is easily distinguished in samples.

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Oil Shows – Fluorescence & Show descriptions

All cuttings sample lithologies should be checked for oil. Not only is oil (hydrocarbons) found in Sandstones and Limestones (50% of the world reservoirs are Limestone), but also in tight Siltstones and Claystones too!!

Tight Siltstones and Argillaceous Siltstones with zero visible porosity can frequently have oil shows, lignite & source rock Claystones and Carbonaceous Claystones can also be packed with kerogens and oil.

When testing tight Sandstone, Siltstone and Claystone lithologies, the lack of permeability in the rock means simple solvent cut test with will not give results even if the lithology is exhibiting quite a strong direct fluorescence (DF) (occasionally rare pinpoint diffuse CF may be seen from broken cutting surfaces).

When testing these lithologies it is CRITICAL the CRUSH cut test is performed – discussed in later slides.

Fluorescence Oil fluorescence is brought about by the excitation of electrons by ultraviolet light from their ground state to a higher energy level and the subsequent return of the electrons to their ground state accompanied by the emission of a quantum of energy perceived as colors. Which is a fancy way of saying a photon is emitted at a different energy level. What does the fluorescence colour tell us? The fluorescence color observed depends on the API gravity of oils.

Dry gas no fluorescence

Gas/condensate white to blue-white, frequently "spotty"

35-45º API blue-white to light yellow

25-35º API light yellow - dark straw yellow

15-25º API dark straw yellow - orange brown

less than 15º orange brown - no fluorescence

Mineral Fluorescence Mineral fluorescence is distinguished from hydrocarbon fluorescence by the lack of cut fluorescence – in most cases. The diagnostic natural fluorescence colours are shown below:

Mineral Colour of Fluorescence Amber bright yellow to white (occasional cut) Dolomite subtle purple-white Calcite variety of colours from dull yellow and dull

brown to distinctive orange Limestone generally little or no fluorescence Feldspars variable bright yellowish white to white

when partial weathering to Clay may occasionally exhibit a slight cut caused by the clay dissolving in the solvent.

Lignite blue-white Chert dull brown/yellow

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Oil Show Description Procedure (WBM Systems) Reagent Cut Test Any samples exhibiting fluorescence should be treated with a solvent such as Trichloroethane (now illegal as it is carcinogenic), or more commonly Iso-Propanol. Discuss. The colour resulting from the addition of the solvent to a dried sample is known as the “cut” when viewed in natural (white) light. When viewed in ultraviolet light (UV), the colour is described as “cut fluorescence”. It is very important that lithology and percentages are stated & if a stain, cut or ring is invisible, say so, rather than not saying anything Sometimes WSG’s are known just to write a show description for a specific cutting sample depth, without reference to what lithology or giving a percentage of the lithology that contains oil show An example of how a correct show should be described is: 70% SANDSTONE: medium light grey to light olive grey, etc… SHOW in SANDSTONE: 80 to 90% with etc… Oil Shows should be described in 7 distinct stages. 1) Smell the sample Get your nose into the sample tray and describe any hydrocarbon odour This may range from heavy, characteristic of low gravity oil, to light and penetrating as for condensate. Describe as weak, moderate/light, strong/heavy or no odour 2) Cuttings in white light (visible staining) The amount by which cuttings and cores will be flushed on their way to surface is largely a function of their permeability. In very permeable rocks the drill cuttings retain only a small amount of oil. Often bleeding oil and gas may be observed in cores, and sometimes in drill cuttings, from relatively tight formations. Using the binocular microscope search the tray and described as visible, with colour and form, or invisible. Give percentages of the tray that contains oil staining. Examples of this would be: SHOW in (70%) SANDSTONE: strong HC odour, 80 to 90% with even to locally patchy visible brownish black oil stain… OR SHOW in (70%) SANDSTONE: 20% with spotted visible black free globular oil… OR SHOW in (50%) SANDSTONE: 100% with even pale brown visible oil stain… OR SHOW in (70%) SANDSTONE: No visible oil stain… 3) Cuttings under UV light Place the whole sample tray under the fluoroscope for examination. Describe fluorescence, colour, intensity and form. Also, this is important, please refer to the percentage of the tray exhibition UV shows. Examples of this would be: SHOW in (70%) SANDSTONE: strong HC odour, 80 to 90% with even to locally patchy visible

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brownish black oil stain, 90 to 100% even, moderately bright to bright yellowish gold direct fluorescence (DF). OR 40% bright milky yellowish white spotted to locally patchy DF… OR 100% Even dull orange brown DF… OR Trace (1-2%) pinpoint very bright straw yellow DF… OR No DF… 4) Solvent Cut under white light Select some suitable cuttings where visible light oil staining is evident or UV DF. Place aggregates in white spot tray and add drops of solvent. Describe cut as visible, with colour and speed of cut, or no cut. **The speed of the solvent cut coming from a cutting aggregate is an indication of the permeability of the formation** Examples of this would be: SHOW in (70%) SANDSTONE: strong HC odour, 80 to 90% with even to locally patchy visible brownish black oil stain, 90 to 100% even, moderately bright to bright yellowish gold direct fluorescence (DF), instant dark brownish black cut/tea… ** You may have seen or heard this expression before? ‘TEA’ is used to describe the colour of a solvent cut in white light** OR thick black flashing tea (cut)… OR slow blooming (or steaming) pale brown tea cut… OR very pale brown diffuse cut… OR NO cut/tea… Solvent Cut under white light TOTAL colour chart

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5) Solvent Cut under UV light Examine an aggregate in the fluoroscope for cut fluorescence, also examine an aggregate that has been dried then crushed Reservoirs with low permeability may not show a cut fluorescence but will show a crush cut fluorescence. Describe fluorescence, intensity and speed of cut fluorescence/crush cut fluorescence or say no CF. Examples of this would be: SHOW in (70%) SANDSTONE: strong HC odour, 80 to 90% with even to locally patchy visible brownish black oil stain, 90 to 100% even, moderately bright to bright yellowish gold direct fluorescence (DF), instant dark brownish black tea cut, instant flashing bright yellowish white cut fluorescence (CF)… OR moderately bright slow blooming (or pinpoint steaming) yellowish green CF… OR very slow pale diffuse milky bluish white CF… OR trace diffuse moderately bright milky white CF, instant flashing moderate milky white crush cut fluorescence (CCF)…OR no CF/CCF Blooming vs. Streaming 6) Ring under UV light Allow the solvent to evaporate and describe any residual ring fluorescence. Describe intensity, thickness of the residual ring and colour. Examples of this would be: SHOW in (70%) SANDSTONE: strong HC odour, 80 to 90% with even to locally patchy visible brownish black oil stain, 90 to 100% even, moderately bright to bright yellowish gold direct fluorescence (DF), instant dark brownish black tea, instant flashing bright yellowish white solvent cut (SC), moderately bright thick solid yellowish gold residual UV ring… OR moderately bright thin veneer to locally spotted residual UV ring… OR pale fine spotted milky white to yellowish white pinpoint residue…

Note: keep a reference sample of the solvent –

some exhibit slight direct fluorescence

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OR no residual UV ring 7) Residue under white light Allow the solvent to evaporate and describe residue. Describe as visible with colour or invisible. Examples of this would be: SHOW in (70%) SANDSTONE: strong HC odour, 80 to 90% with even to locally patchy visible brownish black oil stain, 90 to 100% even, moderately bright to bright yellowish gold direct fluorescence (DF), instant dark brownish black tea, instant flashing yellowish white solvent cut (SC), moderately bright thick solid yellowish gold residual UV ring, thick even brownish black residue… OR thin moderate brown veneer residue… OR trace pale brown to light tan ring residue… OR no residue… Dead Oil

There has been much confusion, inconsistency and misunderstanding concerning the usage of this term.

It has been used to describe oils that are either very waxy and solid, non-producible or immobile. All of those definitions are misleading and deceptive.

In addition, it has never been clear whether or not so-called “dead oils” exhibit fluorescence and cut fluorescence.

In view of the above the term “dead oil” should only be used to describe thermally dead, solid hydrocarbons that DO NOT fluoresce. Whenever the term is used, qualifying data should be given.

Oil Show Description Flowchart

Cut No Cut

ADD SOLVENT

Note Percentage of lithology fluorescing

Note colour, form and intensity of

fluorescenceNote colour and

speed in of cut in visible light

Note colour and speed in of cut in

UV light

Note colour of cut fluorescence and ring fluorescence

Note colour of residue in white

light

Crush some dry sample - spot tray

or mortar and pestle

Add solvent -repeat the process

for samples exhibiting cut

Record any cut as ‘crush cut’ in description

Smell the sample tray - note the odour

Note colour of crush cut fluorescence

and ring fluorescence… etc

NB: crush cut ring F should be seen on blotting paper but for quick look interpretation -crushed DRY sample in spot

tray will suffice

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Give reference to Oil distribution inside the rock

In your description it is very useful to comment on where the oil/visible light staining/fluorescence is distributed within the lithology…i.e. is the free oil/staining/fluorescence: 1. Coating on grains. 2. Free globular in rock matrix (intergranular) 3. Intercrystalline, vuggy (carbonates). 4. In fractures (very important state the depth of oil invasion within the fractures). 5. OR evenly (uniformly) dispersed (source rock Claystones).

Loss of volatiles

For best results and consistency it is best to test the samples for shows as soon as they are collected.

The reason behind this is that some light grade oils and condensates will be lost over time by evaporation.

This shouldn’t be and issue for the WSG as it is advanced prior to entering or during drilling of a target reservoir that they spend the majority of your time in the mudlogging unit.

Oil Show Description (SOBM Systems)

For obvious reasons, it is very difficult to ascertain any Oil Shows in cuttings drilled with OBM.

When you look at a sample tray of cuttings drilled with OBM under the fluoroscope the whole sample tray will fluoresce.

Great care must be taken reporting ANY shows to your SOG (town) and on your Complog/Litholog.

The ability to see ‘REAL’ shows will largely depend on the nature (API gravity) of the real oil.

Masking is the term we use to describe what the OBM does to the real oil shows – it MASKS them!

The OBM (even after washing with detergent) will tend to coat the cuttings with a film of oil. As stated previously if there is ANY porosity or permeability in the cuttings (e.g. drilled

Sandstones and Silstones), then during the drilling process and the cuttings transit from TD to surface in the annulus, permeable/porous cuttings with be FLUSHED to some extent by the hydrostatic pressure and flow of the mud etc.

This process can TOTALLY MASK the real oil shows in the cuttings. In general, you will only be able to distinguish real oil shows if the fluorescence/visible light

oil staining is significantly different from the OBM. The rule being that identification of real oil shows vs. OBM is easier when the real oil is a

lower API gravity that the base oil in the OBM. i.e heavier lower API grade, darker (API gravity of 15-25° API).

Most modern OBM give quite a distinct moderately bright yellowish green fluorescence. TEPI OBM seems to give a dull orange DF.

Dry gas no fluorescence

Gas/condensate white to blue-white, frequently "spotty"

35-45º API blue-white to light yellow

25-35º API light yellow - dark straw yellow

15-25º API dark straw yellow - orange brown

less than 15º orange brown - no fluorescence

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Examination Process & Tips

For WBM cuttings follow EXACTLY the same procedure and description technique as you would do with cuttings drilled with water based mud (WBM).

BUT… Keep samples of BASE OIL and MUD in the fluoroscope for comparison. Change the reference mud sample at each shift change – there may have been

additions/changes to the mud since your last sample examination. Perform various cut fluorescence tests of the sample of mud and base oil – keep these

as reference, note visible light colour, UV fluorescence, colour of cut and residue under UV and visible light.

It can be useful to report these show for comparison on your DGR. In OBM, quite often the visible light oil stain is easier to see than the UV fluorescence.

Thoroughly check samples for visible free oil in pore spaces. If running LWD logs use them as reference and pay particular close attention to samples

that LWD resistivity is high. Sandstones AND Claystones. Increasing resistivity (over the normal compaction trend) can be an indication of entering a source rock formation.

Examining Core Chips/SWC (OBM)

Real oil shows can easily be determined from core samples and SWC. There will be a ‘flush zone’ on the outer surface of the cores which will be a function of

the formations porosity and permeability. It will be clearly visible under the fluoroscope. When examining chips or whole SWC try to liberate some of the fresh formation for

examination – be careful to avoid any contamination as this will affect the overall cut. WBM contaminants

Be aware that in some modern WBM systems it is quite common for synthetic oil products to be added. These generally act as drilling torque reducers/lubricators.

GlydrillTM being one of these products and is run at 5% in the mud system. This can give a significant contamination mud therefore to the drilled cuttings as well. When drilling with these products, keep reference samples of the Glydrill and mud as if

you were drilling in a OBM regime.

Other Contaminants – PIPE DOPE

What’s that? Pipe dope (essentially thick grease) is applied to the drill pipe during connections,

occasionally the dope may end up in the sample. It will generally give a golden brown dull fluorescence and will occur as greasy blobs.

Recording Oil Shows on the Litholog

As well as a description of the Oil Show being included after the lithology description it is also required to annotate the litholog/mudlog with black bars over the intervals exhibiting shows.

These are normally on a grading scale of 1-3 or 1-6 depending on the oil company. Discuss.

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Other tests for Oil Shows – Emulsion ‘Pop’ Test WBM

You can also see shows in the mud system, commonly smell hydrocarbons in the shaker, flowline area and even see clear hydrocarbons floating on top of the mud (pits, flowline).

Samples of fresh mud from the flowline can be collected and poured into a tray, inspect the mud samples under the fluoroscope for shows and on some occasions oil may be seen ‘popping’ at the surface of the mud.

Then add some water to the mud which lowers the viscosity of the mud and separates the mud from the oil. By this method small samples of oil can be skimmed of the top of the mud.

Finally the mixture can be placed in a bottle and shaken. The results should be monitor and the results described. Light grade oils are liable to evaporate so the sample should be closely monitored.

You can repeat the process with bulk wet cuttings.

Emulsion ‘Pop’ Test – Classification table

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Example #1

SHOWS in 100% SST: no noticeable odour on core, 30% light creamy yellowish staining, 80% even moderately bright yellowish white direct fluorescence, dull very slow diffuse milky white solvent cut from aggregate, instant diffuse white crush cut, very pale yellow tea, moderate spotted milky white residual ring in UV light, nil to rare trace pale brown residue.

Example #2

SHOWS in 100% medium SST: strong petroliferous odour, 70% spotted dark black visible oil staining and local greasy coating on grains, free solidified waxy oil seen in pore spaces, 100% even very dull direct fluorescence, dull very slow diffuse yellow solvent cut, dark brown black tea, thick moderately bright yellow residual ring in UV light becoming dull brown black with time, thick brown black oil residue.

Example #3

SHOWS in 100% SST: moderate petroliferous odour, 70% light creamy yellow to locally dark patchy brown visible staining, 90% moderately bright to bright milky yellow direct fluorescence, instant flashing milky white solvent cut from loose grains and aggregate, very pale yellow tea, moderate spotted milky white residual ring in UV light, nil to very pale yellow residual veneer.

Example #4

SHOWS in 100% Fine grained SST: no noticeable odour(looks wet/waterized?) no visible staining, possible trace spotted traces of black bituminous flecks? 80% dull with trace <10% pinpoint spotted bright white direct fluorescence, moderate slow diffuse milky white solvent cut, no tea, faint weakly spotted milky white residue in UV light, nil in visible light, possibly some mineral Feldspar fluorescence?

cuttings descriptions-clastic

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