FE 02Basic Mud Logging Seminar

Seminar ObjectivesHow hydrocarbons are formedThe search for oil and gasPresent the parts of the rigOutline the steps taken in drilling and completing a wellOverview of basic mud logging operations

How Are Hydrocarbon Deposits Formed And Trapped?

Life and Geologic TimeThe presence of C,H,N and O created the conditions of life on EarthFirst bacteria appeared 3.5 billion yrs agoOil and gas derived entirely from decayed organic matter

Death, Decomposition and BurialMost organic waste is destroyed by bacteriaSome are deposited in oxygen-poor aquatic milieus and are protected from bacterial actionSedimentary layers sink naturally or are buried deeper by tectonic forces

MaturationMaturation is the cooking of organic matter trapped in sedimentary layersIncreasing heat and pressure causes kerogen to formSource rocks are rocks that contain sufficient substances to generate oil and gasWhen a source rock starts to generate oil or gas it is said to be mature

KerogenDark-coloredIndefinite and complex mixture of compounds with large molecules containing mainly H and C but also O, N, and SInsoluble in water and petroleum solventsIt may consist of compacted organic materialSource of most hydrocarbons generated

Oil WindowSpecial environment where oil is generatedThe temperature in the oil window:65 and 150 C

Gas WindowSpecial environment where gas is generatedTemperature range: 120 to 225 C

Oil and Gas Windows

MigrationMigration is the tendency to move about and away from the source rock.A slow process2 types: (primary migration) and (secondary migration) Triggered by natural compaction and the processes of oil and gas formationOil and gas migrate upward and through porous and permeable beds

Primary Vs. Secondary MigrationPrimary Migration release of oil from kerogen and its movement in the narrow pores and capillaries of the source rockSecondary Migration movement of hydrocarbons away from the source rock

Porosity and PermeabilityPorosity is the amount of void space in a rockPermeability is the degree of interconnectivity of these voids (pores)

EntrapmentOil and gas fields need to be trapped in permeable reservoir rocks and capped by a seal called a cap rock

Reservoir RockA subsurface body of rock having sufficient porosity (usu. 5% to 30%) and permeability to store and transmit fluids. Contains little, if any, insoluble organic matterMost hydrocarbons are found in clastic reservoirs (sandstones and siltstones) followed by carbonate reservoirsPetroleum accumulations are rarely found in shales, igneous and metamorphic rocks

Traps2 main types of traps:Structural trapsStratigraphic trapsTraps must exist before hydrocarbon generation ceases

Structural TrapsTraps formed by Earth movementsExamples: Fault trapsAnticlinal traps (80% of the worlds oilfields are in this category)Rocks are domed over rising salt masses

Stratigraphic TrapsTraps that result when the reservoir bed is sealed by other beds or by a change in porosity or permeability within the reservoir bed itself Examples:a tilted or inclined layer of petroleum-bearing rock is cutoff or truncated by an essentially horizontal, impermeable rock layer petroleum-bearing formation pinches out reservoir bed is surrounded by impermeable rock change in porosity and permeability in the reservoir itself

A Final WordOnly about 2% of the organic matter dispersed in sediments become petroleumOnly 0.5% of it gets accumulated in reservoirs

End of Topic

The Search for Oil

Buried TreasureOil seeps used for medicinal purposes, lighting fuel, caulking boats and for military purposesIndustrial Revolution saw a need for better source of fuelFirst oil well completed by Edwin Drake in 1859

Buried TreasureOil exploration involves science, creativity, and also good luck

The Decision to ExploreOil Exploration is undertaken by a variety of people for a variety of reasons such as:Governments Development of local resources reducing dependence upon othersDevelopment of industry / EmploymentPrivate CompaniesFinancial Gain

Government Run CompaniesExamples of National Oil Companies include:

PTTThailandPertaminaIndonesiaPetronasMalaysiaPetroBrasBrazil

Private Oil CompaniesMajorsShellExxon MobilBP-Amoco-ArcoChevron-TexacoIndependents (range from Multi-national to local)LasmoGulf ResourcesUnocal

Economic FrameworkProduction Sharing Contracts (P.S.C.)A system whereby a Government leases acreage for Private Oil Companies or National Oil Companies to explore.Economic Terms vary dependant upon a wide variety of factors including:Country tax systemLocation of acreageDuration of agreementAnd many more !

Where to Drill?This decision is made at a variety of levels:Global:Which Country/Basin should we drill in?Economic Terms?Political Stability?Availability of prospective acreage?Market / Infrastructure?This decision is part of a companies overall strategy, made by upper management and New Ventures explorationists.

Where to Drill?Local:Which basin?Which province / block?Which structure?Where on the structure?Which formations are prospective?

Sedimentary BasinsAreas of extensive and prolonged subsidence where large accumulations of sediments occur and are hosts of hydrocarbon deposits600 known sedimentary basins of which 160 sedimentary basins have yielded oil but only 26 are significant producersExploration has occurred in another 240 basins, but no discoveriesThe most predominant sedimentary basin is the Arabian-Iranian basin (>20 super giant fields)

Sedimentary Basins

The Job of the GeologistObserve, explore and record any clues related to hydrocarbonsReconstitute a scenario to identify places of oil/gas accumulationsWork with other specialists

The Job of the GeophysicistInitially use gravimetry and magnetometry and later seismic surveysFirst 2 are regional in scale, less costly than seismic and are used to identify potentially oil-bearing sedimentary basins

GravimetryGravity surveys can be used to map the extent or depth of sedimentary basins or even individual hydrocarbon prospects.Gravimetry - the measurement of gravitational acceleration over an area, usually presented as a map or profile of Bouguer or free-air anomalies

MagnetometryGenerally performed from the air (aeromagnetic survey)Measures variations in the magnetic field. This gives an idea of the depth distribution of crystalline terrains that have NO chance of containing any oil.

Seismic SurveyMeasure the travel time of sound waves generated by a shock applied to the ground or seaMore costly than gravimetry and magnetometryBut essential to discovering oil and gas fields

Seismic SurveySignal emitted by vibrator truckReflected waves are received by geophonesData transmitted to laboratory truck

Offshore SeismicSeismic vesselHydrophones

More data obtained offshore than onshore

The Job of the GeophysicistUse gravimetry, magnetometry and later seismic surveysCreate an image of the subsurface deposits and their deformations, to help geologists identify traps.In coordination with the geologist they interpret the image to extrapolate the geometry of the trap.

Seismic InterpretationIsochronic lines points on the ground at which the waves take exactly the same time to be reflected / refracted at the surface3-D Seismic MapsSeismic profileDepth section based on seismic profile

Arriving At The Decision

Where to DrillThe decision where to drill is based partly on science and partly on gambling.The site for a wildcat (exploration well) is usually above the thickest part of the stratum thought to contain hydrocarbons.

End of Topic

(1) Billions of years ago, the sun fuelled life on earth. The presence on Earth of four simple elements (carbon, hydrogen, nitrogen and oxygen) created the conditions for life. They combined to form amino acids. (2) From the first living cell to ourselves (via algae, ferns, protolepidodendrons, trilobites, ammonites, dinosaurs and, later, mammals), millions upon millions of plant and animal species have sprung up and then vanished; billions upon billions of creatures have lived and died. (3) Oil and gas are derived almost entirely from decayed animals, plants and bacteria. Many special circumstances have to be in place for hydrocarbons to form.

(1) Being made up of carbon, hydrogen, nitrogen and oxygen, most organic waste is destroyed and digested by bacteria. (2) But some was deposited on the beds of inland seas, lagoons, lakes, river deltas and other oxygen-poor aquatic milieus, and were thus protected from bacterial action. In these areas vast amounts of plant material accumulate. Bacteria breaking down this material may use up all the available oxygen, producing a stagnant environment that is unfit for larger grazing and scavenging animals. The plants, bacteria and the chemicals derived from their decay become buried in silts and muds and are preserved.(3) Organic matter mingled with sediment (sand, salt, etc.), and then accumulates in layers over many millions of years, the oldest layers being buried beneath more recent ones. By their sheer mass, these sedimentary layers sink naturally. The continuous action of plate tectonics at work in the Earth's mantle breaks up these layers and precipitates them still more deeply into the Earth's crust.LINK: This provides the organic materials, the so-called protopetroleum, for later diagenesis into true petroleum.

(1) The further these sedimentary layers sink, the higher the temperatures and pressures rise. Here the remains of living things are cooked. (2) Temperatures within the Earths crust increase with depth causing sediments, and any organic material they contain, warm up. This causes fats, waxes and oils from algae, bacteria spores and cuticle (leafs skin) to link and form dark specks of kerogen. ______________________ (AFTER KEROGEN)(4) As the source rock becomes hotter, long chains of hydrogen and carbon atoms break from the kerogen, forming waxy and viscous heavy oil. At higher temperatures, shorter hydrocarbon chains break away to give light oil and then gas. Light oil is the more valuable crude. This process is called thermal degradation and cracking.

(Process in which heavy HC molecules are broken up into lighter ones)When the source rock starts to generate oil or gas it is said to be mature. The most important hydrocarbons are gas, oil, oil containing dissolved gas, and gas condensate. Gas condensate is light oil, which is gaseous at high underground temperatures and pressures.

Pix of kerogen (upper) and coal and woody kerogen (from cellulose and woody parts of plants) (bottom)(2) Chemically it is a complex mixture of hydrocarbon compounds of large molecules, containing hydrogen, carbon, oxygen, nitrogen, and sulfur. (3) Kerogen is the chief source of oil in oil shales. Oil is formed from kerogen by heating.

Depending on the amount and type of organic matter, oil generation occurs during the mature stage at depths of about 760 to 4,880 metres (2,500 to 16,000 feet) at temperatures between 65 and 150 C. This special environment is called the "oil window." In areas of higher than normal geothermal gradient (increase in temperature with depth), the oil window exists at shallower depths in younger sediments but is narrower. Maximum oil generation occurs from depths of 2,000 to 2,900 metres. Below 2,900 metres primarily wet gas, a type of gas containing liquid hydrocarbons known as natural gas liquids, is formed.

A pix of an oil window from U Conn

(1) Very often oil and gas have a mind of their own. Due to their physical properties they have a tendency to move about from the source rock and away from it. This is called migration. ___________________(BREAK: AFTER PRIMARY VS SECONDARY MIGRATION)

(2) Migration is a slow process, with oil and gas travelling perhaps only a few kilometers over millions of years.(3) Many things can trigger migrationIt can be triggered both by natural compaction of the source rock and by the processes of oil and gas formation. (Natural compaction): Most sediments accumulate as a mixture of mineral particles and water. As they harden to become rock, some water is expelled and dispersed. If the rocks contain oil or gas, this is also expelled. (Process of oil and gas formation): As hydrocarbon chains separate from the kerogen during oil and gas generation, they take up more space and create higher pressure in the source rock. This way, oil and gas ooze through minute pores and cracks in the source rock and thence into rocks where the pressure is lower.(4) Oil, gas and water migrate through permeable rocks

This means that liquids and gas can freely move about cracks and pore spaces between the rock particles that are interconnected, and are large enough to permit fluid movement. Fluid cannot flow through rocks where these spaces are very small or are blocked by mineral growth; such rocks are called impermeable. Oil and gas also migrate along large fractures and faults, which may extend for great distances.(4) Oil and gas are less dense than rock and water and therefore migrate upwardBeing lighter than water, they tend to rise toward the Earth's surface. Much oil is dispersed in isolated blobs through large volumes of rock. But when large amounts become trapped in porous rocks, gas and oil displace water and settle out in layers due to their low density. Water is always present below and within the oil and gas layers. If there's nothing to stop them, they ultimately seep out through the surface, or solidify as bitumen. (1) Oil and gas fields need to be contained in a seal designed by the nature. This can only happen if hydrocarbons have become trapped in permeable reservoir rocks, such as porous sandstone or fractured limestone. (1) It is unlikely that the vast quantities of oil now present in some reservoir rocks could have been generated from material of which no trace remains. Therefore, the site where commercial amounts of oil originated apparently is not always identical to the location at which they are ultimately discovered.(1) Porosities in reservoir rocks usually range from about 5 to 30 percent, but all available pore space is not occupied by petroleum. A certain amount of residual formation water cannot be displaced and is always present.(3) The porosity and permeability of carrier and reservoir beds are important factors in the migration and accumulation of oil. Most petroleum accumulations have been found in clastic reservoirs (sandstones and siltstones). Next in number are the carbonate reservoirs (limestones and dolomites). Accumulations of petroleum also occur in shales and igneous and metamorphic rocks because of porosity resulting from fracturing, but such reservoirs are relatively rare. _____________________ (BREAK: AFTER STRATIGRAPHIC TRAPS)In reality most traps are formed by more complex sequence of events, and cannot be classified so rigidly. For example, in (e) the reservoir rock was first folded and eroded, then sealed by an impermeable rock which was deposited later over the eroded structure. (2) In order to trap migrating oil and gas, structures must exist before hydrocarbon generation ceases. It could take, for example that sea trap structures can exist 125 million years ago, but the were not filled with oil until 100 million years later.LINK: What are the chances of a geologist ever suspecting the existence of this reservoir, a few million years later?

A fault trap occurs when the formations on either side of the fault have been moved into a position that prevents further migration of petroleum. For example, an impermeable formation on one side of the fault may have moved opposite the petroleum-bearing formation on the other side of the fault. The impermeable layer prevents further migration of petroleum.An anticline is an upward fold in the layers of rock, much like an arch in a building. Petroleum migrates into the highest part of the fold, and an overlying bed of impermeable rock prevents its escape. (80% of the worlds petroleum fields are in this category)

Stratigraphic traps are traps that result when the reservoir bed is sealed by other beds or by a change in porosity or permeability within the reservoir bed itself. (1) (Medicinal use): Small surface occurrences of petroleum in the form of natural gas and oil seeps have been known from early times. The ancient Sumerians, Assyrians, and Babylonians used crude oil and asphalt ("pitch") collected from large seeps at Tuttul (modern-day Hit) on the Euphrates for many purposes more than 5,000 years ago. The ancient Egyptians first used liquid oil as a medicine.(Military purposes): The Persians used incendiary arrows wrapped in oil-soaked fibres at the siege of Athens in 480 BC.(Lighting): industrial art of distillation into illuminants was developed by Arabs(2) The Industrial Revolution brought on an ever-growing demand for a cheaper and more convenient source of lubricants as well as illuminating oil. It also required better sources of energy. Liquid petroleum, on the other hand, was a more easily transportable source of energy.(3) Edwin L. Drake completed the first well on August 1859 in northwestern Pennsylvania. This set the groundwork for the petroleum industry and ushered the closely associated modern industrial age.

Exploration for new sources of oil is one of the most exciting and demanding jobs. It involves elements of science, skill, creativity and among other things pure chance and good luck. Indeed, it is one thing to believe that a formation or structure may bear oil or gas and another to prove it. Before an oil company undertakes the enormous cost of drilling, geologists and geophysicists gather additional data and carry out further studies to decide if oil and gas could be present and where the traps are located.

(1) various national Oil Companies also operate, and while constrained by economics they also have the directive to develop local resources and help improve the economy by providing jobs and reducing the dependence upon other nations.(2) Oil exploration is undertaken primarily by Private Companies for Financial gain.----------------------The oil companies are in the business at a variety of levels, exploring and producing oil and gas, developing markets for oil and gas products and producing and selling the products.Not all Oil Companies are involved in all levels of the business.Some only explore and produce while others explore, produce, refine, transport and sell right down to the petrol station, some are only interested in selling or refining products. Singapore Petroleum is an example.

P.S.C. system is widely used in the industry.Terms vary, dependent upon factors such as the individual country system, hence some countries are financially more attractive than others.Within a particular regime incentives may be given for remote or frontier areas, deepwater areas where exploration costs are prohibitive under a normal scheme.

Companies decide for a variety of reasons where they are going to drill, what type of area is attractive to them.

Political stability is a key issue hence activity may slow due to political upheaval, or countries be unattractive due to their political regime.

Economic terms vary between countries, both within the PSC system and for the availability of potential markets.

Maturity of exploration also affects the decision, known areas of hydrocarbon production are obviously easy to assess for the presence of hydrocarbons as opposed to frontier areas that may have no previous exploration activity.

The decision to enter a particular country may be based on many factors, not just geological prospectivity.Within a country, various basins may be present, the decision on where to explore is fine-tuned to a specific basin, then a particular block. This may be based on what is available at the time. Companies can also farm-in and take over a percentage of an existing block.Further exploration results in the development of plays an exploration scenario including source, reservoir and trap that is unproven and not evaluated fully to be worthwhile drilling.A play type is then evaluated until it has reduced risk of failure and becomes a prospect.The prospect may then be drilled, or ranked with other prospects until a suitable prospect is selected to drill.

Within a given prospect, certain horizons/rocks will be the designated potential zones of interest / targets / objectives. These are ultimately expected to be the reservoirs which will be produced.

(A map of Arabian Iranian basin) (1) The geologist's job is to observe, explore and scrupulously record any clue to the possible presence of hydrocarbons below ground. Geologists examine rocks and take samples to ascertain their nature and date the strata from which they were taken. (2) They then seek to reconstitute a scenario that may have been written 4 billion years ago. They attempt to trace the history of these deposits and their lateral variations in order to identify places where oil might have become "trapped." All of this involves a mixture of laboratory analysis and observation in the field.They also perform other tasks, including picking the best exploration permits, locating explorations wells over these traps, and appraising fields following discovery.______________________________Like drilling engineers, geologists work with a variety of other specialists, e.g.:geophysicistsreservoir engineersgeochemists: some of these specialize in mineral geochemistry, analyzing the reservoir rocks to evaluate the quality of the porosity containing the oil. Others specialize in organic geochemistry, their job being to characterize the nature and origin of the oil. Together, they evaluate the capacity of the rock to release the oil imprisoned within it (i.e. its productivity).Others specializing in particular fields or techniques. Oil companies sometimes call in outside experts, including academics and private consultants with leading edge techniques or equipment. They are used occasionally, as companies do not need them continuously.-Computer engineers help geologists in using software to visualize cross-sections, maps and other geological data.ADDITIONAL:Oil companies employ teams of geologists with a wide variety of specialties:Sedimentologists, for example, study the structure and arrangement of sedimentary strata, while those specializing in biostratigraphy date the period at which rocks were deposited by observing the fossils they contain. Specialists in structural geology measure the rock's deformation, and finally petroleum geologists use all of their colleagues' data in order to describe the geometry and characteristics of the oil reservoir. The key concept behind the exploration for hydrocarbons is the petroleum play - consisting of a number of ingredients; (a) a reservoir unit, (b) a petroleum charge system, (c) a topseal or caprock, (d) a trap, and (e) a timely relationship between the previous four factors. Petroleum geologists try to find out enough about these 5 ingredients to enable them to make sensible decisions about the likelihood of a play being successful.

(1) Now it's the geophysicist's turn to study the physical properties of the subsoil. A variety of methods are used at this stage, and a comparison of their results serves to enrich the geologist's findings.Geophysicists use a variety of methods, depending on the geological object they wish to visualize.

ADD:Bouguer anomaly The remaining value of gravitational attraction after accounting for the theoretical gravitational attraction at the point of measurement, latitude, elevation, the Bouguer correction and the free-air correction (which compensates for height above sea level assuming there is only air between the measurement station and sea level). This anomaly is named for Pierre Bouguer, a French mathematician (1698 to 1758) who demonstrated that gravitational attraction decreases with altitude.

ADD:aeromagnetic survey Measurements of the Earth's magnetic field gathered from aircraft. Magnetometers towed by an airplane or helicopter can measure the intensity of the Earth's magnetic field. The differences between actual measurements and theoretical values indicate anomalies in the magnetic field, which in turn represent changes in rock type or in thickness of rock units.

Once the contours and depth of the sedimentary basin have been ascertained, seismic refraction, and more often seismic reflection techniques, are used. These methods measure very precisely the travel time of sound waves generated by a shock caused by dropping a heavy weight, a mechanical vibrator, a bubble of high-pressure air; ... or via an explosion applied to the ground or in the sea. 1.The shock wave is reflected or refracted by the geological strata back to the surface, where it is picked up by a a highly-sensitive microphone called a geophone (or hydrophone at sea). The way in which the waves are propagated varies as they pass through the different strata.2.The geophysicist at the surface listens to the echo of these waves and records them. 3.If the point from which the sound wave was generated, and the speed at which it travels through the different rocks are known, then one can deduce the depth of the stratum from which the wave was reflected. 4.By plotting all of the points identified on a grid covering the topographical map, one can map the depths of the main strata.BRIEF:A surface shock generates sound waves which are refracted and reflected underground. The way in which the waves are propagated varies as they pass through the different strata. Using a highly-sensitive microphone known as a "geophone," the geophysicist at the surface listens to the echo of these waves and records them. And since ships can travel easily in all directions, seismic measurement is in fact easier at sea than on land. The geophysicist can thus obtain more data offshore than onshore and a more precise three-dimensional image, once the data have been processed. (2) Geophysicists apply a certain number of physical principles in reconstituting geological strata and their deformation, in the form of vertical sections, maps plotting the value of specific parameters, and, increasingly, computer images. (3) They interpret the image to extrapolate the geometry of the petroleum trap. Interpretation is carried out in constant collaboration with geologists, who are able to identify the type of formation and describe its characteristics.LINK: Oil companies rarely have the necessary equipment and personnel to conduct field measurements themselves. They therefore contract with outside geophysical service companies to conduct seismic campaigns. The oil companies' in-house geophysicists oversee these campaigns. The geophysicist's seismic recordings are fed into powerful computers. The terrain is mapped by means of isochronic lines. This method yields two and three-dimensional images of the underground strata, and the resulting seismic maps serve to determine whether certain strata are likely to contain hydrocarbons.

ADD:two-dimensional seismic data (2D)A group of 2D seismic lines acquired individually, as opposed to the multiple closely spaced lines acquired together that constitute 3D seismic data.three-dimensional seismic data (3D)A set of numerous closely-spaced seismic lines that provide a high spatially sampled measure of subsurface reflectivity. Typical receiver line spacing can range from 300 m [1000 ft] to over 600 m [2000 ft], and typical distances between shotpoints and receiver groups is 25 m [82 ft] (offshore and internationally) and 110 ft or 220 ft [34 to 67 m] (onshore USA, using values that are even factors of the 5280 feet in a mile). Bin sizes are commonly 25 m, 110 ft or 220 ft. The resultant data set can be "cut" in any direction but still display a well sampled seismic section. The original seismic lines are called in-lines. Lines displayed perpendicular to in-lines are called crosslines. In a properly migrated 3D seismic data set, events are placed in their proper vertical and horizontal positions, providing more accurate subsurface maps than can be constructed on the basis of more widely spaced 2D seismic lines, between which significant interpolation might be necessary. In particular, 3D seismic data provide detailed information about fault distribution and subsurface structures. Computer-based interpretation and display of 3D seismic data allow for more thorough analysis than 2D seismic data.four-dimensional seismic data (4D)Three-dimensional (3D) seismic data acquired at different times over the same area to assess changes in a producing hydrocarbon reservoir with time. Changes may be observed in fluid location and saturation, pressure and temperature. 4D seismic data is one of several forms of time-lapse seismic data. Such data can be acquired on the surface or in a borehole.

All these results are aggregated and studied. Geologists, geophysicists, petroleum architects, together with drilling, production and reservoir engineers all supply data to economists and financial planners. By juggling figures, parameters and probabilities, they seek to work out a possible strategy for developing the reservoir in the event of confirmation of the presence of hydrocarbons..Each member of the exploration team has contributed to the performance of the mission. By collating and comparing their experience, know-how and findings, their ultimate conclusions are the result of a team effort. Those conclusions are stated briefly: No: the chances of a result are too slim; or... Yes: the "prospect", i.e. this highly promising reservoir, is worth taking a gamble. The team is prepared to "pay to see," making the decision to drill. Geologists, geophysicists and reservoir engineers have concluded there is a "prospect" or possible producing zone. But to find out whether there really are hydrocarbons trapped in the rock, they are going to have to drill down to that zone..

The best location for the siting of the drill rig is determined based on the existing state of knowledge of underground conditions and the topography of the terrain. This is generally sited vertically above the thickest part of the stratum thought to contain hydrocarbons. The drilling team often operates under difficult conditions. The well is generally sunk to a depth of between 500 and 4,000 meters. In a few cases it may go beyond 6,000 meters, and one has even gone to a depth of 10 kilometers, or 30,000 feet.