RESERVOIR GEOSCIENCEPDB 2012

ASSIGNMENT 1

Name : Thevaruban S/O RagunathanID : 19645Course : Petroleum EngineeringGroup: 2

Sedimentary rocks make up about three-quarters of the rocks at theEarths surface. They form at the surface in environments such as beaches, rivers, theocean, and anywhere that sand, mud, and other types of sediment collect. Sedimentary rocks preserve a record of the environments that existed when they formed. By looking at sedimentary rocks of different ages, scientists can figure out how climate and environments have changed throughEarths history.Fossilsof ancient living things are preserved in sedimentary rocks too.To fully understand sedimentary rocks, a few terms must be known. For example, crystallization is process of formation of solid crystals. Next, diagenesis is the change of sediments or existing sedimentary rocks into a different sedimentary rock during and after rock formation, at temperatures and pressures less than that required for the formation of metamorphic rocks. Lithification is the process in which sediments compact under pressure, expel connate fluids, and gradually become solid rock. Metamorphism is the change of minerals or geologic texture in pre-existing rocks, without melting into liquid magma (a solid-state change).Weathering is the breaking down or dissolving of rocks and minerals on Earths surface. Besides, subduction is the process that takes place at convergent boundaries by which one tectonic plate moves under another tectonic plate and sinks into the mantle as the plates converge.Many sedimentary rocks are made from the broken bits of other rocks. These are called clastic sedimentary rocks. The broken bits of rocks are called sediment. Sediment is the sand you find at the beach, the mud in a lake bottom, the pebbles in a river, and even the dust on furniture. The sediment may, in time, form a rock if the little pieces become cemented together.There are other types of sedimentary rocks whose particles do not come from broken rock fragments. Biochemical sedimentary rocksare made ofmineral crystalssuch ashaliteand gypsum formed by chemical processes. The sediment particles oforganic sedimentary rocks are the remains of living things such as clamshells, plankton skeletons, dinosaur bones, andplants.Chemical sedimentary rocks in the meanwhile are made up off when large area of water evaporates and dry up. This large area of water then becomes more concentrated and minerals begin to precipitate. The resulting rocks are called evaporates.

Examples of sedimentary rocksClassification of Sedimentary RocksSediments can be broadly divided into two groups which are clasticknown as the residues of weathering andthe second one is chemical/biochemical precipitateswhich are products of crystallization of ions that were dissolved during weathering.The dominant constituents of sedimentary rocks are: Quartz- (SiO2) one of the most abundant minerals in the exposed continental crust, and an extremely hard, resistant and chemically stable mineral. Calcite- (CaCO3) a major constituent of limestone (a very common sedimentary rock type) and a common cementing agent in shale and sandstone (clastic rocks). Clay- residual product developed from weathering of silicate minerals. Rock fragments- relatively unweathered remnants of weathering.Other important constituents of sedimentary rocks are: dolomite- ((Ca,Mg)CO3) may replace calcite in limestone feldsparandmica- relatively unweathered residues haliteandgypsum- (NaCl, CaSO4*2H2O) evaporites organic matter- e.g. coalSedimentary rocks are characterized bylayering. Layering is produced by physical or chemical changes that occur in their environment of deposition. Layers can be found in sedimentary rocks in a wide variety of scales, from much less than a millimetre to many meters. Layering and other structures within sedimentary rocks provide important clues as to their origin. Many of the specimens cannot be seen to have obvious layering due to the small size of the samples relative to the layering. In your description of the rocks, make a note of any evidence for layering.Textureis an important aspect of identifying and describing sedimentary rocks. The important textures you will be examining today are: clastic- made up of grains crystalline- interlocking crystals resulting from crystallization skeletal- composed of fossil shellsRock textures can be classified into 5 types, which are: Grain Size Grain Sorting Grain Shape Grain Roundness Crystalline (Coarse, Fine)

Sandstones are classified according to the percentage of matrix present and based on their composition. The best way to characterise sandstones is by using the Pettijohn classification of sandstone chart:

Transportation of sediments

There are 5 types of transporting agents, which are:

Water (Most important) Air Gravity Ice Dense sediment and water mixturesThere are 2 types of fluid flow which are laminar flow and turbulent flow. The different between this two fluids are flow velocity, bed roughness and type of fluid. In addition, there are three mechanisms involved in transporting the particles known as traction, saltation and suspension.

Another name for sediment transport is sediment load. The total load includes all particles moving as bed load, suspended load, and wash load.

To understand the relationship between particle size and velocity, the use of the Hjulstorms diagram is required.

As from the diagram, it can be seen that as for clay and silt, the smaller the grain, the higher the velocity required to erode it. This is due to the high cohesive force between the grain particles which requires more energy to break it down.Sedimentary structures can be found at the bed surface as well as within the beds. There are 5 categories of sedimentary structures which are: Erosional Depositional Post-depositional Diagenetic Biogenic.On the surface, bedding surface structure can be found such as Ripples shrinkage cracks parting lineation rain spot impressions tracks and trails Just below the surface, bedding under surface structures such as flute casts groove casts tool marks load casts Scours and channels. Internal sedimentary structures contain Bedding Lamination Graded bedding Cross-stratification Massive bedding

PorosityPorosity is the void space in between the rocks. The higher the porosity, the higher the rocks capacity to store fluid. There are three types of porosity: Effective Porosity ------ The pores which are interconnected Ineffective Porosity ---- The pores which are isolated and the fluids inside it cannot be retrieved Total Porosity---- The total pores the rock contain.

Catenary pores are those which has more than one throat passage (effective porosity). Cul-de-sac pore has only one throat passage. Closed pores has no throat pores (ineffective porosity).

In a rock formation, there are two types of porosity which are the primary porosity and the secondary porosity. Primary porosity is developed at the deposition stage where it contains intergranular pores of clastic /carbonates, intercrystalline and fenestral pores of carbonates and usually more uniform than induced porosity. The factors affecting primary porosity are particle spherecity and angularity, packing and sorting.

Secondary porosity developed by geologic processes after deposition (diagenitic process). The secondary porosity contains grain dissolution in sandstones / carbonates, vugs and solution cavities in carbonates and fracture development in some sandstones and carbonates. Factors affecting secondary porosity are cementing particle, overburden stress and vugs, dissolution and fractures.

Sandstone Porosity Types:

Integranular

Micropores Disolution FracturesCarbonate Porosity Types: Interparticle Intraparticle Intercrystal Moldic Fenestral Fracture VugPermeabilityThe ability of a fluid to flow. Absolute permeability is the ability of a fluid to flow when one fluid is present. Effective permeability is the ability of a fluid to flow when more than one fluid is present. Relative permeability is the ratio of effective permeability to absolute permeability. Tortuosity is the property of curve being twisted. Permeability is parallel to the bedding planes (horizontal permeability). Factors affecting permeability are: Grain size Grain sorting Grain roundness DiagenesisDiagenesisis the change ofsedimentsor existingsedimentary rocksinto a different sedimentary rock during and after rock formation (lithification), at temperatures and pressures less than that required for the formation ofmetamorphic rocks.It does not include changes fromweathering and it is any chemical, physical, or biological change undergone by asedimentafter its initial deposition. This process excludes weathering andmetamorphism. These changes happen at relatively low temperatures and pressures and result in changes to the rock's originalmineralogyand texture. There is no sharp boundary between diagenesis andmetamorphism, but the latter occurs at highertemperaturesandpressuresthan the former. After deposition, sediments are compacted as they are buried beneath successive layers of sediment and cemented by minerals that precipitate fromsolution. Grains of sediment,rockfragments andfossilscan be replaced by other minerals during diagenesis.Porosityusually decreases during diagenesis, except in rare cases such asdissolution of minerals anddolomitization. The study of diagenesis in rocks is used to understand the geologic history they have undergone and the nature and type of fluids that have circulated through them.

Three stages of diagenesis: Eodiagenesis------ takes place at shallow depths Mesodiagenesis--- occurs at deep burial Telodiagenesis---- uplift of buried sediments into the system of meteoric waters.

There are 5 main diagenetic processes; Compaction Dissolution Cementation Replacement Recrystallization

REFERENCES

Boggs, S. (2009). Petrology of sedimentary rocks (2nd ed.). Cambridge: Cambridge University Press.

Glossary: Diagenesis. (n.d.). Retrieved November 14, 2015, from https://wwwf.imperial.ac.uk/earthscienceandengineering/rocklibrary/viewglossrecord.php?Term=digenesis

Nichols, G. (1999). Sedimentology and stratigraphy. Oxford: Blackwell Science.

Sedimentary Rocks. (n.d.). Retrieved November 14, 2015, from http://geology.com/rocks/sedimentary-rocks.shtml

Sediment Transport and Deposition - Environmental Measurement Systems. (n.d.). Retrieved November 14, 2015, from http://www.fondriest.com/environmental-measurements/parameters/hydrology/sediment-transport-deposition/

The Solid Earth System. (n.d.). Retrieved November 14, 2015, from http://eesc.columbia.edu/courses/ees/lithosphere/labs/dep_environs.html

Tucker, M. (1981). Sedimentary petrology: An introduction. New York: Wiley.