Sediment Deposition and Diagenesis PDF

1 UNIVERSITY OF GUYANA SEDIMENT DEPOSITION AND DIAGENESIS BY JOSEPHINE MAXIMUS  2 INTRODUCTION  Deposition is the laying down and accumulation o...

1 UNIVERSITY OF GUYANA SEDIMENT DEPOSITION AND DIAGENESIS BY JOSEPHINE MAXIMUS  2 INTRODUCTION  Deposition is the laying down and accumulation of sediment.  Sediment can be transported as pebbles, sands, mud, and salts dissolved in water. Dissolved minerals may later be deposited by organic activity (e.g., as sea-shells) or by evaporation. The depositional processes leave their record in the sediment in the form of sedimentary structures and textures UNIVERSITY OF GUYANA  3 PROCESSES  Sediments can be deposited by a wide range of transporting agents, including:  Wind : Tidal currents and storm currents. Waves  The growth of animal skeletons, as in reefs.  Flowing water, as in streams.  Direct precipitation of minerals as in evaporites (Insitu materials). UNIVERSITY OF GUYANA  4 ENVIRONMENT OF DEPOSITION  Continental depositional environments: fluvial and glacial systems, lakes, and the aeolian sand seas of deserts  Transitional environments: deltas, lagoons, tidal flats, sabkhas, beaches, and barriers  Open marine environments: shallow shelves and inland seas, and bathyal– abyssal sites of pelagic, hemipelagic, and turbidite sedimentation. Many of these sediments possess distinctive characteristics, which can be used to recognize their equivalents in the geological record. UNIVERSITY OF GUYANA  5 CONTINENTAL ENVIRONMENT UNIVERSITY OF GUYANA  6 COASTAL AND MARINE ENVIRONMENT UNIVERSITY OF GUYANA  7 SEDIMENT DIAGENESIS  Diagenesis is the physical, biochemical, and chemical changes that occur within sediments after deposition.  Diagenetic processes include a) compaction, b) dissolution, c) cementation d) recrystallization and replacement. These processes change the texture, structure, and mineralogy of sediments. Commonly increasing the bulk density and reducing the porosity and permeability.  End Product: Sedimentary Rock UNIVERSITY OF GUYANA  8 SEDIMENT DIAGENESIS CONT’D  Sedimentary rocks will experience  During the early stages of dissolution, replacement, and diagenesis, sediments undergo oxidation. compaction, water expulsion,  thinning of beds, and porosity loss. The results include the solution of Concomitant and subsequent carbonate cement, alteration of modifications may include feldspars to clay minerals, cementation by minerals such as oxidation of iron carbonate quartz and carbonates, formation of minerals to iron oxides and pyrite other authigenic minerals, including to gypsum, and solution of less clay minerals, and dissolution of stable minerals. less-stable minerals. These diagenetic processes bring about important physical, mineralogical, and chemical changes in original UNIVERSITY OF GUYANA depositional assemblages.  9 STAGES OF DIAGENESIS There are three stages of diagenesis.  Eodiagenesis: The principal  changes that occur are bioturbation, Eodiagenesis – the earliest stage, mineralogical changes, and which takes place at very shallow compaction. depths  Mesodiagenesis: The processes in  Mesodiagenesis – occurs during mesodiagenesis are physical and deep burial chemical compaction, cementation,  Telodiagenesis – an uplift of buried dissolution by pore fluids, mineral sediment into the system of replacement, and clay mineral meteoric waters. authigenesis. UNIVERSITY OF GUYANA NECESSARY CONDITIONS FOR DIAGENESIS 10  Temperature Very important parameter in the diagenetic process. Increases with depth. Diagenesis occurs in a temperature (T) range of roughly 0–300oC. Normal geothermal gradients near the Earth’s surface are approximately 25–30oC per km. 300oC is reached at a depth of about 10 km. Extreme cases: 100oC /km in areas of high heat flow, and may be as low as 10oC /km in areas of low heat flow. UNIVERSITY OF GUYANA  DIAGENESIS CONT’D 11  Pressure Diagenesis occurs at pressures (P) of up to 1-kilobar Hydrostatic Pressure and Lithostatic pressure. A hydrostatic gradient for pure water reaches a pressure of 1 kilobar at a depth of approximately 10 km. The lithostatic gradient for Earth material with a density of 2,750 kg/m3 reaches a pressure of 1 kilobar at a depth of approximately 3.5 km. UNIVERSITY OF GUYANA  PROCESSES 12  As sediments are progressively buried, the temperature and lithostatic pressure increase. Buried grains are continually immersed in pore fluids of variable composition that may be stagnant or flowing.  Sediment burial kick-starts a series of diagenetic processes that include (but are not limited to): - Physical Changes- Compaction(physical/mechanical) Chemical changes – Chemical dissolution-Cementation-Mineral Replacement –Recrystallization- Organic Matter Transformation UNIVERSITY OF GUYANA  13 COMPACTION  Compaction is accompanied by physical and chemical rearrangement and the formation of new minerals. The weight of the overburdened sediment and rock causes a reorganization of the packing of grains. The degree of compaction is controlled by such factors as grain shape, sorting, original porosity and amount of pore fluid present. UNIVERSITY OF GUYANA  14 PHYSICAL COMPACTION CONT’D  Physical compaction refers to the reorganization of sediments by reorientation, deformation, and breakage of grains.  Due to increasing overburden pressure during sediment burial  The volume of the sediment is decreased and the bulk density is increased  Pore space is reduced and water is expelled. Physical compaction is most important in sediments that have the Plagioclase feldspar has been fractures parallel to its twin planes through highest initial porosity e.g. loose sands. compaction and the adjacent quartz have been forced into the plagioclase. UNIVERSITY OF GUYANA  15 PHYSICAL COMPACTION CONT’D Simple mechanic rearrangements of spherical sediments can reduce porosity UNIVERSITY OF GUYANA from 48% to 28%  16 PHYSICAL COMPACTION CONT’D Rate of mechanical compaction of sandstone vs shale UNIVERSITY OF GUYANA  PHYSICAL COMPACTION CONT’D 17  Also known as pressure solution This process is associated with dissolution at grain-to-grain contacts. This process leads to sutured (interpenetrating) grain-to-grain contacts and jagged stylolites. Common in carbonate rocks and many sandstones. UNIVERSITY OF GUYANA  18 STYLOLITES  Stylolites: Sedimentary structures consisting of a series of relatively small serrations (alternating, interlocked, tooth-like columns) of stone.  Representation of rock mass loss to pressure dissolution Common in sandstone and carbonates. UNIVERSITY OF GUYANA Stylolite in limestone  19 EFFECT OF COMPACTION ON PACKING  Grains start as tangential contacts. As the sediment is buried and compacted, grains are rotated and pushed closer, and pore space is reduced, forming long contacts. With further compaction, softer grains are compacted around harder ones, and grains start to dissolve in the pore water forming concavo-convex contact. As grain contact dissolution continues, boundaries become irregular or sutured, and original grain shapes are destroyed. Eventually, all the contacts are sutures and porosity destroyed. UNIVERSITY OF GUYANA  20 EFFECT OF COMPACTION ON RESERVOIR QUALITY  Reservoir quality can be defined by the amount of hydrocarbon that can be stored and the flow of HC within the reservoir. Porosity and permeability are some key parameters in determining a good or bad reservoir quality. Some other factors that may affect the quality of the reservoir are organic content (TOC), thermal maturity, fluid saturations, and more. UNIVERSITY OF GUYANA  21 CHEMICAL DIAGENESIS  This is the process by which chemically unstable grains are preferentially removed, leaving vugs (holes) that may comprise of parts of grains, whole grains, or large volumes (caves) as in carbonate and evaporite karst.  This is distinguished from pressure solution because it is not associated with localized pressure.  Relatively soluble minerals such as carbonates and evaporites are most susceptible to chemical dissolution.  Feldspars and other aluminosilicate minerals in sandstones and shales are also commonly dissolved.  Dissolution leads to increased porosity. UNIVERSITY OF GUYANA  22 CHEMICAL DISSOLUTION Formation of stalactite and stalagmite= speleothems UNIVERSITY OF GUYANA  CHEMICAL DISSOLUTION OF CARBONATE 23 Zone of subsurface water in coastal environment Arid, semi-arid and humid environment James and Choquerie, 1984 UNIVERSITY OF GUYANA  24 CHEMICAL DISSOLUTION CONT’D Semi-arid environment-Southern Middle east-Saudi Arabia(arid environment) Europe UNIVERSITY OF GUYANA  CHEMICAL DISSOLUTION CONT’D 25 UNIVERSITY OF GUYANA  CEMENTATION 26  Cementation is the chemical or biochemical precipitation of a new mineral in the pore space of sediments.  The pore space may be primary or produced partly by dissolution  Cementation may be: - concentrated in local areas known as nodules or concretions - concentrated in certain layers - or spread more homogeneously throughout the sediment.  Cementation is most significant in coarse-grained sediments with high initial porosity and permeability.  Cementation leads to a massive reduction in porosity. UNIVERSITY OF GUYANA  CEMENTATION 27  Concretions form within sedimentary rock after the material has been laid down.  Water circulating within the rock will pick up and dissolve chemicals from the surrounding rock. These chemicals then precipitate out around some small nucleus in the rock.  Concretions are generally harder than the surrounding material, so they are often exposed to erosion. UNIVERSITY OF GUYANA  28 TYPES OF CEMENTING MATERIALS CONT’D  Overgrowth: formed by the precipitation of the same mineral, e.g., quartz or calcite.  Poikilotopic: Result when cement minerals completely envelop the grains.  Isopachous: Grows on all surfaces within pores, e.g., sparry calcite.  Meniscus: Forms when cement precipitates occur from water flowing down through the cement. UNIVERSITY OF GUYANA  29 TYPES OF CEMENTING MATERIALS UNIVERSITY OF GUYANA  30 TYPES OF CEMENTING MATERIALS CONT’D UNIVERSITY OF GUYANA  31 EFFECT OF CEMENTATION ON RESERVOIR QUALITY UNIVERSITY OF GUYANA  MINERAL REPLACEMENT AND 32 RECRYSTALLIZATION  The replacement of an existing mineral with a new one  Dissolution of one mineral + precipitation of the new one.  The most common is chemical sediments such as carbonates, evaporites, etc. - notable: limestone to dolomite.  Common replacement reactions in terrigenous sediments: - the transformation of smectite to illite in shales - the replacement of potassium feldspar and calcium feldspar by sodium feldspar (albitization) in sandstones.  Recrystallization- The formation of new crystal structure while retaining the basic chemical composition, e.g., Aragonite recrystallizing to calcite. UNIVERSITY OF GUYANA  33 MINERAL REPLACEMENT  DOLOMITIZATION: Dolomite is formed when magnesium-rich water (sea water), the mineral CaMg(CO3)2 replaces the calcite(CaCO3) in the rock. UNIVERSITY OF GUYANA  34 ORGANIC MATTER TRANSFORMATION  Organic-matter transformations in sediment initially occur as bacteria feast upon buried organic matter – forming biogenic methane gas.  A series of complicated maturation processes at depth results in the production of oil and gas.  Terrestrial plant material is also progressively altered during burial to give lignite and coal. UNIVERSITY OF GUYANA  35 FACTORS CONTROLLING DIAGENESIS The main factors that control diagenesis are: (1) the chemical composition and texture of the starting sediments. (2) the composition of the pore fluids. (3) the temperature, pressure, and pH, which control the chemical stability of the mineral phases. (4) the porosity and permeability. UNIVERSITY OF GUYANA  CHEMICAL COMPOSITION AND 36 TEXTURE OF STARTING MATERIALS The shapes and sizes of the grains in these different sediment types have important implications for compaction. Most terrigenous sands and gravels are composed of spherical to ellipsoidal grains of quartz, feldspars, and rock fragments. Mica grains are plates. Siliciclastic clay minerals in mud are micron-diameter plates. Carbonate sands include spherical to ellipsoidal peloids and ooids, but also oddly shaped skeletal fragments. UNIVERSITY OF GUYANA  37 CHEMICAL COMPOSITION OF PORE FLUID  Important near-surface (early diagenetic) pore fluids include diluted meteoric (chemical weathering) water and seawater.  Below a few kilometers depth, many sedimentary basins contain so called oilfield brines.  Salinities that range from 10,000 to >300,000 mg/l.  The most important ions in the majority of these brines are Na+ , Ca2+ , and Cl- , which distinguishes them chemically from the common near- surface pore fluids. UNIVERSITY OF GUYANA  38 PRESSURE TEMPERATURE AND PH  Various chemical reactions in sediments vary with T and P.  The pH controls mineral reactions in the subsurface.  Quartz and calcite are two of the most common cements in sedimentary rocks of all compositions. The solubility of quartz increases with increasing temperature, whereas the solubility of calcite decreases with increasing temperature.  The solubility of CO2 increases with increasing pressure. UNIVERSITY OF GUYANA  39 POROSITY AND PERMEABILITY  Porosity and permeability tend to be correlated in sedimentary rocks.  Cementation and compaction reduce the porosity and permeability in a sediment or sedimentary rock, whereas dissolution increases them.  Depending on the grain size and sorting, the original porosity of modern terrigenous sands ranges from 0.25 to 0.55.  The porosity of modern skeletal carbonate sands ranges from 0.40–0.70.  In general, permeability increases with mean grain size in unconsolidated sediments but is greatly influenced by the diagenetic change. UNIVERSITY OF GUYANA  40 UNIVERSITY OF GUYANA  41 SUMMARY UNIVERSITY OF GUYANA  42 QUESTIONS UNIVERSITY OF GUYANA