Diagenesis

Diagenesis is defined in the Glossary of Geology (Jackson, 1997) as “All the chemical, physical, and biologic changes undergone by a sediment after its initial deposition, and during and after its lithification, exclusive of surficial alteration (weathering) and metamorphism….

From: Developments in Petroleum Science, 2013

Volume 2

R.C. Selley, in Encyclopedia of Geology (Second Edition), 2005

The Boundaries of Diagenesis

Diagenesis begins as soon as sediment is deposited. Fossilized beer bottles and other anthropogenic detritus found in modern ‘beach rock’ limestone picturesquely illustrate this. Ancient evidence of early diagenesis is further confirmed by the occurrence of intraformational conglomerates containing clasts, not only of contemporaneous limestone, but also of siderite-cemented sandstones and shales. As sediment is buried more deeply, temperature and pressure increase and, ultimately, diagenesis merges into metamorphism, with shale becoming slate, sandstone becoming quartzite, and limestone becoming marble. Field observation and laboratory experiments demonstrate that the boundary between these rock types, and hence diagenesis and metamorphism, is gradational. The sequence, deposition → diagenesis → metamorphism, is not a ‘one-way street’, however. At any time while sediment is on its way to metamorphism, it may be uplifted to the surface again. Rocks returned to the surface show a reversal of the trend of porosity decreasing with burial, and its enhancement by both physical and chemical processes. The term ‘epidiagenesis’ was applied by Fairbridge in 1967 to the diagenesis resulting from uplift and weathering. Epidiagenesis is of little significance in shales. It is, however, of great importance in sandstones and carbonates, because of the way in which it restores porosity and permeability to rocks that had previously lost these features. When buried beneath an unconformity, these epidiagenetically enhanced zones may provide excellent petroleum reservoirs. Epidiagenesis is also well known to mining geologists, being responsible for the ‘gossan’ sulphide ore bodies, such as those of Rio Tinto, Spain. Figure 1 delineates the boundaries of diagenesis.

Figure 1

Figure 1. Diagram to show the interfaces and pathways of diagenesis.

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Carbonate Reservoirs

Clyde H. Moore, William J. Wade, in Developments in Sedimentology, 2013

Introduction

Diagenesis during a rising sea level should be dominated by marine water. The sedimentological setting—that is, whether the site is a ramp, land-tied shelf, or isolated platform— is critical to the style of diagenesis and resulting porosity modification that might ensue. Again climate plays a major role in modifying the water responsible for diagenesis. During transgression under arid conditions, marine waters can be evaporated and may become a very active diagenetic fluid with major consequences for porosity modification. In the following discussion, we will establish diagenesis/porosity models for each of the three sedimentological settings, under both humid and arid climatic conditions.

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Experimental Analysis

Caineng Zou, in Unconventional Petroleum Geology (Second Edition), 2017

2.1.1 Composition and Functions of the Reservoir Diagenesis Modeling System

Diagenesis physical modeling systems, which include high-performance indices such as high temperature (the maximum being higher than 550°C), high pressure (maximum lithostatic pressure being 275 MPa or the formation depth of 10,000 m), and fluid supply (acidic or alkaline media, the fluid pressure being 120 MPa), are capable of continuously performing 100-day-long or longer compaction diagenesis and corrosion model experiments. Test parameters are recorded and acquired in real time. The test process is controlled by a computer program. A remote alarm is provided. A reservoir diagenesis physical modeling system consists of the following four parts.

1.

Six reactor furnaces: The implementer of the model experiment consists of six reaction kettles connected in parallel with heating furnaces. The six reactor furnaces have the same composition and the same heating and pressurization ranges. Each part is composed of a tank, a heating furnace, a reaction kettle, a sample tube, a seal kit, upper and lower pressure supplies, and fluid supply pipelines.

2.

Pressure supply unit: The pressure supply unit of the system consists of two pressure mechanisms with airbag buffer capacities that provide the sealing pressure and lithostatic pressure of each reaction kettle.

3.

Fluid supply unit: The unit that supplies acid, alkali, and other reaction fluids needed for rock diagenesis consists of an ISCO high-pressure pump, a hydraulic press, an intermediate vessel, a stokehole outlet gas/liquid collector, and a pipe system.

4.

Control assembly unit: The control assembly consists of an industrial PC, a temperature–pressure control box, and a remote signal transmitter. The reservoir diagenesis modeling software in the computer, the working program for implementing the model experiment, is capable of setting the temperature, pressure, operating time, flow rate, and velocity. It also records various experimental data such as the pressure, temperature, and time in real time.

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Traps and Seals

Richard C. Selley, Stephen A. Sonnenberg, in Elements of Petroleum Geology (Fourth Edition), 2023

7.8.1.5 Diagenetic Traps

Diagenesis plays a considerable role in controlling the quality of a reservoir within a trap. As discussed in Chapter 6, solution can enhance reservoir quality by generating secondary porosity, whereas cementation can destroy it. In some situations, diagenesis can generate a hydrocarbon trap (Rittenhouse, 1972). Oil or gas moving up a permeable carrier bed may reach a cemented zone, which inhibits further migration (Fig. 7.40(A)). Conversely, oil may be trapped in zones where the solution porosity locally developed in a cemented rock (Fig. 7.40(B)). Secondary dolomitization can generate irregular diagenetic traps as dolomite occupies less space than the original volume of limestone.

Fig. 7.40. Configurations for diagenetic traps caused by (A) cementation, (B) solution, and (C) shallow-oil degradation.

Diagenetic traps are not formed by the solution or precipitation of mineral cements. As oil migrates to the surface, it may be degraded and oxidized by bacterial action if it reaches the shallow zone of meteoric water. It is known that this tarry residue acts as a seal, inhibiting further up-dip oil migration (Fig. 7.40(C)). The Shuguang oil field in the Liaohe Basin is an example of a diagenetic trap caused by shallow-oil degradation (MaLi et al., 1982).

Traps that originate purely from diagenesis are rare, although there are probably a number of diagenetic traps around the world whose origin has gone unrecognized, and many are yet to be found (Wilson, 1977). However, many traps form due to a combination of diagenesis and one or more other causes. This type of origin is particularly true for the subunconformity traps discussed in the next section.

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Sediments, Diagenesis and Sedimentary Rocks

K.L. Milliken, in Treatise on Geochemistry (Second Edition), 2014

9.7.2 The Realm of ‘Late Diagenesis’

Diagenesis’ refers to the physical and chemical processes that affect sedimentary materials after deposition and before metamorphism and between deposition and weathering. The effects of diagenetic processes on rock properties such as porosity and the degree of lithification are progressive. It is therefore difficult to draw absolute boundaries between diagenesis and the adjacent segments of the rock cycle. Divisions within diagenesis are even less readily drawn. Diagenesis includes many chemical and physical processes that are also active during deposition, weathering, and metamorphism. Crystalline rocks are also affected by subsolidus aqueous reactions that resemble diagenesis (e.g., Sprunt and Nur, 1979; Clauer et al., 1989; Ramseyer et al., 1992; Kominou and Yardley, 1997).

‘Late diagenesis’ of siliciclastic materials here refers to processes that postdate the initial stages of consolidation through compactional grain rearrangement and early cementation that occurs in fluids that have a clear affiliation with the depositional environment. It is essentially synonymous with ‘mesodiagenesis’ (Morad et al., 2000). Late diagenesis takes place in the subsurface under conditions of elevated temperature (~50–300 ºC). In general, late diagenesis occurs at depths greater than 1.5 km. As a result of fluid–rock interactions, pore fluids during late diagenesis differ in important ways from fluids associated with depositional environments (e.g., Land and Macpherson, 1992b; Hanor, 1994).

This chapter is concerned especially with chemical processes that create pervasive modifications of volumetrically significant rock components and impart chemical and textural modifications that can be detected in the bulk rock. Processes of similar type and magnitude may occur at lower temperatures and at shallower depths in zones having either enhanced fluid flow (e.g., in fault zones) or high geothermal gradients (e.g., adjacent to intrusions). Chemical and physical modifications during late diagenesis are not as severe as those during metamorphism. Processes encompassed by this review take place in rocks that have retained their textures as deposits of particulate debris. ‘Grain,’ in the context of sandstones and shales, has a specific genetic connotation that is distinct from ‘crystal.’

Much of our knowledge about late diagenesis has come from the study of samples obtained by drilling for oil and gas. Cenozoic–Mesozoic sedimentary accumulations 10–12 km thick occur in many basins, but wells that penetrate below 6 km are rare. As a result, the full transition into greenschist metamorphism has never been studied in the context of simple burial under conditions of ‘normal’ geothermal gradients (in the range of 20–30 °C km−1). Knowledge about late diagenesis in the realm of 200–300 °C comes almost entirely from tectonically uplifted and deformed regions or from settings of anomalously high geothermal gradients. This leaves us with great uncertainty regarding the relative importance of burial effects (primarily thermal and compactional) and conditions imposed during dynamic tectonism and uplift on a variety of rock modifications.

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Sediments, Diagenesis, and Sedimentary Rocks

K.L. Milliken, in Treatise on Geochemistry, 2003

7.07.2. The realm of “Late Diagenesis”

Diagenesis” refers to the physical and chemical processes that affect sedimentary materials after deposition and before metamorphism and between deposition and weathering. The effects of diagenetic processes on rock properties such as porosity and the degree of lithification are progressive. It is therefore difficult to draw absolute boundaries between diagenesis and the adjacent segments of the rock cycle. Divisions within diagenesis are even less readily drawn. Diagenesis includes many chemical and physical processes that are also active during deposition, weathering, and metamorphism. Crystalline rocks are also affected by subsolidus aqueous reactions that resemble diagenesis (e.g., Sprunt and Nur, 1979; Clauer et al., 1989; Ramseyer et al., 1992; Kominou and Yardley, 1997).

“Late diagenesis” of siliciclastic materials here refers to processes that postdate the initial stages of consolidation through compactional grain rearrangement and early cementation that occurs in fluids that have a clear affiliation with the depositional environment. It is essentially synonymous with “mesodiagenesis” (Morad et al., 2000). Late diagenesis takes place in the subsurface under conditions of elevated temperature (∼50–300 °C). In general, late diagenesis occurs at depths greater than 1.5 km. As a result of fluid–rock interactions, pore fluids during late diagenesis differ in important ways from fluids associated with depositional environments (e.g., Land and Macpherson, 1992b; Hanor, 1994).

This chapter is concerned especially with chemical processes that create pervasive modifications of volumetrically significant rock components and impart chemical and textural modifications that can be detected in the bulk rock. Processes of similar type and magnitude may occur at lower temperatures and at shallower depths in zones having either enhanced fluid flow (e.g., in fault zones) or high geothermal gradients (e.g., adjacent to intrusions). Chemical and physical modifications during late diagenesis are not as severe as those during metamorphism. Processes encompassed by this review take place in rocks that have retained their textures as deposits of particulate debris. “Grain,” in the context of sandstones and shales, has a specific genetic connotation that is distinct from “crystal.”

Much of our knowledge about late diagenesis has come from the study of samples obtained by drilling for oil and gas. Cenozoic–Mesozoic sedimentary accumulations 10–12 km thick occur in many basins, but wells that penetrate below 6 km are rare. As a result, the full transition into greenschist metamorphism has never been studied in the context of simple burial under conditions of “normal” geothermal gradients (in the range of 20–30 °C km−1). Knowledge about late diagenesis in the realm of 200–300 °C comes almost entirely from tectonically uplifted and deformed regions or from settings of anomalously high geothermal gradients. This leaves us with great uncertainty regarding the relative importance of burial effects (primarily thermal and compactional) and conditions imposed during dynamic tectonism and uplift on a variety of rock modifications.

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Origin of Coal as Gas Source and Reservoir Rocks

Romeo M. Flores, in Coal and Coalbed Gas, 2014

Gas from Peat to Coal Reservoir: Generation During Diagenesis

Diagenesis follows the biochemical stage of peatification and begins with inundation of the peat by deposition of detrital sediments (Kim, 1978). As burial by the sediments and subsequent subsidence continue microbial activity is terminated by increased temperature and pressure as well as by accumulation of toxic substances. The effects of diagenesis toward coalification of peat organic matter cannot be accessed during early burial conditions but can be tested in the laboratory. Because the microbial degradation of cellulose and lignin start off coalification, Kim (1978) used sterile samples of these organic matter and wood inoculated with microorganism to study gas formation analyzed by gas chromatography in the laboratory, which resemble the biogenetic phase of peatification. The result of the study is summarized in Figure 3.24, which shows CH4 and CO2 are the dominant gas products from all the samples. Carbon dioxide and H2 were detected first, followed by CH4, and over a period of several months the gas composition average 95% of CH4 and 5% for CO2. However, additional analysis by Kim (1978) indicates that the gas retained in peat is predominantly CH4 is retained in peat with some CO2 and heavy hydrocarbon gases.

FIGURE 3.24. Biogenic gases (e.g. methane and carbon dioxide) produced from wood, cellulose, and lignin samples inoculated by microorganisms. CH4 = methane; CO2 = carbon dioxide; H = hydrogen; pct = percent.

Source: Adopted from Kim (1978).

In order to compare the biogenic gas formation during peatification to diagenetic phase of gas formation during early coalification, Kim (1978) used coal samples of various rank (e.g. lignite, subbituminous, bituminous) from various U.S. coal mines to analyze gas generation with increased temperature. The volume of hydrocarbon gases (e.g. commonly methane, isobutane, and pentane) increased as temperature increased but correspondingly CH4 decreased at higher temperature. The generated hydrocarbon gas during the experiments contained average of more than 95% CH4 at 35 °C, 50% at 125 °C, and 30% at 150 °C (Figure 3.25). Thus, the demethylation during diagenesis occurs at a very low temperature (35 °C) and the reaction may involve more complex reactions with heavy hydrocarbons.

FIGURE 3.25. Methane emitted from coal with increased temperatures (from 35 to 150 °C) under laboratory conditions. CH4 = methane.

Source: Adopted from Kim (1978).

Although the laboratory experiments of Kim (1978) proved that CH4 is generated from cellulose and lignin during biogenic phase of peatification and diagenetic phases of early coalification, sustaining gas generation and accumulation in the peat coal reservoir may not be a fait accompli. With increasing diagenesis the lignin and polysaccharide content of peat decreases but the content of humic substances increases. Cellulose is still found in peat but is absent from brown coal (e.g. lignite), which is formed at a more advance phase of diagenesis. At the end of diagenesis the resulting brown coal contains no carbohydrates and the amount of relatively unaltered lignin has decreased to <10% (Killops & Killops, 2005). Thus, the cellulose and lignin, which are the common source of CH4 according to Kim (1978) may be depleted toward the advance phase of diagenesis.

Gas preservation from peat to coal may rely entirely on rapid deposition, burial, and entrapment by impermeable seal rocks (e.g. mudstone, shale). These factors are controlled by the depositional environments, and tectonic and eustatic settings associated to the peat accumulation. For example, peat accumulation in alluvial floodplains can be inundated and buried gradually by sediments during intermittent flooding and/or rapidly during flash flooding. In addition, rapid sediment burial of peat accumulation may develop in river deltas coupled with subsidence and transgression by the sea.

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Methods in Geomorphology

C.A. Woodward, C.R. Sloss, in Treatise on Geomorphology, 2013

Glossary

    Diagenesis

    The physical or chemical transformation of deposits such as sediments or soil by geological processes. Heat and pressure are the two main geological forces that can alter sedimentary deposits altering their physical and chemical composition.

    Gyttja

    A term derived from Swedish meaning dredging, mud, slush, or silt. In geomorphology this term is used to refer to sediment composed of (partially) decomposed plant and animal remains and fine nonorganic sediment deposited in standing water.

    Indurated

    To make hard or harden. A sedimentary rock that is hard may be said to be indurated, for example, an indurated limestone.

    Thixotropic

    Thixotrophy is a property of gels or fluids that are thick (or viscous), but become less viscous over time when shaken, agitated, or stressed.

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Palaeosoils and Relict Soils

Nicolas Fedoroff, ... Zhengtang Guo, in Interpretation of Micromorphological Features of Soils and Regoliths (Second Edition), 2018

9 Diagenesis

Diagenesis of recent palaeosoils (Middle Pleistocene to Holocene) usually only concerns biological activity and organic matter (Chichagova, 1995). As soon as a soil is buried, the soil fauna feeding on plant residues disappears. For instance, earthworm populations may disappear, followed by the consumption of earthworm excrements by mites, replacing them with mite excrements. Fresh organic fragments in a buried soil are progressively humified and tend to disappear, whereas charcoal fragments are preserved, especially if they are ferruginised. In much older palaeosoils, buried by less than a few hundred metres and not affected by tectonic movements or thermal activity, microscopic characteristics are weakly altered or unaltered by diagenesis (Retallack, 1991). In lithified palaeosoils, changes are more important, but pedogenic characteristics can still be recognised in thin sections (Retallack & Wright, 1990).

Groundwater can act as a major pedogenic factor when it occurs near the soil surface, but it can also induce diagenetic processes in buried soils, similar to near-surface processes (Gibling & Rust, 1992). Development of diagenetic attributes depends on the rate of groundwater circulation, redox conditions and ionic concentration. Textural intercalations can occur at great depth where groundwater moves along faults. Soluble salts may be either dissolved or precipitated. Some iron and manganese oxide impregnations in palaeosoils, unrelated to other pedofeatures, are undoubtedly of diagenetic origin.

Pressure resulting from deep burial combined with circulating groundwater leads to collapse of voids and ultimately to the development of a massive microstructure (Sheldon & Retallack, 2001).

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