Diagenesis

Diagenesis will either increase or decrease porosity and permeability and cause a marked change of reservoir behaviour compared to an unaltered sequence. 

The diagenetic processes relevant to field development are compaction, cementation, dissolution and replacement. 

Compaction occurs when continuous sedimentation results in an increase of overburden which expels pore water from a sediment package. Pore space will be reduced and the grains will become packed more tightly together. Compaction is particularly severe in clays which have an extremely high porosity of some 80% when freshly deposited. 

In rare cases compaction may be artificially Initiated by the withdrawal of oil, gas or water from the reservoir. The pressure exerted by the overburden may actually help production by squeezing out the hydrocarbons. This process is known as compaction drive and some shallow accumulations in Venezuela are produced In this manner in combination with EOR schemes like steam injection. 

If compaction occurs as a result of production careful monitoring is required. The Ekofisk Field in the Norwegian North Sea made headlines when, as a result of hydrocarbon production, the pores of the fine-grained carbonate reservoir collapsed and the platforms on the seabed started to sink. The situation was later remedied by inserting steel sections into the platform legs. Compaction effects are also an issue in the Groningen gas field in Holland where subsidence in the order of one meter is expected at the surface. 

2 Microbial degradation of organic matter during diagenesis 

The chemical residues from microbial degradation, if not rapidly assimilated, undergo condensation reactions 

As we have just seen, the classical model for the formation of kerogen involves the condensation of the various products of microbial degradation that escape 

Carbonate sediments deposited in shallow marine environments are often exposed to the influence of meteoric waters during their diagenetic history. Meteoric diagenesis lowers 8 0- and 8 C-values, because meteoric waters have lower 8 0-values than sea water. For example. Hays and Grossman (1991) demonstrated that oxygen isotope compositions of carbonate cements depend on the magnitude of depletion of respective meteoric waters. 5 C-values are lowered because soil bicarbonate is C-depleted relative to ocean water bicarbonate. 

Physicochemical reactions within the sea-sediment sphere tend to reach equilibrium. Those reactions that are so rapid that they occur prior to burial in the bottom sediments are referred to as halmyrolysis (e.g. formation of clay aggregates), while those that take place in the upper part of the sediment are termed early diagenesis . The diagenetic processes include cementation, compaction, diffusion, redox reactions, transformation of organic and inorganic material, and ion exchange phenomena. A short 

Cementation is the precipitation of a binding material around grains, thereby filling the pores of a sediment. Among the processes mentioned, Berner (1971, p. 97) states that  

Thus the initial state of compaction results in a collapse of the more unstable original structures and a rearrangement of the particles so that a tighter packing is obtained. Increased packing will expel the interstitial water from the sediment and decrease the 

For the upper part of the sediment, where the sea-sediment interaction is most predominant, steady-state compaction is an acceptable assumption as porosity and compaction undergo a linear change during burial. 

The process of diffusion is discussed in Chapter 4 and in detail by Crank (1956). Diffusion in a sediment is complicated by the presence of particles in the fluid medium. Diffusion is thus retarded, and a calculation of sediment diffusion must also include the terms porosity, represented by n, and tortuosity. Since the value of tortuosity for natural sediments is seldom known, it is more convenient to use the term formation factor or lithological factor , denoted L, which takes into account everything but porosity. Tick s diffusion constant D, is replaced by the whole 

sulfide levels in interstitial waters increase. A number of elements form insoluble sulfides, which under these anoxic conditions are precipitated and retained within the sediments. A notable example is the accumulation of pyrite, FeS2, but also Ag, Cu, Pb and Zn are enriched in anoxic sediments in comparison with oxic ones. 

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Carbonate reservoir rock is usually found at the place of formation ( in situ ). Carbonate rocks are susceptible to alteration by the processes of diagenesis. 

Keywords reservoir structures, faults, folds, depositional environments, diagenesis, geological controls, porosity, permeability... 

In the following section we will examine the relevance of depositional environments, structures and diagenesis for field development purposes. 

Carbonate rocks are not normally transported over long distances, and we find carbonate reservoir rocks mostly at the location of origin, in situ . They are usually the product of marine organisms. However, carbonates are often severely affected by diagenetic processes. A more detailed description of altered carbonates and their reservoir properties is given below in the description of diagenesis . 

Diagenetic Healing late precipitation of minerals on or near the fault plane has created a sealing surface (see diagenesis for more detail). 

Carbonate reservoirs are usually affeoted to varying degree by diagenesis. However the process of dissolution and replacement is not limited to carbonates. Feldspar for instance is another family of minerals prone to early alterations. 

The number of injectors required may be estimated in a similar manner, but it is unlikely that the exploration and appraisal activities will have included injectivity tests, of say water injection into the water column of the reservoir. In this case, an estimate must be made of the injection potential, based on an assessment of reservoir quality in the water column, which may be reduced by the effects of compaction and diagenesis. Development plans based on water injection or natural aquifer drive often suffer from lack of data from the water bearing part of the reservoir, since appraisal activity to establish the reservoir properties in the water column is frequently overlooked. In the absence of any data, a range of assumptions of injectivity should be generated, to yield a range of number of wells required. If this range introduces large uncertainties into the development plan, then appraisal effort to reduce this uncertainty may be justified. 

At the development planning stage, a reservoir mode/will have been constructed and used to determine the optimum method of recovering the hydrocarbons from the reservoir. The criteria for the optimum solution will most likely have been based on profitability and safety. The model Is Initially based upon a limited data set (perhaps a seismic survey, and say five exploration and appraisal wells) and will therefore be an approximation of the true description of the field. As development drilling and production commence, further data is collected and used to update both the geological model (the description of the structure, environment of deposition, diagenesis and fluid distribution) and the reservoir model (the description of the reservoir under dynamic conditions). 

An alternative description of iUite—smectite mixed-layer clays begins with megacrystals of smectite that incorporate smaller packets of iUite (163). These constituents are observed as mixed-layer minerals in x-ray analysis. Diagenesis increases the percentage of iUite layer and with increasing alteration the mixed-layer mineral takes on the characteristics of an iUite dominated iUite—smectite. 

W. D. KeUer, "Diagenesis of Clay Minerals—A Review," Proceedings of the 11th National Conference of Clays and Clay Minerals, 1962, Pergamon Press, Inc., New York, 1963. 

General scheme of kerogen evolution from diagenesis to metagenesis in the van Krevelen diagram. 

Diffusion as referred to here is molecular diffusion in interstitial water. During early diagenesis the chemical transformation in a sediment depends on the reactivity and concentration of the components taking part in the reaction. Chemical transformations deplete the original concentration of these compounds, thereby setting up a gradient in the interstitial water. This gradient drives molecular diffusion. Diffusional transport and the kinetics of the transformation reactions determine the net effectiveness of the chemical reaction. 

Berner, R. A. (1980). Early diagenesis - a Theoretical Approach." Princeton University Press, Princeton, NJ. 

Lynn, D. C. and Bonatti, E. (1965). Mobility of manganese in diagenesis of deep-sea sediments. Marine Geol. 3,457-474. 

Manias, W. G., Covey, M., and Stallard, R. E. (1985). The effects of provenance and diagenesis on clay content and crystallinity in Miocene through Pleistocene deposits, southwestern Taiwan. Petrol. Geol. Taiwan 173-185. 

Degens, E. T. and Mopper, K. (1976). Factors controlling the distribution and early diagenesis of organic material in marine sediments. In "Chemical Oceanography" (J. P. Riley, ed.), Vol. 6, pp. 59-113. Academic Press, New York. 

Emerson, S. and Widmer, G. (1978). Early diagenesis in anaerobic lake sediments II. Equilibrium and kinetic factors controlling the formation of iron phosphate. Geochim. Cosmochim. Acta 42,1307-1316. 

Ingall, E. and Jahnke, R. (1997). Influence of water-column anoxia on the elemental fractionation of carbon and phosphorus during sediment diagenesis. Mar. Geol. 139, 219-229. 

Froelich, P. M., Klinkhammer, G. P., Bender, M. L. et al. (1979). Early oxidation of organic matter in pelagic sediments of the eastern equatorial Atlantic suboxic diagenesis, Geochem. Cosmochim. Acta 43, 1075-1090. 

Price, T.D., Blitz, X, Burton, J. and Ezzo, J.A. 1992 Diagenesis in prehistoric bone Problems and solutions. Journal of Archaeological Science 19 513-529. 

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