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3.1.3 Diagenesis

The term diagenesis describes all chemical and physical processes affecting a sediment after deposition. Processes related to sub-aerial weathering and those which happen under very high pressures and temperatures are excluded from this category. The latter are grouped under the term "metamorphosis'. Diagenesis will alter the geometry and chemistry of the pore space as well as the composition of the rock. Many of these changes are controlled by the oxidising potential (ell) and the acidity/alkalinity (pH) of the pore-water which circulates through the formation. Consequently, the migration of hydrocarbons and the displacement of water out of the pore system may end or at least retard diagenetic processes. 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. Compaction reduces porosity and permeability. As mentioned earlier during the introduction of growth faults, if the expulsion of pore water is prevented, overpressures may develop. Cementation describes the "glueing" together of components. The "glue" often consists of material like quartz or various carbonate minerals. They may be introduced to the system by either percolating pore water and/or by precipitation of minerals as a result of changes in pressure and temperature. Compaction may for instance lead to quartz dissolution at the contact point of individual grains where pressure is highest. In areas of slightly lower pressure, e.g. space between the pores, precipitation of quartz may result (Fig. 3.11 ).

Figure 3.11 Destruction of porosity by cementation

This kind of pressure solution / precipitation is active over prolonged periods of time and may almost totally destroy the original porosity. Precipitation of material may also occur in a similar way on the surface of fault planes thus creating an effective seal via a process introduced earlier as diagenetic healing. Dissolution and replacement. Some minerals, in particular carbonates, are not chemically stable over a range of pressures, temperatures and pH. Therefore there will be a tendency over geologic time to change to a more stable variety as shown in Figure 3.12.

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