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Abstract: Relationships between Sediment Compaction, Fluid Flow, Overpressure and Oil Migration: Examples from the North Sea and Haltenbanken, Offshore Norway

BJØRLYKKE, KNUT
Department of Geology, University of Oslo, Oslo, Norway

Compaction of sediments follows the rules of soil and rock mechanics when there is little mineral dissolution and precipitation and is a function of the effective stress and the compressibility. In siliciclastic sediments compaction is mostly mechanical at shallow depth, but below 2-3 km (>70-100°C) compaction becomes increasingly chemical and is mainly controlled by temperature rather than effective stress. Mechanical compaction of larger volumes of mudstones must be very slow because of the low permeability and hence a long time is required for water to escape. Overpressure reduces the effective stress and therefore causes a negative feedback on compaction and the rate of fluid flow.

The average fluid fluxes in sedimentary basins are controlled by the rate of compaction (porosity reduction) and phase changes like dehydration of clay minerals and generation of petroleum which are driven by temperature and to a much lesser extent by effective stress. For a given compaction-driven pore-water flux the pressure gradients reflect the permeability distribution in the overlying sediments.

During progressive mechanical and chemical compaction sedimentary rocks are normally consolidated and have a ductile response to stress at low strain rates. This makes it difficult to transmit plate tectonic stress through subsiding sediments. Ductility also implies that faults and fractures are not likely to remain open except at pore pressures close to fracture pressure.

Studies from Haltenbanken, offshore Norway and the North Sea show that reservoirs have leaked petroleum at the top of the structures when pressure compartments have reached fracture pressure. There is evidence that oil migrated through fault zones that later at greater depths became pressure barriers due to diagenetic cementation (welding) along the fault zone and also in fine grained sandstones (carrier beds).

Oil migration should therefore not always be predicted from the present pressure compartmentalization.

In the North Sea pressure compartments are also controlled by low permeability shales (seals) and lateral drainage limited by faulting.

Calculations suggest that the permeability of the top seal may be less critical for the development of overpressure than the lateral drainage affected by faulting.

Compaction-driven fluid flow is very slow particularly in the deeper parts of a basin (>3 km) and is mainly a function of the rate of heating rather than changes in effective stress. This makes episodic flow rates less likely even at fracture pressure.

Modeling of compaction-driven flow is limited by the accuracy of input parameters, particularly the porosity distribution.

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