--> ABSTRACT: Episodic Overpressure and its Control on Reservoir Diagenesis, by Richard E. Swarbrick; #91020 (1995).

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Episodic Overpressure and its Control on Reservoir Diagenesis

Richard E. Swarbrick

Conditions which have favoured the generation of present overpressure in sedimentary basins around the world have also operated in the geological past. Evidence for palaeo-overpressure is found in a variety of features and rock textures observed in outcrop in uplifted basinal sequences. Some basins may also have experienced multiple phases of overpressure interspersed with periods when pressure returned to hydrostatic, at least in the aquifer zones and including potential reservoir section.

Fluctuating pressure with time in reservoirs may exert an important control on reservoir quality, in particular the timing and amount of pore reduction and secondary pore enhancement. Increasing pore pressure will tend to increase solubility of most cements. Hence when conditions favour overpressure in the reservoir section (e.g. rapid burial leading to fluid retention; fluid expansion due to gas generation) there will be a tendency towards dissolution. The opposite will occur during pressure dissipation when the overpressuring mechanism is removed or there is failure of the seal. Under conditions of declining pressure cementation is favoured, and may be strongly influenced by gas exsolution in basins where hydrocarbons are being generated. An analogue for pressure control on cementat on comes from active fault zones where polyphase cements result from fluctuating pressure, with cementation during rapid pressure dissipation during earthquake rupture.

Pressure modelling coupled with palaeo-overpressure estimation from fluids trapped in diagenetic cements from the Brent reservoir, Northern North Sea, suggest reservoir diagenesis at near hydrostatic reservoir conditions, but immediately after a phase of regional overpressure.

AAPG Search and Discovery Article #91020©1995 AAPG Annual Convention, Houston, Texas, May 5-8, 1995