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We used a basin-scale model to reconstruct the development of overpressures in a sedimentary basin. The area of application is the Venture gas field, Nova Scotia, Canada. It is in an overpressured zone in which sedimentation rates have been relatively low since 90 Ma, and where large horizontal stresses have been evidenced.
We investigated the importance of overpressure mechanisms using the Institut Francais du Petrole Temispack model, which integrates compaction, hydraulic fracturing, fluid flow, heat transfer, and the formation and migration of hydrocarbons.
We first constructed a reference model using geological, geochemical, and physical data. Second, we studied sensitivity of the model to several input parameters and, from this, deduced additional constraints. The modeled permeability along faults and of shaly beds interlayered in the reservoirs must be very low to enable both the local and regional distributions of overpressure and gas to be reproduced.
Several runs of the reference model studying the relative importance of possible overpressuring mechanisms and the relative contribution of plausible source rocks to gas accumulations showed that gas generation and accumulation, and lateral compression contribute little to overpressuring, which is mainly accounted for by compaction disequilibrium in spite of low sedimentation rates. Faults and shaly beds play an important role in maintaining the overpressure.
Gas generation in the reservoir units or close to them does not contribute significantly to the field gas accumulations. Gas sources are likely to be found in the underlying formations.
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