QUANTITATIVE MODELS OF VERY HIGH FLUID PRESSURE - THE POSSIBLE ROLE OF LATERAL STRESSES

Because of the occurrence of very large observed overpressure in a wel l located in SE Asia, numerical modelling has been undertaken to evalu ate evolution of the sediments. The intensity of the excess pressure, close to lithostatic pressure, and the likely relation to induced, ope n-fracture anomalies, as well as evidence of large fluid transfers, al l lead to the suggestion that the tectonic regime is related to the ge neration of high excess pressure. To verify this hypothesis, a 2D flui d-flow/compaction model (GEOPETII) was used to test whether high exces s pressure could be obtained without considering the effect of lateral stresses. While high excess pressures are possible, even if undercomp action is the only process considered, the distribution of fluid press ure with depth does not correspond to that observed, and the migration of fluids cannot be reproduced because of the extremely low permeabil ity of the sediments. Accordingly, the model has been modified so that the effect of lateral stresses can be investigated through: (1) the v alue of the fracturing coefficient which controls the maximum overpres sure that it is possible to generate; (2) the sensitivity of the syste m to a possible compressive event, which could squeeze the sediments l aterally so that the rate of fluid expulsion would be greater than if the fluids were driven only by the mechanical load of sediments. Final ly, the role of faulting or fracturing is discussed as a possible mech anism responsible for the upward transfer of fluid which can lead to a n increase of excess pressure at depths shallower than the depth where excess pressure was first generated, a mechanism also leading to high excess pressure. Moreover, this mechanism allows for a good reproduct ion of observed excess pressure distributions.