Stratigraphic variation of transport properties and overpressure development in the Western Foothills, Taiwan
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Overpressure, fluid pressure higher than hydrostatic pressure, has developed below the middle Miocene formations in the north-central Western Foothills of Taiwan. To study the mechanism by which overpressure is generated and maintained in the Taiwan oil fields, we estimated the fluid pressure history and overpressure distribution by using a one-dimensional basin model incorporating laboratory-approximated hydraulic parameters. Transport properties of outcropping sedimentary rocks were measured at effective pressures of 5 to 200 MPa. All parameters showed apparent stratigraphic variation, decreasing with increasing burial depth. Permeability showed the strongest sensitivity to depth, decreasing by 6 orders of magnitude to 10-20 m2 at the bottom of the basin. A critical sealing layer was not identified in the geologic column. The basin model incorporates overburden loading due to sediment accumulation, aquathermal expansion of water, the dehydration reaction of expandable clay to nonexpandable clay, and oil generation. Predicted overpressure was generated dramatically from 3 Ma, when the accumulation rate increased rapidly as a result of tectonic collisions in the area. If we assume a fluid influx from the bottom of the basin, the predicted overpressure is consistent with the observed overpressure, implying that continuous inflow from depth, possibly along the décollement or normal faults, may be the main cause of overpressure generation in this area. Stratigraphic variation of transport properties, which decrease with depth, also influences overpressure trends in the Western Foothills, where overpressure is generated only in deeper horizons. The clay mineral distribution estimated by a kinetic smectite–illite dehydration model is consistent with the observed mineralogical data.
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