EPISODIC FLUID EXPULSION FROM GEOPRESSURED SEDIMENTS

Numerical simulation of fluid expulsion from geopressured sediments using a one-dimensional finite difference model shows that large volumes of fluid can be transported through a fractured seal during short-lived (<100 year) expulsion events. In this model, abnormal fluid pressures are maintained by an impermeable seal. When the fluid pressure exceeds 85% of the lithostatic pressure, a Vertical fracture opens in the seal, providing a permeable pathway from the geopressured sediments into the overlying section. Fluid pressure in the seal decreases as fluid flows through the fracture, closing the fracture as sediments in the seal compact. Once the fracture closes, the seal remains impermeable until fluid flowing from deeper in the section increases fluid pressure to the fracture criteria, a process that takes 10 000-500 000 years. Fractures emplaced in relatively incompressible seals, such as compacted and/or cemented sediments, remain open for 20-50 years; fluid fluxes through the seal integrated over the life of these events are 1-5x10(7) kg/m(2), and temperatures in the overlying sediments are increased by 5-20 degrees C. These temperature anomalies decay to less than half their original value in 300 years, suggesting that the thermal anomalies observed in wells that are attributed to fluid expulsion formed in response to a very recent or currently active event.