HEPPARD, P. D., M. L.ALBERTIN, B. E.WAGNER, and D. J. BETTENCOURTT, Amoco Production Company, Houston,Texas

Abstract: Success and Failure in Pressure Prediction Using Seismic Velocities in Deep Water Environments

Pore pressure prediction is a significant tool for reducing cost and improving safety in exploratory drilling, and in finding viable hydrocarbon seals. Common methods for predicting pressure depend on the accuracy of interval velocities derived from seismic data, the validity of the conversion to pore pressure, and an understanding of the geologic conditions of the area.

Velocity sensitivity analyses to test velocity resolution as a function of depth and interval thickness, data quality assessment, and an understanding of the limitations of the velocity analysis are all key components in determining how appropriate the velocity data are for pressure prediction. However pressure analyses must also take into account rock type variations, unconformities, uplift, and the redistribution of overpressure within porous and permeable beds such as sandstone.

At King, deep water Gulf of Mexico, the anisotropic Miocene section caused us to overestimate interval velocities and underpredict pore pressure. Lack of velocity sensitivity at depth prohibited us from detecting a rapid velocity decrease in the deep, highly overpressured Oligocene and Cretaceous section. In deep water Trinidad, uncertain pressure implications of slow Pleistocene clastic sediments led to two predictions based on different geologic assumptions. In the Nile Delta the fast velocity of the sand dominated, shallowest section could be misleading for prediction. An additional complexity is the presence of thin sandstone reservoirs with variable lateral continuity and structural relief. The pressure in these porous and permeable reservoirs must be estimated by redistributing the overpressure observed in the bounding shales throughout their entire areal extent.

AAPG Search and Discovery Article #90923@1999 International Conference and Exhibition, Birmingham, England