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