Geopressure prediction from automatically‐derived seismic velocities

Abstract

The phenomenon of geopressure is essentially stratigraphic in nature. In most cases, its occurrence correlates strikingly well with some mappable geologic characteristics, such as lithology changes, sediment deformation, and faulting. High‐precision velocity estimates can be made from the apparent amplitude variations with offset (AVO) that result from moveout errors, even if the seismic data itself lacks any intrinsic AVO. These velocity estimates provide us with an opportunity to estimate cross‐sections and 3‐D volumes of the gradient of pore pressure with depth from surface seismic data. These cross‐sections and volumes may be obtained through the estimation of seismic interval velocities as a function of depth, subtraction of the shale compaction trend, and the calibration of trend deviations in terms of pore‐pressure gradients. When viewed in combination with stacked seismic sections, the pore‐pressure gradient sections provide the interpreter added information about the hydrogeology of the sediment. In this paper, we show examples of pressure gradients caused by a lithology change, sealing faults, and fluid migration flows. Pressure gradient cross‐sections are also extremely useful for the design of mud densities and casing prior to spudding a well.