The consequences of ignoring rock properties when predicting pore pressure from seismic and sonic velocity

Abstract

Rock properties are shaped by the geological history of the basin into which they have been deposited. The environment of deposition (and eroded hinterland provinces) controls the mineralogy and resultant rock type, while geological processes such as uplift, denudation, or constant deposition and burial will all shape the development of porosity in rocks. These factors may all play a part, to some degree, in the development of any abnormal pressure (herein termed overpressure; pressure in excess of the hydrostatic pressure) within formations. The more processes that have occurred, the more complicated the task will be to accurately predict this anomalous pressure. Typically, velocity data such as seismic interval velocity and sonic logs are used with industrystandard algorithms in all geological settings and lithology types to predict pore pressure. This paper will set out why standard pore pressure algorithms must only be applied after the rock properties have been assessed fully and examples are given of the circumstances where velocity data, both sonic and seismic, may never allow an accurate pressure assessment. Brief mention will also be made of why additional care must be taken before using seismic velocity to predict pore pressure.