New Insights into the Pore Pressure Estimation from Sonic Velocity Data

Pore pressure prediction is of fundamental importance in petroleum exploration and production operations. Because there is no direct method to measure pore pressure in shale, the indirect method (the equivalent depth method) for estimating pore pressure from sonic velocity data is widely used. It has been found in the application of the equivalent depth method in several Chinese sedimentary basins that the calculated pressures from sonic logging data are very consistent with the DST or RFT pressures measurements in some basins and are very divergent in others. The consistence of calculated and measured excess pressures in the basin with different excess pressure generation mechanisms such as in the cases in Tarim basin, Bohai Bay basin and Jiuquan basin showed that the pressure calculated by the equivalent depth method accounts for all excess pressure generation mechanisms such as disequilibrium compaction, tectonic compression, aquathermal pressuring, hydrocarbon generation and clay diagenesis. The divergence of calculated and measured excess pressures was found in the organic-rich source rocks in Triassic in Ordos basin and in Cretaceous in Songliao basin. Compared with common shales, organic-rich shales are composed of more solid organic matter, which can influence the sonic velocity values. Our study shows that the sonic velocity decreases dramatically when the TOC is higher than 2%. So the pore pressures calculated by the equivalent depth method from sonic velocity data may much higher than the actual pressures in organic-rich shale. The use of the equivalent depth method may mislead in predicting pore pressure of organic-rich shale without considering the influence of total organic carbon content.

AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California