Pore Pressure Prediction for a Paleozoic Marine Shale in South China

Pore pressure prediction of shale gas reservoir is essential for reserves evaluation and drilling-related geohazard prevention. For young shale sediments, P-velocity has direct correlation association with effective stress (overburden stress minus pore pressure). Trend methods, such as Eaton's method and Flemings’ method, have been widely used and have achieved reliable and accurate results. For old shale gas reservoir, such as the Paleozoic marine shale in South China, effective stress is not the only direct factor affecting P-velocity. To achieve a suitable prediction for old shale reservoir, we first use modified Eaton's method to predict pore pressure of target shale with sonic log referencing mud weight. Then, we calculate overburden stress and bulk modulus of target shale with density, P-velocity and S-velocity logs. Through cross plotting P-velocity, effective stress and bulk modulus, we find that they have a polynomial correlation association and fit them with a polynomial function. After that, we use well-log constrained seismic inversion to extract the density and P-velocity cubes. Finally, we estimate the pore pressure of target shale with the fitted polynomial function and seismically inverted P-velocity and density. The result shows the estimated pore pressure gradient of the target shale is consistent with the log-derived values.