Geological characteristics and abnormal pore pressure prediction in shale oil formations of the Dongying depression, China

Pore pressures in shale oil formations in the Dongying depression of the Bohai Bay Basin are highly overpressured. Understanding overpressure principles not only can reveal the mechanism of shale oil generation to provide prospects for shale oil exploration and production, but also can reduce potential drilling risks and provide insights for sweet spot identification. Overpressure generation mechanisms are analyzed based on petrophysical data, core analysis, sedimentation rate, oil saturation, and smectite and illite data. We find that undercompaction, oil generation, and smectite and illite transformation are primary causes of overpressures. Undercompaction is the major cause of overpressures in the Es3 formations because of their rapid deposition rate of 300-700 m/m.y. For the overpressures primarily generated by undercompaction, sonic transit time can be used to predict shale pore pressure with correctly selected depth-dependent normal compaction trend. In the Es3 and Es4 formations, overpressure data are consistent with the increase of vitrinite reflectance and oil saturation, indicators of oil generation. Additionally, in shallow formations, starting at about 2000 m, the smectite to illite transformation occurs, which makes rock property changes with depth. A composite normal compaction trendline is proposed to account for the smectite to illite transformation. Using well log data, the shale oil formations can be identified and there are two shale oil formations in the studied area, and a shale oil geo-marker is found. At this geo-marker, the pore pressure gradient reaches the highest value. This high pore pressure behavior is beneficial for shale oil production because of a higher pore pressure corresponds to a higher production.