Mechanism and distribution prediction of abnormal high pressure of the Paleocene Shahejie Formation in Linnan Sag,Huimin Depression

In the process of oil and gas exploration,various abnormal high pressures were tested in the Paleogene Shahejie Formation in Linnan Sag,Huimin Depression.The uncertainty of overpressure origin would reduce the reliability of pressure prediction.This study elaborated both the characteristics of fluid pressure in porous sandstones and the well log responses of overpressured mudstones through such data as drill stem tests (DSTs),mud density and logging. Besides,based on acoustic and density logs in normal and abnormal high pressured intervals,we established an effective stress-log response plot to deduce the origin of overpressure in the Shahejie Formation,and predicted the distribution of abnormal pressures in the study area.The results show that overpressures in the sandstone reservoirs of the Shahejie Formation mainly occur in the 3rd (Es3) and 4th (Es4)members of the formation with a depth over 3 000 m,and their maximum excessive pressures are 23.82 MPa and 14.04 MPa,respectively.The logging responses of overpressured mudstones typify undercompaction,usually exhibiting abnormally high interval transit time,abnormally low density,and high neutron poro-sity.Overpressures in the formation mainly originate from overpressure transmission among neighboring mudstones induced by disequilibrium compaction.Most overpressure data points follow the trend of loading curves,and only the overpressure data points in local deep depression zones(with burial depth being more than 4 000-4 300 m)follow unloading curves, possibly due to the pressure increase with hydrocarbon generation resulting from a higher maturity (Ro is within 0.9%-1.05%)of the deeply buried organic-rich mudstones.Besides,fluid pressures calculated with equilibrium depth method match well with the pressure data obtained from DSTs. This verifies the conclusion that the abnormal high pressures in the formation are fundamentally caused by disequilibrium compaction. The excessive pressures of the Es3 show a ring-like distribution surrounding the core of the sag and progressively lower towards and become normal in its neighboring slopes and uplifts. These research results will be of valuable guidance for pre-drilling pressure assessment in Linnan Sag. © 2018, OIL & GAS GEOLOGY Editorial Board. All right reserved.