Overpressure generation by disequilibrium compaction or hydrocarbon generation in the Paleocene Shahejie Formation in the Chezhen Depression: Insights from logging responses and basin modeling

Abstract

Abundant oil resources associated with overpressure have been confirmed to be present in the Paleogene Shahejie Formation in the Chezhen Depression. Pore pressure strongly influences the migration and trapping of hydrocarbons and impacts the safety of drilling operations. The origin of the overpressure and the relationship between overpressure and hydrocarbon migration are not clearly understood in the Chezhen Depression, which has brought difficulties in prospect evaluation and during drilling. According to the measured pressures, mud weights, logging responses, and basin modeling results, the possible overpressure mechanisms in the Shahejie Formation in the Chezhen Depression are revealed, and the distribution of overpressure and its implications for oil migration dynamics are discussed. The overpressure in the Shahejie Formation is mainly concentrated in Es3 and Es4, and the distribution of overpressure in reservoirs is complex. The disequilibrium compaction of rapidly buried Paleocene mudstones is a common overpressure mechanism in the Chezhen Depression, and the overpressure points in acoustic transit time-vertical effective stress, density-vertical effective stress and acoustic transit time-density cross plots follow the loading curves. Hydrocarbon generation has played an important role in overpressure generation at depth in the center of the Chezhen Depression. The acoustic transit time of high-overpressured intervals shows more anomalies than the density measurements, and the overpressure points fall on the unloading curves in rock property-vertical effective stress cross plots. However, overpressure due to hydrocarbon generation rarely occurs in isolation and always appears simultaneously with disequilibrium compaction. The transfer of overpressure often complicates the pressure distribution in sandstones, leading overpressure and normal pressure to occur at similar depths. The overpressure in the Chezhen Depression promoted oil migration and improved the efficiency of oil accumulation. In particular, overpressure was the main force driving the migration of the oil generated in the Es3u downward to the Es4 reservoirs. Knowledge of the mechanisms and distributions of overpressure has great significance for further exploration in the Chezhen Depression and other extensional basins.