Paleo-pressure evolution and its origin in the Pinghu slope belt of the Xihu Depression, East China Sea Basin

The Xihu Depression in the East China Sea Basin is one of the prime gas and light oil-producing regions of eastern China. Overpressure evolution and origin in the sandstone reservoirs of the Pinghu (E(2)p) Formation in the Pinghu slope belt of the Xihu Depression were investigated through a combination of fluid inclusion analyses and basin numerical modeling, combined with measured pore pressures (drill stem and wireline formation tests), well loggings, gas geochemistry, and casting thin section data. Four oil inclusion types, exhibiting different API gravities and hydrocarbon gas inclusions, were identified, using inclusion petrography, microscopic fluorescence, and Raman spectra. Homogenization and ice melting temperatures of coeval aqueous inclusions, determined using microtherrnometry, combined with burial-thermal histories determined by basin numerical modeling, suggested multiple episodes of oil and natural gas charge in the E(2)p reservoirs, from the Miocene to the present day. Pore pressure evolution history, reconstructed with fluid inclusion PVTx analysis, indicated that the Pinghu Formation experienced three phases: overpressure generation, followed by overpressure release, and then further overpressure generation. The first phase of weak overpressure was generated during burial in the middle-late Miocene. Currently, it is hard to definitely determine the origin of the first phase of overpressuring. Nevertheless, it can be speculated that the origin may be related to disequilibrium compaction, oil generation and tectonic compression based on burial, tectonic and hydrocarbon generation histories. Following this, a sudden uplift of formations led to overpressure release in the late Miocene, and then, the second phase of moderate overpressure generation occurred during burial in the late Pliocene and Quaternary. Based on well-log signatures, vitrinite reflectance data of mudstones and gas geochemistry, combining with seismic analysis, the second phase of overpressuring was interpreted to have been mainly caused by fluid expansion, which resulted from local gas generation by kerogen maturation and long-distance vertical and lateral pressure transfer.