Intense fluid overpressure in the eastern slope of the Yinggehai Basin, South China Sea

The gas-bearing Yinggehai Basin in the northern South China Sea is characterized by high pressure and temperature conditions. The Miocene strata in the eastern slope exhibit intense overpressure with a pressure coefficient exceeding 2.2. Understanding the characteristics of overpressure and clarifying its causes are very important for natural gas exploration and development in reservoirs with intense overpressure. This study identifies and characterizes the pressure structure in the eastern slope of Yinggehai Basin by utilizing formation pressure data from formation tests (MDT) and drill pipe tests (DST) in combination with conventional logging data (acoustic wave, density, and resistivity). The pressure distribution exhibits a distinct three-stage ladder structure. Overpressure initiates in the Lower Pliocene Yinggehai Formation, with a pressure coefficient of 1.2. The formation pressure increases sharply in the underlying Upper Miocene strata (1^st member of the Huangliu Formation), with a pressure coefficient ranging from 1.4 to 1.8. The pressure coefficient increases to between 2.1 and 2.3 in the 2^nd member of the Huangliu Formation.

We utilized the logging curve combination method, Bowers effective stress method, and Bowers acoustic-density crossplot method to differentiate between two types of overpressure origins: loading and unloading type, and further evaluated their contribution rate to overpressure. Loading overpressure primarily accounts for the overpressure observed in the Yinggehai Formation, contributing to pore pressure with a range from 51% to 93%. In contrast, the overpressure in the Huangliu Formation is predominantly of the unloading type, contributing to pore pressure from 35% to 46%. Additionally, we analyzed the genetic mechanism of overpressure, utilizing lithology, subsidence rate, organic matter maturity, and seismic attribute data. We find that the loading overpressure in the Yinggehai Formation is attributed to unbalanced compaction of mudstone due to rapid sedimentation (sedimentation rate > 500m/ Ma). The unloading intense overpressure in the Miocene strata is most likely caused by the vertical transmission of overpressured fluid along faults and fractures. Our findings provide implications for complex pressure structure, overpressure evaluation, and genetic mechanisms in sedimentary basins.