Variation of system openness and geochemical features in overpressured sandstones of the Yinggehai Basin, offshore South China Sea

Hydrocarbon discoveries in recent years in the Upper Miocene of the Yinggehai Basin show differences in overpressure magnitude, gas and mineral components. Overpressure close to or reaching the leakage threshold has induced high system openness for one field, but the overpressure of another field remains to be safely closed. A new vision to investigate the interplay between the system openness and the gas chemical and mineralogical features were conducted. The gas components in the high openness system are rich in high maturity hydrocarbons and inorganic CO2, which both have heavier carbon isotopes. Low maturity gases with lighter carbon isotopes are dominant in the low openness system. The abnormal carbon isotope ratio between iso-butane and n-butane in the high openness system confirms that early low maturity leakage has generated abnormal compositional and isotopic fractionation. Although the sedimentation process was uniform in the two systems, the high openness system has rare K-feldspar grains and chlorite, but it is rich in illite/smectite mixed layers (I/S). Precipitation is characterized by ankerite and kaolinite in the high openness system, whereas the low openness system primarily consists of calcite. The strong dissolution of K-feldspar grains and chlorite should have released abundant potassium and silicon. The high abundances of I/S and kaolinite, which are both sensitive to potassium and the rarity of quartz overgrowths in the high openness system, imply that the leakage of potassium/silicon from the sandstones has induced a mass imbalance between dissolution and precipitation. Even the sandstone porosity was slightly increased by the mass imbalance in the high openness system. The risk of hydrocarbon expulsion during brine leakage cannot be ignored.