Abstract:

This article discusses the specific characteristics and mechanisms of natural gas accumulation and leakage in overpressured sequences. Based on the pressure, stress regimes and the petroleumbearing properties, three types of overpressured reservoirs (filled, underfilled, unfilled or drained) are recognized. The charging history and gasbearing properties of all drilled traps in the Yinggehai and Qiongdongnan basins are closely related to overpressure. The newly discovered DF13-1 gas field is a filled overpressured gas reservoir, whereas the YA21-1 structure represents unfilled or drained overpressured trap with waterphase pressure close to the fracture pressure of the top seal. The accumulation or leakage of natural gas in overpressured traps is controlled by the minimum horizontal stress, the intensities of waterphase pressure relative to seal fracture pressure, and the geomechanical properties of top seals. In the Yinggehai and Qiongdongnan basins, fault and diapercontrolled overpressured traps may contain relatively short gas columns compared to lithologic traps with similar burial depth and waterphase excess pressure. The depth to the crest of lithologic traps played an important role in determining the pressure state and gasbearing properties of the reservoirs. Effective prediction of the pressure and stress combined with the analysis of the geomechanical properties of the seals is an important way to reduce exploration risk.

Key words: overpressure, pressurestress coupling, seal fracturing, accumulation/leakage mechanisms, hydrocarbon retention capacity, Yinggehai and Qiongdongnan basins