Abstract:

The Quaternary of Sanhu area in Qaidam basin is rich in low gas-saturated reservoirs. It is difficult to identify low gas-saturated reservoir because of the obscurities and diversities of the log response features. The authors are aiming to explore the origin of low gas-saturated reservoir from both macro and micro perspectives, after comprehensively analyzing the sedimentary environment, structural feature as well as diagenesis and compaction. With reference to this study, the low gas-saturated reservoir can be mainly attributed to the low-amplitude structural trap, and the weak hydrodynamic environment and late accumulation should be taken into consideration, as well. Due to the distinct influence of sedimentary environment, diagenesis and compaction in the vertical, the micro origins of low gas-saturated reservoir are different in the shallow and deep formation. However, the characteristics of high clay mineral content, high shale content and high formation water salinity are the same, meanwhile, the carbonate and pyrite are also widely distributed in the whole formation. In the shallow formation, the weak seal capacity of shale cap rock is the critical variation. By contrast, in the deep formation, the high irreducible water saturation should take more responsibilities.

Key words: Qaidam basin, Quaternary of Sanhu area, low gas-saturated reservoir, origin, low-amplitude structure, shale cap rock, irreducible water saturation, petroleum gas