General depositional features of the carbonate platform gas reservoir of the Lower Triassic Jialingjiang Formation in the Sichuan Basin of southwest China: Moxi gas field of the central basin

摘要

The Lower Triassic Jialingjiang Formation gas reservoir in Sichuan Basin of southwest China is a representative carbonate platform reservoir. However, it has not been well studied due to relatively less production in comparison with the reservoir in the well-known underlying Upper Permian Changxing and Lower Triassic Feixianguan formations. Thus, to provide supplement to the hot study of carbonate platform reservoirs in the Sichuan Basin, the authors describe the general depositional features of the carbonate platform gas reservoir of the Jialingjiang Formation based on a case study of the representative second member of the formation (termed as Jia 2) in the Moxi gas field of the central basin. The features mainly include depositional setting, lithology, depositional structure, depositional sequence and reservoir space. These results lead to a conclusion that the Jialingjiang Formation reservoir (the second member in particular) is not of a tidal flat deposition in an intertidal (to supratidal) environment as previously suggested but of restricted and evaporative carbonate platform deposition in a subtidal environment. The tidal flat-like (i.e., platform flat) facies occur only in the Jia 2~2-B layer. Moreover, the restricted-evaporative carbonate platform facies can be further divided into 5 subfacies and 23 microfacies, of which the facies of grain shoal and dolomitic flat are relatively favorable for the development of reservoirs. Their depositional model, distribution and evolution were further tentatively suggested. The facies are subject to paleotomography and sea level variations. These results also have general implications for the carbonate platform reservoirs elsewhere.