Advances and trending topics in sedimentary reservoir research of the Upper Triassic Xujiahe Formation,Sichuan Basin

The Upper Triassic in the Sichuan Basin is rich in both conventional and unconventional gases, the temporal and spatial distribution and enrichment of which are controlled by sedimentary reservoirs under an isochronous stratigraphic framework. Exploration activities in the basin have drawn greatly from sedimentary reservoir research and highlighted its scientific and strategic significance. Progresses made in the domain can be summarized as establishment of symmetrical and less symmetrical sequence stratigraphic models and their evolutionary patterns of the foreland basin based on previous Upper Triassic sequence stratigraphic frameworks. The Ma'antang Formation to the third member of Xujiahe Formation are marine deposits, and the fourth to the sixth member of Xujiahe Formation are continental deposits, though still under the effect of sea transgression. The Ma'antang Formation mainly consists of reef-shoal facies, lagoon facies, tidal flat lake and delta facies; the Xiaotangzi Formation mainly consists of barrier sand bar-lagoon facies with small deltas developed around basin. The second, fourth and sixth members of Xujiahe Formation are dominated by alluvial fans, fan deltas and braided river delta facies, the third member consists of costal-delta facies, and the fifth member is mainly composed of lacustrine-delta facies. Five major provenances existed during the Late Triassic in Sichuan Basin, including Qinling ancient land, Dabashan ancient land, Songpan-Ganzi fold belt, Kangdian ancient land, and Jianglan ancient land, with each providing sediment inputs of temporal and spatial differences in terms of nature, scope and extent. Sediments were largely sourced from the north part of the basin during the deposition of the second and third members of Xujiahe Formation. Tight sandstone reservoirs in the Xujiahe Formation are closely linked to compaction which caused great primary porosity loss and the quartz overgrowth in Ⅱ and Ⅲ phases that further reduced the primary porosity. The closed underground fluid environment also made its contribution to the formation of tight reservoirs. Sweet spots in the formation were controlled by microfacies, dissolution, chlorite lining, and fractures, and thus making the underwater branch channels at delta fronts the most promising targets for tight gas discovery. With major progresses in shallow water delta research and shale gas exploration in recent years, shallow water deltas, fine-grain sedimentology and deposition mechanisms, sedimentary process simulation and fine-grained reservoir characterization of the Upper Triassic in Sichuan Basin, have become the trending topics and future research directions. © 2021, OIL & GAS GEOLOGY Editorial Board. All right reserved.