Gas source analysis of the Sinian Dengying Formation in the Penglai gas field and its petroleum geological significance

The natural gas resources in the Sinian system of the central Sichuan paleo-uplift are abundant, making it a key area for increasing reserves and production in the deep layers of the Sichuan Basin. In recent years, significant breakthroughs have been achieved in the Penglai gas field, which is located in the slope region of the paleo-uplift, with major discoveries in the Sinian Dengying Formation. However, research on the natural gas source of the Dengying Formation reservoir remains unclear, which limits the in-depth understanding of the distribution characteristics of the gas reservoir and the selection of the future exploration direction and target. Based on the latest geochemical data from natural gas and reservoir bitumen, this study clarifies the gas source characteristics of the Dengying Formation reservoir in the Penglai gas field and identifies the hydrocarbon contribution from the third member of the Dengying Formation. Research indicates that natural gas from the Dengying Formation reservoir in the Penglai gas field is characterized by heavy δ13C2 values, large differences between δ13C2 and δ13C1, light δ2HCH4 values, and high 40Ar/36Ar ratios. These characteristics are significantly different from the overlying Cambrian gas reservoirs. This is closely related to the contribution of source rocks from the third member of the Dengying Formation, in addition to the Qiongzhusi Formation source rock. The source rocks of the third member of the Dengying Formation, which are stable and widely distributed in the central Sichuan Basin, were deposited under oxidizing water condition. This depositional setting also resulted in heavy δ13C values and low V/(V+Ni) ratios in the reservoir bitumen. Based on the δ2HCH4 values of the natural gas and the V/(V+Ni) ratios of the reservoir bitumen, the contributions of the source rocks of the third member of the Dengying Formation to the Dengying Formation reservoir in the Penglai gas field were estimated. The results showed that the contributions accounted for 38% and 47%, respectively, revealing the significance of the third member of the Dengying Formation in the formation of the Dengying Formation gas reservoirs. This important insight provides crucial guidance for estimating future resource potential and optimizing exploration direction in the study area.