The study on the source of the oil seeps and bitumens in the Tianjingshan structure of the northern Longmen Mountain structure of Sichuan Basin, China

摘要

The Tianjingshan structure of the northern Longmen Mountain is located in the north of the Sichuan province, China. Two primary source rocks are present in this region, the overmature Infracambrian-Lower Cambrian source rocks and the mature Upper-Permian source rocks. Oil seeps and solid bitumens occur extensively in the Mesozoic and Paleozoic marine strata, indicating good exploration prospects in this area. However, oil-to-source correlation in the Tianjingshan structure is difficult due to both high maturity of the Lower Cambrian source rocks and severe biodegradation of oil seeps and bitumens. In this work, one oil seep from the Permian Qjxia Formation and two severely degraded bitumens from the Cambrian strata in the Tianjingshan structure were correlated with their potential source rocks. Hydropyrolysis was used to release biomarkers bound to kerogen of the overmature Lower Cambrian source rock. The released biomarkers were used for the oil-to-source correlation together with those indexes resistant to thermal degradation and biodegradation. Based on the geochemical data and the petroleum geological settings of the area, it can be inferred that the Cambrian, Ordovician and Silurian bitumens with light bulk isotopic values (commonly δ~(13)C < -34.0‰) were sourced from the Infra-cambrian -Lower Cambrian source rocks, while the oil in the Permian reservoir was initially charged by the oil from the Infracambrian-Lower Cambrian source rocks and then later charged by the oil from the Upper-Permian source rocks. The Infracambrian-Lower Cambrian source rocks are the major oil source in the Tianjingshan structure. The Upper-Permian source rocks only partly contributed to the reservoirs in the neighbouring strata.