Discovery of Sinian source rocks in proterozoic in the Tarim Basin, NW China

As one of three cratonic basins in China, the Tarim Basin has the most abundant oil and gas resource, and its Precambrian petroleum system is a new region for hydrocarbon exploration. After conducting numerous field investigations in the Tarim Basin, we discovered the Sinian source rocks with 150m of thickness in Kuruktag area. These source rocks have total organic carbon (TOC) contents between 0.6% and 1.6%, with equivalent vitrinite reflectance (%Ro) between 1.23% and 1.42%. In addition, dark mudstone and a large amount of reservoir bitumen is present in the Upper Sinian outcrops in Aksu region. Biomarkers analysis indicates that reservoir bitumens in the Sinian and Cambrian strata were derived from the Sinian dark mudstones. Meanwhile, biomarkers of the reservoir bitumens from well ST1 in the Lower Cambrian Xiaoerbulake Formation indicate that the bitumen was originated from the Sinian source rocks and subsequently experienced biodegradation influence. Furthermore, a special hydrocarbon charging event with homogenization temperature of 50–60 °C has been elucidated from coeval aqueous phase of oil inclusions from in well ZS5, indicating that the oil and gas charging event in the Sinian source rocks occurred throughout the basin. As a result, the Sinian petroleum system in the Tarim Basin may contain high-quality source rocks, which contribute to the massive amount of Lower Paleozoic oil and gas resources in the basin. This study has important practical significance for oil and gas exploration of the Precambrian petroleum system in the Tarim Basin. •Hydrocarbon generation from Sinian source rocks has been found in the Tarim Basin.•Low-temperature fluid inclusion in Cambrian can only originate from precambrian.•Biomarkers confirm that Sinian source rocks once generated hydrocarbons in history.