Geochemical oil and source correlation between crude oil of well Fusha 8 and source rocks of Permian Pusige Formation in southern margin, piedmont of southwestern Tarim Basin

To further expand the oil and gas exploration in the southern margin of the piedmont of the southwestern Tarim Basin, this study focuses on the newly discovered oil from well Fusha 8 and source rocks from wells Fusha 2 and Yang 1 in the Permian Pusige Formation of the Fusha-Keliyang structural belt. Organic geochemical oil and source correlation research was conducted. Through pyrolysis, vitrinite reflectance testing, gas chromatography (GC), GC-Mass Spectrometry (MS), and n-alkane monomer hydrocarbon isotope analysis, the geochemical characteristics of different layers of source rocks in the Permian Pusige Formation were characterized from aspects such as basic geochemical properties, molecular biomarkers, and monomer hydrocarbon carbon isotope compositions. The study revealed variations in hydrocarbon generation potential across different layers of source rocks from the Pusige Formation. Also, oil sources in well Fusha 8 were preliminarily explored. Results showed that the upper part of the upper section and the middle section of the Pusige Formation had low organic matter abundance, containing type Ⅲ organic matter, and were unlikely to serve as effective source rocks. Also, these source rocks may be contaminated by migrated hydrocarbons from the deeply buried reservoirs. The lower part of the upper section and lower section of the Pusige Formation had relatively higher organic matter abundance, featuring type Ⅱ1 to Ⅲ organic matter in the low mature to mature stages. The lower part of the upper section of the Pusige Formation was deposited in a mildly oxidizing shallow lake environment, and the organic matter inputs were mixtures of terrestrial higher plants and aquatic organisms. Its lower section was deposited in a more reducing deep to semi-deep lake environment, with a higher content of aquatic organisms as inputs. The crude oil from the Lower Jurassic of well Fusha 8 had higher Pr/Ph, C24Te/C26TT, and sterane/hopane ratios and lower C19-C23TT/C30H ratios. It was rich in C29 regular steranes, fluorene, and dibenzofuran, with n-alkane monomer carbon isotope values ranging from -32.0‰ to -29.0‰. These characteristics are consistent with the biomarker and monomer hydrocarbon carbon isotope compositions of the lower part of the upper section of source rocks in the Pusige Formation, confirming a strong geochemical oil and source correlation between them.