Biomarker geochemistry of crude oils and Lower Paleozoic source rocks in the Tarim Basin, western China: An oil-source rock correlation study

In order to select biomarker parameters that have potential to quantitatively distinguish between the Cambrian-Lower Ordovician and Upper Ordovician source rocks in the Tarim Basin with a total organic carbon content >0.5%, 42 Lower Paleozoic shale samples, were collected from a number of wells and outcrops in the Tarim Basin, northwestern China. The biomarkers in the extracts of the source rocks were analyzed by gas chromatography-mass spectrometry. The results show that C23 and C30 were dominant in the tricyclic terpanes and pentacyclic triterpanes, respectively, and C27 and C29 were both dominant in C27-29 steranes of the source rock samples. The distribution of n-alkanes, isoprenoids and terpanes indicate that the source rock samples have been slightly to moderately biodegraded, and they originated from a reducing depositional environment with considerable marine algal input in. Comparatively, Upper Ordovician source rock samples presented lower values of C28/C29 regular steranes and higher values of C29 ααα20S/(20S + 20R) and C29αββ/(αββ+ααα). Abnormally lower values of C29 ααα20S/(20S + 20R) and C29αββ/(αββ+ααα) were observed in the Cambrian-Lower Ordovician strata. The set of data in this study show that homohopane index including C31homohopane/C30hopane, C31-35homohopanes/C30hopane, and the ratios related to the triaromatic steroids including C26−20S/C28−20S TAS, C27−20R/C28−20R, (C26−20R + C27−20S)/C28−20S and C28/(C26+C27) TAS, could serve as potential biomarker parameters to distinguish between the two groups of source rocks in the Tarim Basin. The values of those parameters (in above order) in Cambrian-Lower Ordovician source rocks were <0.6, <1.5, >0.3, >0.5, >0.85, and <1.2, respectively, whereas those in Upper Ordovician source rocks were >0.8, >1.9, <0.3, <0.5, <0.85, and >1.2, respectively. The correlation between the source rock samples and 19 oils obtained from the Tadong, Tabei and Tazhong uplifts in the Tarim Basin show that most of the oils in Tabei uplift are probably from Upper Ordovician source rocks, the oils in Tadong uplift are more likely from Cambrian- Lower Ordovician source rocks, and the Tazhong uplift is more like a mixed oil region with oils from both of the two groups of source rocks.