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Maturity evaluation of highly evolved organic matter is an important problem in oil and gas geochemistry, because conventional organic geochemical proxies are not applicable due to hydrocarbon generation and expulsion. In this study, we investigated the pyrobitumen in the reservoirs of the Sinian Dengying Formation in the Sichuan Basin, China. We focused on aromatic hydrocarbon biomarkers, and the calibration and application of aromatic maturity proxies using pyrobitumen reflectance and Raman spectral parameters. Our results show that only the thiophene compounds are effective maturity indicators, such as MDR (4-/1-MDBT), MDR′ (4-MDBT/(1-MDBT+4-MDBT)), MDR1 (1-MDBT/DBT), 4,6-/(1,4+1,6)-DMDBT, (2,6 +3,6)-/(1,4+1,6)-DMDBT, and [2,1]BNT/([2,1]BNT+[1,2]BNT), as these are largely independent of the optical anisotropy of pyrobitumen The condensation and incorporation of thiophene compounds can lead to an increase in the reflectance of pyrobitumen, which means the thiophene compound parameters are positively or negatively correlated with pyrobitumen reflectance New potential maturity parameters were proposed, including DMDBTs/MDBTs, TeMDBTs/(DBT+MDBTs +DMDBTs+TMDBTs+TeMDBTs), 1-MDBT/MDBTs, and [2,1]BNT/BNTs. This method and these parameters are universally applicable to maturity characterization, especially in rocks that lack vitrinite, contain strongly anisotropic organic matter, and have high contents of thiophene compounds. [ABSTRACT FROM AUTHOR]

Inorganic geochemical approaches (i.e., trace elements) for hydrocarbon–source rock correlations in sedimentary basins are a frontier area of research in petroleum geochemistry. This study used inorganic geochemistry to the over mature petroleum systems in the Sinian (Neoproterozoic) Dengying Formation in the Sichuan Basin of China. Our results show that the Ba N –Ni N –Cu N ternary diagram, V N –Ni N –Mo N ternary diagram and V/Ni–U/Th diagram (related to paleo-productivity and redox conditions), rare earth element (REE)–Y patterns, Ce/Ce*–La/La* diagram, and Ce/Ce*–La/La*–(LREE/HREE) N ternary diagram (related to fluid interactions) can robustly distinguish different source rocks. In particular, V, Ni, and Mo have a strong relationship to organic matter in source rocks, while their occurrence is most likely related to porphyrins in petroleum. Therefore, the V N –Ni N –Mo N ternary diagram can be used for hydrocarbon–source rock correlations. In contrast, Ba, Cu, U, Th, and REE–Y have different and complex geochemical behaviors, limiting the application in hydrocarbon–source rock correlations. The application in the Sichuan Basin shows that the hydrocarbons in the Dengying reservoir were mainly derived from the lower Cambrian Qiongzhusi Formation with minor contribution from the lower Cambrian Maidiping and Sinian Doushantuo formations. Compiled data from Lower Paleozoic crude oils in the Tarim Basin show that the V N –Ni N –Mo N ternary diagram can also be applied to low-maturity crude oil systems. Therefore, V, Ni, and Mo have strong relationship to organic matter in petroleum systems and the potential for hydrocarbon–source correlation, while other elements should be used under given conditions. [ABSTRACT FROM AUTHOR]