The Upper Triassic Chang 9 organic-rich sediments have been considered as effective hydrocarbon source rocks for the Mesozoic petroleum system in the Ordos Basin. Previous studies on the Chang 9 member mostly focused on the influence of their paleoproductivity and paleoredox conditions on the organic matter (OM) enrichment, whereas there are few studies on the influence of the paleoclimate condition and sediment provenance on the OM enrichment. In this study, a series of geochemical analyses was performed on the Chang 9 core samples, and their hydrocarbon generation potential, paleoclimate condition, and sediment provenance were assessed to analyze the effect of paleoclimate-provenance on OM enrichment. The Chang 9 source rocks are characterized by high OM abundance, type I−II OM type, and suitable thermal maturity, implying good hydrocarbon generation potential. Based on the C-values and Sr/Cu ratios, the paleoclimate condition of the Chang 9 member was mainly semihumid. In addition, the Th/Co vs. La/Sc diagram and negative δEuN indicate that the Chang 9 sediments were mainly derived from felsic source rocks. Meanwhile, the paleoweathering intensity of the Chang 9 member is moderate based on moderate values of CIA, PIA, and CIW, which corresponds to the semihumid paleoclimate. The relatively humid paleoclimate not only enhances photosynthesis of the primary producer, but also promotes chemical weathering intensity, leading to suitable terrestrial clastic influx to the lacustrine basin, which is beneficial for OM enrichment.

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Benzonaphthothiophenes (BNTs) were firmly identified in coals by comparison with retention indices reported in the literature. The thermodynamic stabilities of BNTs were calculated by quantum chemical computations, and their stability sequence is as follows:The BNT isomers are ubiquitous in coal samples from the Xihu Depression and the Ordos Basin. This study reveals the effects of thermal maturity on the distributions of BNTs in coals, where the relative abundances of [1,2]BNT and [2,3]BNT to [2,1]BNT were observed to be mainly controlled by thermal maturation conditions. Combining with theoretical calculations of thermodynamic stability, two benzonaphthothiophene maturity indicators, defined as, were proposed. Two preliminary calibrations of BNTR-1 and BNTR-2 against measured vitrinite reflectance (%Ro) were established, and the relationships are as follows: %Rc = 5.5 × BNTR-1/100 + 0.5 (%Ro >0.9) and %Rc = 15 × BNTR-2/100 + 0.5 (%Ro > 0.9), respectively. BNTR-1 and BNTR-2 have good correlations with the widely used molecular maturity parameters. The good correlations with vitrinite reflectance and maturity parameters suggest that BNTR-1 and BNTR-2 are useful indicators in evaluating the maturity for sedimentary organic matter at high levels of thermal stress. The accuracy of the BNT ratio ([2,1]BNT/ [1,2]BNT) must be kept in mind when it is used as a migration tracer of oil generated at a high thermal maturation stage. There is no clear trend for the absolute concentrations of BNTs with increasing pristane/phytane and gammacerane/C 30 hopane values, suggesting that the redox conditions and water salinity during deposition may have little influence on the generation of BNTs. This study can expand the understanding of the distribution and geochemical significance of complex sulfur-containing organic compounds in sedimentary organic matter.

The organic-rich sediments of the Carboniferous in the Shibei Sag, Junggar Basin, China are potential source rocks with good hydrocarbon generation conditions for oil and gas exploration. Thus, it is meaningful to analyze their geochemical characteristics, including the hydrocarbon generation potential, genetic types of kerogen, thermal maturity, the origin of organic matter (OM), and sedimentary environment. The results reveal that the Carboniferous source rocks have fair to excellent hydrocarbon generation potential. They are mainly gas-prone with type III-IIkerogens. Random vitrinite reflectance (Rr) of the studied samples ranges from 0.63 to 1.04%, suggesting a primarily mature stage, which is also supported by the maturity-related hopane and sterane ratios. Stable carbon isotopic compositions, organic petrological macerals, and biomarker characteristics all indicate a predominant higher plant input. Abundant bicyclic sesquiterpenoids and diterpenoid biomarkers in the studied samples suggest the conifer inputs. In particular, high amounts of phyllocladanes, retene, and kauranes suggest the contribution of gymnosperms, pre-gymnosperms, pteridophytes, or bryophytes in swamps. From the early Carboniferous to the late Carboniferous, the environment became more oxic, the water level dropped, and higher plants flourished. Based on the abovementioned results, the Carboniferous source rocks were likely to be deposited in a transitional environment under lacustrine-fluvial conditions. Specifically, source rocks from the Batamayineishan Formation were deposited in a peat environment during the interval of the volcano eruptions. Considering the quantity and maturity of OM and their large thickness, the Carboniferous sediments have a good petroleum exploration potential.

Redox conditions of lacustrine strata assessed by redox proxies with unified thresholds were conflicted in the Ordos Basin. The internal cross-calibration approach of multiple redox proxies, based on the enrichment sequence model of authigenic redox-sensitive element (RSE) governed by redox potential, has recently been proposed to calibrate redox thresholds in marine depositional systems. However, this assessment model often induces an overlapped threshold between anoxic–ferruginous and anoxic–euxinic conditions. Except for the redox potential, the enrichment of RSE is also controlled by its host phase. Thus, we introduced the enrichment degree and occurrence state model controlled by the host phase content into this approach to identify anoxic–euxinic conditions by an enrichment threshold of Mo (or U). Based on this method including double redox assessment models, we calibrated the thresholds of various redox conditions of the Chang 8–Chang 7 Members in the southern Ordos Basin. Subsequently, the effectiveness of previous bi-element proxies, the evolution of redox conditions, and their influencing factors were analyzed based on the calibrated thresholds. Results show that this method is applicable in the lacustrine strata. The cross plot of DOPT (degree of pyritization based on the total Fe and S content) (or SEF)–MoEF (or UEF) (enrichment factor) is suggested for calibrating thresholds of different redox conditions. Previous bi-element proxies are not suggested to be used again. There is a positive redox evolution sequence from oxic–suboxic (suboxidized) to anoxic–euxinic conditions in Chang 8–Chang 7 Members. Hydrothermal activity and paleoproductivity play different roles in the formation of redox conditions.