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A total of 75 shale samples from the Biyang Depression were analyzed with Rock-Eval, total organic carbon (TOC), organic maceral, and gas chromatography–mass spectrometry (GC–MS) techniques to reveal the effect of salinity on source rock formation and its control on the oil content. On the basis of the diversities in salinity and redox conditions reflected by the gammacerane/αβ C30 hopane (G/H) ratio, the pristine/phytane (Pr/Ph) ratio, and the extended tricyclic terpane ratio [ETR = (C28 + C29) tricyclic terpane/Ts)], three types (T1, T2, and T3) of shales were identified, which deposited in the brackish condition (G/H ratio < 0.3), the semi-saline condition (G/H ratio = 0.3–0.6), and the saline condition (G/H ratio > 0.6), respectively. The comparisons among T1, T2, and T3 shales revealed that the salinity had a significant effect on the paleoproductivity of the lacustrine system. Most algae thrive in semi-saline conditions (T2) but are restrained in saline conditions (T3), so that moderate salinity conditions (G/H ratio = 0.3–0.6) are most conducive to the accumulation of algae organic matter (AOM). Although they have similar thermal maturity (0.6–1.1% vitrinite reflectance) and kerogen type (type II), the T1, T2, and T3 shales were developed in different reducing environments indicated by the Pr/Ph ratio and contained variable abundance of organic matter, resulting in remarkable differences in the preservation condition of AOM and the shale oil content within the T1, T2, and T3 shales. The classification and evaluation criteria of the shale oil resource were performed according to three categories based on the oil content (S1) and TOC values. It was revealed that the T1, T2, and T3 source rocks were mainly distributed among these areas of the potential, enriched, and ineffective shale oil resources, respectively, suggesting that shales developed in semi-saline environments deserve to be the most desirable target of shale oil exploration and development at present. Thus, this work may help related industry practitioners acquire valuable information from lacustrine shale systems.

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Shale oil has been found in the Paleogene lacustrine source rocks in the second (Eh3 2) and third beds (Eh3 3) of the third member of the Hetaoyuan Formation from the Biyang Depression of the Nanxiang Basin, eastern China, but the formation environments of the organic-rich shales are still unclear, which restricts the shale oil exploration. Here, this paper discusses the paleoenvironment and its control on the organic-rich shale formation. Our study suggests that shales from the Eh3 2 and Eh3 3 beds display some heterogeneities in total organic carbon (TOC) contents and oil potential as a result of their different paleoenvironments. During Eh3 3 deposition, the paleoproductivity was moderate to high, which was diluted by moderate to high terrigenous detrital matter (TDM) input. During Eh3 2 deposition, the TDM and terrigenous organic matter (TOM) inputs increased. The paleoproductivity was moderate, which was greatly diluted by the high TDM content. The redox conditions were anoxic during the two source rock intervals. Paleoproductivity is the main controlling factor of source rock formation during Eh3 2 and Eh3 3 depositions, which has a strong positive correlation with the burial of organic carbon. However, anoxia did not promote deposition of the richest source rocks. During Eh3 3 deposition, shales with high contents of TOC values and better oil potential could be formed in this environment on account of the moderate to high paleoproductivity. In contrast, during Eh3 2 deposition, high TOM input did not obviously improve the burial of organic carbon but high TDM input significantly diluted the paleoproductivity. The bottom water became semi-saline to hypersaline, but it did not enhance the anoxic environment. Shales with medium TOC values and oil potential could be formed in this environment as a result of moderate paleoproductivity.

The influence of the paleoclimate on the organic matter (OM) accumulation and depositional process in the third member of the Hetaoyuan Formation shales is unclear in the Paleogene lacustrine systems in the Biyang Depression, Nanxiang Basin, Eastern China. Here, inorganic and organic geochemical analyses were performed on samples from a 25 m profile to examine the effects of paleoclimate change on the depositional environment and OM accumulation within this interval. Based on mineralogical and geochemical data from the B1 Well, two major lithofacies assemblages, calcareous mudstone-dominated (LA1) and clay-rich mudstone-dominated (LA2), were identified. Although generally anoxic bottom water was present during sedimentation, subtle differences in depositional environment and OM enrichment were found for the two lithofacies. The mineral composition and multiple geochemical proxies, including detritus index (sum of quartz, clay minerals, and feldspar contents), C-value [∑(Fe + Mn + Cr + Ni + V + Co)/∑(Ca + Mg + Sr + Ba + K + Na)], chemical index of alteration (CIA = 100 × Al 2 O 3 /(Al 2 O 3 + CaO* + Na 2 O + K 2 O), Rb/Sr, V/(V + Ni), Sr/Ba, Pr/Ph, and gammacerane/C 30 hopane ratios all indicated that LA1 was deposited in a stable stratified saline lake with anoxic bottom water in a relatively cool, arid paleoclimate. However, these parameters suggested that LA2 was deposited in a brackish-to-saline lake with anoxic bottom water in a relatively warm, humid paleoclimate. Abundant OM (average total organic carbon (TOC) content 4.24 wt %) was accumulated mainly in LA1, with relatively high primary productivity suggested by high values of P/Ti, Ba/Al, 4-methylsterane/∑C 29 steranes index (4-MSI), ααα20RC 27 /ααα20RC 29 sterane (C 27 /C 29 ), and ααα20RC 28 / ααα20RC 29 (C 28 /C 29 ) sterane ratios. In contrast, in warmer, more humid conditions, the corresponding sediments in LA2 contained less OM (average TOC 2.29 wt %), consistent with ratios indicating lower primary productivity and higher detritus input. We draw the conclusions that the accumulation of higher OM in LA1 was controlled by the combination of paleoproductivity and preservation in anoxic bottom water, while the formation of the LA2 organic-rich mudstone was mainly related to primary productivity, anoxic conditions, and the dilution by terrigenous detrital matter.