Lacustrine Source Rock Deposition in Response to Coevolution of the Paleoenvironment and Formation Mechanism of Organic-Rich Shales in the Biyang Depression, Nanxiang Basin

Abstract

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.

Impact of Paleosalinity, Dilution, Redox, and Paleoproductivity on Organic Matter Enrichment in a Saline Lacustrine Rift Basin: A Case Study of Paleogene Organic-Rich Shale in Dongpu Depression, Bohai Bay Basin, Eastern China

“…In detail, the ( S 1 + S 2 ) ranges within 1–13.19 mg/g with an average of 5.30 mg/g in the Es 3 U shale and within 0.15–11.29 mg/g with an average of 4.86 mg/g in the Es 3 L shale (Tables and ). The result indicates a relatively low content of organic matter in the Es 3 U and Es 3 L shales, which is consistent with the previous studies of the Dongpu Depression. , According to the evaluation criteria of continental lacustrine source rocks, 47.4% of the Es 3 U shale is good source rock, and 48.1% of the Es 3 L shale is good source rock based on a TOC and S 2 cutoff (Figure b).…”

“…The result indicates a relatively low content of organic matter in the Es 3 U and Es 3 L shales, which is consistent with the previous studies of the Dongpu Depression. 32,33 According to the evaluation criteria of continental lacustrine source rocks, 52 47.4% of the Es 3 U shale is good source rock, and 48.1% of the Es 3 L shale is good source rock based on a TOC and S 2 cutoff (Figure 4b).…”

“…On the basis of the above models, some scholars studied the organic matter enrichment conditions in the shales in the Yangtze Platform, , northern South China Sea, and Jianghan Baisn with respect to paleoproductivity, redox conditions, and dilution. Massive shale oil resources were recently discovered in many SLRBs in China, such as the Paleogene shale oil plays in the Dongpu Depression, Bonan Sag, and Dongying Depression , in the Bohai Bay Basin; Biyang Depression in the Nanxiang Basin, Jianghan Basin; − , and the Permian shale oil plays in the Jimusar Sag in the Junggar Basin. , However, studies on the main factors controlling organic matter enrichment in shales of the SLRBs are still lacking. − Recently, some research started focusing on SLRB shales. Ding et al.…”

Climate-Driven Variations in the Depositional Environment and Organic Matter Accumulation of Lacustrine Mudstones: Evidence from Organic and Inorganic Geochemistry in the Biyang Depression, Nanxiang Basin, China

“…Biomarkers are commonly applied to infer the origin of OM in sediments and to indicate significant contribution to primary productivity in lacustrine surface water. 12,60 Sterane distributions reflect primary producers in both marine and lacustrine systems. 61,62 The precursors of 4-methylsteranes are presumed to be 4-methyl sterols, mostly regarded as having been derived from dinoflagellates.…”

“…7−11 It is likely, therefore, that many factors that influence the concentration of organic carbon, including primary productivity, water redox conditions, and dilution of clastic material, control OM accumulation rather than any single factor. [8][9][10]12 Lacustrine sedimentary environments are more sensitive to climatic changes than marine environments due to their limited geographic extent and volume. 13−15 Climate change may have influenced the balance between precipitation and evaporation and driven terrestrial weathering, changing water chemistry, primary productivity, and the amount of terrigenous material input, ultimately resulting in variation in depositional processes and OM accumulation in lacustrine basins.…”