Abstract
Abstract In this study, we conducted systematic analysis of total organic carbon (TOC) content, lipid biomarkers, carbon isotopes of kerogen (δ13Ckerogen), and mineral composition in Niu-ti-tang (Є1n) and Pa-lang (Є1p) shales from Guizhou Province in order to provide a better understanding of the organic sources and marine environmental condition during deposition of the Lower Cambrian shales of South China. The results show that a broad variety of lipid biomarkers, such as n-alkanes, pristane, phytane, terpanes, hopanes, and steranes, are in these shales, which suggests a significant contribution of various paleobios with bacterial microorganisms and algae thriving under a low-salinity and stable anoxic environment. The negative δ13Ckerogen value (minimum −36.4‰) and occurrence of pyrite (1–7.5%) and carbonates (2.4–57.3%) indicate that bacterial sulfate reduction prevailed under anoxic conditions during deposition of the Lower Cambrian shales. Moreover, the difference in mineral and δ13Ckerogen composition between Є1n and Є1p shales might imply significant changes in primary production and paleoocean environments due to sea-level rise, as shown by the higher average TOC content in Є1p shales (2.52%) compared to that in Є1n shales (1.79%). The covariances of TOC content and mineral and δ13Ckerogen composition suggest that the Є1p shales might have been deposited under a higher sea level associated with high primary productivity, compared to Є1n shales. Thus, high primary productivity driven by sea-level rise is suggested to be the main controlling factor on organic matter enrichment in Є1p shales under stable anoxic conditions.
Highlights
The Early Cambrian was a critical interval in geological history and featured profound oceanic and biotic changes, i.e., “Cambrian Explosion” [1, 2]
The total organic carbon (TOC) content ranges from 0.29% to 6.52% and averages 2.52% for the Є1n and Pa-lang (Є1p) shales; for the Є1n shales, the values vary from 0.07% to 7.34% and average 1.79%
The biodiversity, primary productivity, and paleoocean environment have been deciphered based on lipid biomarker composition and δ13Ckerogen values, along with bulk geochemical parameters (i.e., TOC content, Tmax ratio) and mineral composition through the Lower Cambrian Є1n and Є1p shales from Guizhou Province, South China
Summary
The Early Cambrian was a critical interval in geological history and featured profound oceanic and biotic changes, i.e., “Cambrian Explosion” [1, 2] During this critical time interval, organic-rich black shale was deposited during global sealevel rise and oceanic anoxic events [3,4,5,6,7]. A set of organic-rich Lower Cambrian black shales in South China, e.g., Niu-ti-tang (Є1n) and Qiongzhu-si Formations (Є1q), is widely distributed throughout the Yangtze Platform. They have been identified as essential source rocks for shale gas exploration and development in this region because of their considerable thicknesses, large distribution area, relatively high total organic carbon (TOC) content, and thermal maturity [8,9,10]. Shale reservoir characteristics and gas potential, including their
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