Depositional sedimentary environment and hydrothermal controls on organic matter enrichment in the Lower Cambrian Niutitang shale reservoir, South China

The purpose of this research was to examine paleoenvironments, hydrothermal activity, and seawater restriction of the lower Cambrian Niutitang Formation shale gas reservoir in the eastern Xuefeng Uplift and to determine factors affecting organic matter (OM) enrichment. In the studied borehole (X1) in western Hunan Province, the Niutitang Formation can be subdivided into the Niu1, Niu2, and Niu3 members based on geological and geochemical features, including trace element enrichment, lithology and fossil content. Total organic carbon (TOC) values of the Niutitang Formation are variable, averaging 1.5 wt.% in the Niu1 Member, 12.7 wt.% in the Niu2 Member, and 5.1 wt.% in the Niu3 Member. Paleoclimatic changes were responsible for changes in biota, which impacted patterns of OM enrichment. Climate proxies (Chemical Index of Alteration (CIA)), and productivity proxies (Babio, Cu/Al, and Ni/Al) consistently indicate higher paleoproductivity in the Niu2 Member. The Niu1 and Niu2 members were possibly affected by hydrothermal events, whereas hydrothermal activity was absent during deposition of the Niu3 Member. Hydrothermal activity may not only provide nutrients and silica but alsoenhance the reducing condition of the water column. In addition, hydrothermal events may have possibly influenced biological survival in the different environments, which in turn increased their reproduction within the early Cambrian ocean and affected OM production. Redox proxies (MoEF and UEF) imply that the Niu1, Niu2, and Niu3 members were deposited in suboxic, euxinic, and ferruginous environments, respectively. Redox conditions, strongly restricted water environments, and hydrothermal events were conducive to OM enrichment during the early Cambrian.