Depositional environment and hydrothermal controls on organic matter enrichment in the lower Cambrian Niutitang shale, southern China

Abstract

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 Xiangan 1 well 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 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 proxy (chemical index of alteration) and productivity proxies (biogenic Ba, Cu/Al, and Ni/Al) consistently indicate higher paleoproductivity in the Niu2 Member. The Niu1 and Niu2 Members may be affected by hydrothermal events, whereas hydrothermal activity was absent during deposition of the Niu3 Member. Hydrothermal activity may provide nutrients and silica but may also enhance 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 (Mo and U enrichment factors) 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.