Controls on Organic Matter Enrichment in the Lower Cambrian Niutitang Formation, Zhenba Area, Northwestern Upper Yangtze Platform

Organic matter (OM) enrichment is a fundamental control on source-rock quality and shale-gas potential, yet the relative roles of organic matter supply, preservation, and mineralogical dilution remain poorly constrained in the Lower Cambrian Niutitang Formation at the northwestern margin of the Upper Yangtze Platform. Nineteen shale samples from Well ZHD1 in the Zhenba area, on the northwestern Upper Yangtze Platform, were analyzed for total organic carbon (TOC), mineral composition, and major and trace elements. TOC contents range from 0.98% to 9.09%, with an average of 3.82%, while quartz and clay minerals range from 25% to 44% and from 27% to 53%, respectively. Among the sampled intervals, the middle interval (1694.00~1709.75 m) is characterized by the highest TOC contents (4.48~9.09%), accompanied by elevated Siexcess (10.95~11.67), U/Th (2.60~6.84), MoEF (16.83~34.38), and UEF (9.74~23.85). In contrast, Ti/Al and CIA show relatively limited variations across the sampled section. Quartz is positively related to TOC, whereas clay and carbonate minerals show negative relationships with TOC. Paleoproductivity increased from the lower interval to a maximum during the middle interval and then declined during the upper interval. Redox conditions evolved from moderately reducing to persistently anoxic during the middle interval before becoming less reducing. The middle interval records the highest TOC contents, greatest paleoproductivity, and most favorable preservation conditions. The combined Siexcess, mineralogical, and detrital-input evidence is consistent with a significant biogenic contribution to the non-detrital silica, although subordinate hydrothermal or authigenic contributions cannot be completely excluded. The negative relationships of clay and carbonate minerals with TOC are consistent with a net dilution effect at the bulk-rock scale, although potential clay-surface protection of OM cannot be excluded. These results emphasize the coupled effects of productivity and preservation rather than a single dominant control and provide region-specific constraints on OM enrichment in the northwestern Upper Yangtze Platform.