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Geochemical characteristics of the Upper Devonian Shetianqiao Formation in the Shaoyang sag of Xiangzhong depression from Middle Yangtze area, South China: Implications for the depositional environment and organic matter enrichment

2021; Elsevier BV; Volume: 574; Linguagem: Inglês

10.1016/j.palaeo.2021.110448

1872-616X

Lin Chen, Baomin Zhang, Shu Jiang, Xiaohong Chen, Guotao Zhang, Ping Chen, Weibing Lin, Zhanhong Liu,

Hydrocarbon exploration and reservoir analysis

The formation and preservation conditions of the organic matter (OM) in the Upper Devonian Shetianqiao Formation were investigated to assess the mechanism of the OM enrichment in the shale in the Shaoyang sag. The total organic carbon (TOC) contents of the shale range from 0.2% to 4.4%. The shale can be divided into three intervals. The terrestrial input during Interval 2, as indicated by the Al2O3 and Ti proxies, was relatively small. The chemical alteration index (CIA) shows that the Shetianqiao Formation mainly formed in a warm and humid climate, however, climate cooling occurred in Interval 2. Intervals 1 and 3 were mainly deposited in a restricted environment, whereas Interval 2 experienced upwelling. The climate cooling may have been the driver for the upwelling. The UEF, MoEF, and Mo-U covariation diagrams indicate that Interval 2 was mainly deposited under reducing conditions, whereas Intervals 1 and 3 were chiefly formed in an oxidizing environment. The Cu/Al ratios and excess Si (Sixs) suggest that Interval 2 was formed in significantly higher marine productivity than Intervals 1 and 3. The plot of Co (ppm) × Mn (%) versus Cd/Mo, the logarithmic relationship between Co (ppm) × Mn (%) and TOC, and the good correlations between TOC and the productivity proxies suggest that productivity was the primary factor for the OM enrichment. We constructed a depositional model that emphasizes the control of productivity driven by upwelling on the OM enrichment.