Geochemistry and sedimentary environments of the Upper Ordovician Wufeng Formation in Guangyuan, northern Sichuan and Zhenba, southern Shaanxi

The black shales with a great potential in the Upper Ordovician Wufeng Formation from the Guangyuan region, northern Sichuan to the Zhenba region, southern Shaanxi is interpreted as the key horizons for the marine oil and gas exploration in southern China. Referenced to the previous results, the present paper focuses on the analysis of palaeoclimate, palaeodepth and palaeoredox conditions in the study areas. The analytical results including 0.64 for the average V/(V + Ni) ratio; 1.87 for the average U/Th ratio; 2.98 for the average V/Cr ratio, 15.07 for the average Ni/Co ratio and 0.23 for the average MnO/TiO₂ ratios suggest the highly reduced conditions in the marginal shallow sea during the deposition of the Wufeng Formation. The high Ba contents, and low Sr contents and Sr/Ba ratios for the black shale samples from the Wufeng Formation indicate that the Wufeng Formation was once laid down in the fresh water environments. Geochemically, the samples display LREE enrichment. The δCe values vary between 0.77 and 1.57 with an average of 1.11, indicating positively Ce anomalies or slightly negative Ce anomalies. The average Ce/La ratio (2.22) and average δEu (0.59) indicate the dysaerobic to anaerobic shallow marine environments in the study areas.