Source-rock evaluation and depositional environment of black shales in the Triassic Yanchang Formation, southern Ordos Basin, north-central China

A recent drilling operation in the Triassic Yanchang Formation (Tongchuan District of the Ordos Basin, north-central China) reached a black shale bed with an average in-situ gas content of 1.11 g/ml. Subsequent total organic carbon (TOC) content measurements and rock-eval analysis showed that the black shale section was generally classified as excellent source rock, with TOC contents commonly over 12% and an average genetic potential reaching 90 mg HC/g rock. The black shale samples of the Yanchang Formation contain kerogen of types I and II1, indicating the presence of oil source rocks, and the Tmax values range from 438 to 442 °C, which indicate a maturity stage in the early oil window. To reconstruct the depositional environment of the black shales with high TOC contents, various geochemical elements and their ratios were analyzed, yielding a number of interesting results: (i) the paleoclimate indexes CIA, CIW and C-values range from 50 to 72 (mean 65), 56 to 85 (mean 75), and 0.8 to 2.6 (mean 1.5), respectively, indicating humid conditions during the sedimentation period of the black shales; (ii) the redox conditions during the sedimentation of the black shales were dysoxic to anoxic as evidenced by the cross-plot of V/Cr vs. V/(V + Ni); (iii) the water column environment was a deep, fresh-water lake, further confirming the moist paleoclimate and reducing environment; and (iv) the high TOC contents of the black shales are attributed to inputs from hydrothermal fluids as evidenced by the samples falling into or near to the hydrothermal area of the Ni/Cr vs. Ti plot and the Ti vs. V plot.