Influence on lacustrine source rock by hydrothermal fluid: a case study of the Chang 7 oil shale, southern Ordos Basin

Abstract

Hydrothermal fluid activity during sedimentation of the Triassic Yanchang Formation in the Ordos Basin and the impact of said activity on formation and preservation conditions of source rocks have received little attention. Oil yield, major element, trace element, rare earth element, and total sulfur (TS) data from the oil shale within the Yanchang are here presented and discussed in the context of hydrothermal influence. Oil shale samples returned relatively high total organic carbon (TOC), in the range of 4.69%–25.48%. A high correlation between TS and TOC suggests TS in the oil shale is dominated by organic sulfur and affected by organic matter. The low Al/Si ratio of oil shale samples implies quartz is a major mineralogical component. Si/(Si + Al + Fe) values suggest close proximity of the oil shale to a terrigenous source. δEu; Fe versus Mn versus (Cu + Co + Ni) × 10; and SiO2/(K2O + Na2O) versus MnO/TiO2, Fe/Ti, and (Fe + Mn)/Ti are evidence of hydrothermal fluid activity during oil shale sedimentation, and δU and U/Th of the oil shale indicate reducing conditions. The Sr/Ba of oil shale samples suggests fresh-water deposition. The high correlations of Fe/Ti and (Fe + Mn)/Ti with δU, U/Th, and TS demonstrate that hydrothermal fluid activity promotes reducing conditions. Sr/Ba ratios had low correlation with Fe/Ti and (Fe + Mn)/Ti, implying that hydrothermal fluid activity had little impact on paleosalinity. Fe/Ti, (Fe + Mn)/Ti, δU, U/Th, and Cu + Pb + Zn all exhibited high positive correlation coefficients with TOC in oil shale samples, suggesting that more intense hydrothermal fluid activity improves conditions in favor of formation and preservation of organic matter.