Elemental characteristics and paleoenvironment reconstruction： a case study of the Triassic lacustrine Zhangjiatan oil shale, southern Ordos Basin, China

Using trace elements to reconstruct paleoenvironment is a current hot topic in geochemistry. Through analytical tests of oil yield, ash yield, calorific value, total sulfur, major elements, trace elements, and X-ray diffraction, the quality, mineral content, occurrence mode of elements, and paleoenvironment of the Zhangjiatan oil shale of the Triassic Yanchang Formation in the southern Ordos Basin were studied. The analyses revealed relatively high oil yield(average 6.63%) and medium quality. The mineral content in the oil shale was mainly clay minerals,quartz, feldspar, and pyrite; an illite–smectite mixed layer comprised the major proportion of clay minerals. Compared with marine oil shale in China, the Zhangjiatan oil shale had higher contents of quartz, feldspar, and clay minerals, and lower calcite content. Silica was mainly in quartz and Fe was associated with organic matter, which is different from marine oil shale. The form of calcium varied. Cluster analyses indicated that Fe, Cu, U, V, Zn, As,Cs, Cd, Mo, Ga, Pb, Co, Ni, Cr, Sc, P, and Mn are associated with organic matter while Ca, Na, Sr, Ba, Si, Zr, K,Al, B, Mg, and Ti are mostly terrigenous. Sr/Cu, Ba/Al, V/(V+ Ni), U/Th, AU, and δU of oil shale samples suggest the paleoclimate was warm and humid, paleoproductivity of the lake was relatively high during deposition of the shale—which mainly occurred in fresh water—and the paleo-redox condition was dominated by reducing conditions. Fe/Ti ratios of the oil shale samples suggest clear hydrothermal influence in the eastern portion of the study area and less conspicuous hydrothermal influence in the western portion.

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

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.d Eu; Fe versus Mn versus(Cu + Co + Ni) 9 10; and Si O2/(K2 O + Na2 O) versus Mn O/Ti O2, Fe/Ti, and(Fe + Mn)/Ti are evidence of hydrothermal fluid activity during oil shale sedimentation, and d U 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 d U, 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, d U, U/Th, and Cu + Pb + Zn all exhibitedhigh 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.