Inorganic geochemical characteristics and paleoenvironment of Chang 7 oil shale in Yanchang Formation, Tongchuan area, Shaanxi Province

The geochemistry of shale core samples from the 7th member (Chang 7) of Triassic Yanchang Formation were analyzed in order to study the characteristics of sedimentary palaeoenvironment such as salinity, redox condition, depth, hydrothermal activity and palaeoclimate when the Chang 7 member was deposited in the Tongchuan area, Shaanxi Province. The content of Sr is 198.28×10^-6, Sr/Ba is 0.39, V/Ni is 4.50, the equivalent boron content by the Walker method in most samples is less than 200×10^-6, and the lowest value is about 40×10^-6. Based on the above characteristics, it can be concluded that the Chang 7 oil shale was deposited in a brackish fresh water and anaerobic environment. The quantitative determination of cobalt indicated that the sedimentary environment was a lake with a maximum water depth of 40-70 m. The average Sr/Cu ratio is 4.28, which means that the paleoclimate was warm-wet and semi-humid. P, Cu, Co, Ni, V and Rb were enriched. The total REE was low, and the HREE was enriched. Ce showed a negative anomaly. Most of the La/Yb-REE plots were located near the intersection of basalt, granite and sedimentary rocks. All these characteristics strongly confirmed the existence of hydrothermal sedimentation in the lake.