Sedimentary Environment Analysis of Eocene in Pingtai Area of Qaidam Basin, China

Abstract The Pingtai area is a relatively new oil and gas exploration area in Qaidam Basin, China. As one of the most important sandstone reservoir in the Qaidam Basin, Eocene Lulehe Formation and Xiaganchaigou Formation. Based on the petrological characteristics and element geochemistry of core samples, the paleosalinity and paleoclimate changes are discussed. The results show that the clay mineral assemblage of the Lulehe Formation consists of smectite, chlorite, and illite; while the clay minerals in the Xiaganchaigou Formation consist of illite/smectite mixed layer, chlorite, and illite. The paleosalinity calculated using the B content and Sr/Ba and Rb/Sr ratios indicates that the Lulehe Formation was formed in a freshwater environment, and the Xiaganchaigou Formation was formed in alternating brackish water and fresh water environments. The chemical weathering index (CIA), La/Th ratio, and Eu anomaly index indicate that the overall chemical weathering in the Pingtai area was weak, the provenance was relatively stable, and the influence of diagenesis on the clay minerals and the trace element contents was negligible. From the Lulehe Formation to the Xiaganchaigou Formation, the paleoclimate gradually changed from warm and humid to cold with humid-dry seasonal changes, which is consistent with the global decrease in temperature in the Eocene. Moreover, because of the uplift of the Altun Mountain and the global cooling, the rainfall decreased, resulting in the Xiaganchaigou Formation being formed in a higher salinity environment.