Paleogeographic Evolution and Sedimentary Facies Analysis of Lake Basins in the Northwest Area of Qaidam Basin

Based on the observation and description of the systematic core samples, combined with the information on the evolution of lake basins and sedimentary background in the northwest area of Qaidam Basin, it was found that the area as a whole was in a lake environment. Accordingly, the sedimentary facies in the northwest area of Qaidam Basin were divided. The main subfacies were semi-deep lake and shallow lake. Meanwhile, based on the distribution of lithology, the microfacies were further divided into algal mat, beach, gray cloud flat and muddy flat. Among them, the algal mat microfacies was distributed very little, the gray cloud flat and muddy flat microfacies were more developed; the semi-deep lake subfacies also had a special microfacies type - turbidity fan, which mainly developed in the area of saline springs. After the spatial distribution of sedimentary microfacies in Qaidam Basin northwest area was clarified, the sedimentary facies in individual wells were divided. It was found that the sandstone and carbonate layers were stacked vertically to form relatively good reservoir and permeability combinations. Based on the above analysis, Qaidam Basin northwest area has been in a closed water environment for a long time, with no significant change in the base level, but it is very sensitive to climate change, and the frequent small fluctuations in the base level caused by seasonal changes determine the rapid alternation of sedimentary microfacies types.