Sedimentary characteristics and genesis of the salt lake with the upper member of the Lower Ganchaigou Formation from Yingxi sag, Qaidam basin

The Oligocene Lower Ganchaigou Formation (E-3(2)) was deposited in the salinized lake, which developed in the western Qaidam Basin in the Oligocene. The sediments are dominated by micritic dolomites and organic-rich calcareous mudstones, forming the high-quality reservoirs and source rocks. Intersalt and subsalt hydrocarbon-bearing system developed. Extensive geological and geochemical data from the Yingxi Sag and its surrounding areas have been analyzed to identify major rock types, determining petrological characteristics, interpreting depositional environments and establishing models for the evolution of the Basin. Four major lithologies formed in the lake, lacustrine carbonates, mudstones, evaporates and clastic rocks. The main minerals encountered in these sediments are dolomites and calcites (31.2%), clay minerals (28%), quartz and feldspar (25.6%), anhydride (6.3%), halite (5.6%), pyrites (3.3%) and glauberites (2.2%). Eight lithofacies are distinguished; Micritic dolomite facies; Calcareous sandstone facies; Granular Limestone facies; Packstone facies; Biomicrite facies; Laminated calcareous siltstone facies; Calcareous mudstone facies and Evaporite fades. These are divided into the basin marginal environments and the intrabasinal environments, which are further grouped into five zones: the bench slope zone, the gentle slope zone, the steep slope zone, the underwater ridge zone and the semi-deep to deep lake zone. The facies sequence developed through the early, middle and late sedimentary and evolutionary stages, which corresponded to brackish lake (carbonates and calcareous silts), saline lake (anhydrite-rich carbonates and mudstones) and hypersaline lake (thick-layer halite and thin-layer carbonates). These changes resulted from the chemical, biological and physical sedimentary controls that resulted from variations of climatic, geomorphological and tectonic influences over time. An important result of this history is that both intersalt and subsalt hydrocarbon-bearing system are developed in these rocks.