205 / 2019-06-20 04:01:22

Multi-temporal and -spatial deposition of evaporites in Paleogene Lower Ganchaigou Formation, West Qaidam Basin, China

evaporite; mixed deposits; lacustrine facies; diagenetic evaporite; Lower Ganchaigou Formation; Qaidam Basin

Evaporites in the geological record is of great significance for providing valid information of palaeoclimate and hydrochemistry, which is also a natural mineral resource and usually associated with petroleum reservoirs. This paper presents a complex mixed deposit in the Paleogene Lower Ganchaigou Formation that recorded various evaporites with multiple formation periods and spatial distributions. Three types of evaporites are discriminated by petrographic and mineralogical features, including layered halite (Tp1), euhedral crystals of glauberite and anhydrite (Tp2), and glauberite and anhydrite lumps (Tp3). Mineralogical and geochemical data indicate that the evaporites were deposited from a semi-open to semi-closed saline lacustrine environment, with intermittent influence by terrigenous and freshwater supplies. The Tp1 evaporite was precipitated from hypersaline water during the late stage of lake salinization due to high-density saline water sinking and converging. The Tp2 evaporite was initially precipitated from mesosaline water during the middle stage of lacustrine basin shrinking, and subsequently underwent diagenetic adjustment and overgrowth in shallow burial environment. The Tp3 evaporite was deposited in early-formed karst vugs during penecontemporaneous to shallow burial, in form of cement-filling precipitated from high saline pore fluids. Generally, the lacustrine water evolution and palaeogeomorphology have controlled the temporal and spatial variant of evaporites deposition, shown by the upward depositional sequences of carbonate, anhydrite, glauberite, halite, and also the transverse distribution of Tp1 dominated in the palaeogeomorphic lowlands, Tp2 in shallow water of slope area and Tp3 in relative highlands. The development of Tp3 evaporite was closely related to karst vugs, implying a newfound type of reservoir space when half or none occluded. These results indicate multiple depositional history of evaporites in the Paleogene Lower Ganchaigou Formation and provide new constraints on sedimentary evolution, evaporites distribution, and possible a new guidance for high quality reservoir estimation and prediction.