LAUSR

Abstract Saline lacustrine source rocks in China provide the resource basis for many giant oil fields. Here, we report the first discovery of such source rocks in the piedmont area of the northern Tianshan Mountains in the southern Junggar Basin of NW China. These source rocks are Cretaceous and Paleogene in age. Organic geochemistry and trace-element data suggest that these mudstones were formed in a brackish–saline lacustrine environment, characterized by deep-water and reducing conditions, and in a warm and dry paleoclimate. Evidence for this environment includes mudstone Pr/Ph values of 0.4, and negative Ce and Eu anomalies. The total organic carbon (TOC) contents of these Cretaceous and Paleogene mudstones are typically >0.5%, with maximum values of 1.45% and 2.22%, respectively, signifying moderate to high organic matter abundance. Rock-Eval and kerogen analyses suggest that the organic matter was dominated by type II kerogen. The mudstones are generally thermally mature, suggesting favorable conditions for hydrocarbon (particularly oil) generation. Oil–source correlation confirms the oil generation of these two sets of source rocks, whose development are controlled mainly by reducing environments. These Cretaceous and Paleogene mudstones should therefore be a target for regional exploration in the southern Junggar Basin in addition to the known gas-producing Jurassic rocks. This oil-bearing potential is unique amongst the Early Cretaceous and Eocene–Oligocene central Asian petroliferous basins, as the eastern portion of this realm is considered to host predominantly Jurassic coal-based source rocks, giving rise to numerous large gas fields rather than oil resources.