High microbial gas potential of Pleistocene lacustrine deposits in the central Qaidam Basin, China: An organic geochemical and petrographic assessment

Thick Pleistocene fine-grained rocks in the central Qaidam Basin, China, are regarded as the principal source rocks for microbial gas there. Here, for the first time, a detailed investigation on the organic geochemistry and petrology of this sedimentary sequence is presented. Two different, immature, lacustrine source rocks facies are present in the study area. Organic facies A samples with high TOC (4.1–25.3 wt%) and TS (1.5–3.7 wt%) contents mainly developed under a suboxic to anoxic freshwater column at the lake margins. This shallow water fluvio-lacustrine facies, accounts for only a small proportion of the whole sedimentary sequence, but has the highest petroleum generation potential. In contrast, organic facies B samples having lower TOC (0.5–1.1 wt%) and TS (0.14–1.0 wt%) contents were deposited in a more oxic, brackish-lacustrine water body. However, exceptionally, anoxic conditions were present in sediments that were sealed by algae mats.