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

• Published in: International journal of coal geology 2021-09, Vol.245, p.103818, Article 103818
• Main Authors: Qiao, Jinqi, Grohmann, Sebastian, Baniasad, Alireza, Zhang, Chen, Jiang, Zhenxue, Littke, Ralf
• Format: Article
• Language: English
• Subjects: Accumulation rate; Macrophytes; Microbial gas source rocks; Organic geochemistry; Organic petrology; Qaidam Basin
• ISSN: 0166-5162; 1872-7840
• DOI: 10.1016/j.coal.2021.103818

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. The majority of the interval is comprised of gas prone, mixed type II-III kerogen derived mainly from aquatic plants. Organic facies A is interpreted to be derived from abundant macrophytes (e.g. non-marine algae, submerged angiosperms) around the lake margin. By contrast, the organic matter in organic facies B mainly stems from saltwater algae, with some additional bacterial contribution. Terrestrial higher plants are subordinate in both organic facies. The organic carbon accumulation rate was high compared to organic matter-rich Quaternary marine sediments. Cold and dry climate conditions and high burial rates favored methanogenesis via carbonate reduction in the sediments. •First geochemical and petrographic study of Pleistocene source rocks, Qaidam Basin•Two lacustrine organo-facies representing marginal and more basin-center deposits•Cold climate, high sedimentation and Corg accumulation rate favored methanogenesis•High microbial gas potential is assessed for about 90% of the whole succession.