Oligocene-Miocene source rocks of the Zhongcang Basin: Implications for hydrocarbon potential differentiation between lake basins in Central Tibet

The Oligocene-Miocene Dingqinghu Formation is widely distributed in lake basins of the Bangong-Nujiang suture zone (BNSZ), central Tibet and has been reported as an important source rock layer. However, limited works have been conducted in the Zhongcang Basin. To gain knowledge on hydrocarbon potential of this area and further investigate the organic matter (OM) accumulation mechanisms of source rocks in the BNSZ basins, samples collected along two measured sections from the Zhongcang Basin were subjected to sedimentological, organic geochemical and petrographic analyses. Sedimentological analyses indicate that all samples were deposited in prodeltic to deep lake environments. Total organic carbon (TOC) values and pyrolysis data suggest a poor to fair hydrocarbon source potential of the middle Dingqinghu Formation, with the exception of OM-rich samples locate in a marker bed of the upper part. Vitrinite reflectance (Ro) values of the upper part and middle-lower part samples range from 0.41 to 0.50 and 0.52 to 0.61, indicating immature and marginal mature stages, respectively. These maturity levels were also supported by specific maturity indices of sample extracts, such as ratios of C31 22S/(22S + 22R) homohopanes and C29 ββ/(αα + ββ) steranes. Bulk geochemistry, maceral composition of kerogen, and extract biomarkers demonstrate that OM-rich samples were deposited in fresh to brackish waters with type II-III kerogen mainly derived from algae and higher plants; while poor to fair samples have less higher plant contribution, but relatively high bacterial content that probably originated from more saline environments. The paleoclimate-induced environmental reorganization is likely responsible for the differential OM accumulation in the Zhongcang Basin; relatively wet climates would facilitate primary productivity of the lake as well as development of swamps on the delta, resulting in deposition of mixed terrestrial and aquatic OM with higher TOC, whilst dry conditions would restrict the development of river systems and promote the establishment of saline environments, causing decreased total OM input but increased proportion of halophilic bacterial OM. In addition, we summarized published data of the Dingqinghu Formation in the BNSZ basins and preliminarily reconstructed paleogeography of central Tibet. Two types of drainage patterns were confirmed in this area. A long-lived hierarchical drainage system is developed in the Lunpola area, while single lake systems were prevalent in other basins, including the Zhongcang Basin. The hierarchical drainage pattern could minimize clastic dilution effects during tectonic active and/or relatively wet periods, and prevent the establishment of hypersaline environments during the dry period. The drainage center in this kind of hierarchical system has lower sedimentation accumulation rate and could receive much more aquatic OM at any climatic conditions. We argue that tectonic-induced drainage organization primarily controlled the hydrocarbon potential differentiation between lake basins of central Tibet.