Characteristics of Mesozoic source-reservoir-cap rock association and hydrocarbon prospect evaluation in the Coqen basin, Tibet

Located in central Tibetan Plateau, the Coqen basin is the second largest marine hydrocarbon-bearing basin in Tibet Plateau, covering an area of 140000 km2. Data obtained in recent years were combined with previous studies to investigate the characteristics of source rock, reservoir, and cap rocks. The results show that total organic carbon (TOC) content in limestone of Langshan Formation is very low. The organic matters were composed of typeⅠ-Ⅱ1 kerogen and have reached the mature stage. The argillaceous rocks of Chuanba Mmember of Duoni Formation contain higher TOC and are composed mostly of type Ⅱ2-Ⅲ kerogen which has reached the high mature to over mature stage. The TOC content of argillaceous rocks in Jienu Group is lower. The organic matter is mainly type III kerogen at the high mature to over mature stage. The newly discovered dolomites located in Xueshangle, Geji and Nongwa show excellent reservoir capability characterized by low porosity and low permeability, with the development of some medium porosity and permeability reservoirs. Regionally, mudstone of Duoni Formation and limestone of Langshan Formation have good sealing properties as the cap rocks. Therefore, there exist two source-reservoir-cap rock assemblages characterized by self-source, self-reservoir and self-sealing. However, Mesozoic exploration targets are extensively exposed and poorly preserved. It is suggested that the Upper Paleozoic target strata should be considered as the next exploration objects.