Abstract:

The Upper Permian Longtan Formation provides most source rocks for many large- and medium-sized gas fields in the Sichuan Basin. Due to their advanced thermal evolution， the source rocks are beyond ordinary thermal simulation experiments or chemical kinetics methods that are commonly resorted to while defining the hydrocarbon generation and expulsion characteristics of less thermally evolved source rocks. This hinders the exploration and development of oil and gas generated from the formation. To deal with the problem， this study proposes a model suitable for analyzing the characteristics of hydrocarbon generation and expulsion， evaluating hydrocarbon potential and predicting most promising hydrocarbon accumulation for these source rocks by using the hydrocarbon potential method based on pyrolysis， TOC content analysis and maturity data of source rocks from major petroliferous basins in China. The model shows that the formation has great hydrocarbon generation potential. TypesⅡand Ⅲ organic matter in the formation initiate hydrocarbon generation process when the vitrinite reflectance （R_o） reaches 0.5 %， and start to expel hydrocarbon when R_o reaches 0.8 % and 0.9 %， respectively. The original hydrocarbon generation potential indexes for the two types are 570 mg/g and 200 mg/g respectively. The formation has up to the present the capacity of generating 746 billion tons of hydrocarbon， of which 384 billion tons of hydrocarbon are expelled and 362 billion tons of hydrocarbon are left behind. Shale gas resource volume is 24.6 TCM in the formation， of which 3.7 TCM of gas is recoverable. This predication indicates that exploration shall be focused on finding conventional gas in the north part of the basin， on both conventional and unconventional gases in central and northeast parts of the basin， and on shale gas in the east and south parts of the basin.

Key words: hydrocarbon potential method, hydrocarbon generation/expulsion amount, shale gas resource volume, play fairway prediction, source rock, shale gas, Longtan Formation, Upper Permian, Sichuan Basin