Quantitative analysis for the contribution rate of the natural gas from accumulated-organic matter-type coal-measure source rocks: taking the triassic Xujiahe Formation in Yuanba area, northeastern Sichuan Basin as an instance

As the exploration of unconventional oil and gas and terrestrial natural gas in northeastern Sichuan developing fast, it becomes more important to clarify the contributions of different types of source rocks. This paper takes the Upper Triassic Xujiahe Formation coal-measure source rocks in Yuanba Area, Northeastern Sichuan Basin as an instance. In general, for the Xujiahe Formation source rocks in Yuanba area, dispersed-organic matter-type source rocks have about an average content of 90%, while accumulated-organic matter-type 10%. Kerogen microscopy analysis demonstrates that the organic matter type of dispersed-organic matter-type source rocks is dominated by type II2 and type III, with about 55% and 45%, respectively, while accumulated-organic matter-type dominated by type III. Isotopic and composition characteristics of natural gas demonstrate that, for the Yuanba Xujiahe Formation reservoir, the content of coal-derived gas is about 70%, while oil-associated gas about 30%. Through quantitative calculation, accumulated-organic matter-type source rocks contribute about 60% of the Yuanba Xujiahe Formation reservoir.