Abstract
There are plentiful natural gas resources in Jurassic in the Sichuan basin, where a lot of gas fields (reservoirs) have been discovered, and the natural gas is mainly alkane gas. Comprehensive study on carbon isotopic characteristics indicated that Jurassic alkane gas in Sichuan Basin can be divided into two types, coal-formed gas and oil-type gas. As the majority of the natural gas, coal-formed gas distributed in Central and Northwest gas districts and was mainly from the underlying coal-measure hydrocarbon source rocks in Xujiahe Formation. The carbon isotopic series were mainly normal, while a minority were reversed. The reversal ones in Northwest gas district were caused by mixing of gases from the same source rocks with different maturity, while those in Central gas district were caused by mixing of oil-type and coal-formed gases. Oil-type gas was only discovered in Bajiaochang gas field in Central oil and gas district, and derived from self-generation and self-accumulation in Lower Jurassic, with normal carbon isotopic series and without any reversal. Comprehensive analysis suggested that the carbon isotopic characteristics of Jurassic alkane gas were different according to different regions and layers. The δ13C1 and δ13C2 values of Jurassic alkane gas in central area were significantly lower than those in northwestern area for the influence by oil-type gas generated from Lower Jurassic Ziliujing Formation.
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