Impact of microorganism degradation on hydrocarbon generation of source rocks: A case study of the Bozhong Sag, Bohai Bay Basin

The discovery of the Bozhong 19-6 gas field, the largest integrated condensate gas field in the eastern China in 2018, opened up a new field for the natural gas exploration deep strata in the Bohai Bay Basin, demonstrating there is a great potential for natural gas exploration in oil-type basins. The ethane isotope of the Bozhong 19-6 condensate gas is heavy, showing the characteristics of partial humic gas. In this paper, aimed at the source rocks of the Bozhong 19-6 gas field in the Bohai Bay Basin, the characteristics of the source rocks in the Bozhong 19-6 structural belt were clarified and the reason are explained from impact of microorganism degradation on hydrocarbon generation of source rocks why the condensate oil and gas had heavy carbon isotope and why it showed partial humic characteristics was explored based on the research of parent materials. The following conclusions were obtained: The paleontology of the Bozhong 19-6 structural belt and its surrounding sub-sags is dominated by higher plants, such as angiosperm and gymnosperm. During the formation of source rocks, under the intensive transformation of microorganism, the original sedimentary organic matter such as higher plants was degraded and transformed by defunctionalization. Especially, the transformation of anaerobic microorganisms on source rocks causes the degradation and defunctionalization of a large number of humic products such as higher plants and the increase of hydrogen content. The degradation and transformation of microorganism don’t transform the terrestrial humic organic matter into newly formed “sapropel” hydrocarbons, the source rocks are mixed partial humic source rocks. As a result, hydrogen content incrased and the quality of source rocks was improved, forming the partial humic source rocks dominated by humic amorphous bodies. The partial humic source rocks are the main source rocks in the Bozhong 19-6 gas field, and it is also the internal reason why the isotope of natural gas is heavy.