Oil & Gas Geology ›› 2022, Vol. 43 ›› Issue (1): 175-185.doi: 10.11743/ogg20220114

• Petroleum Geology • Previous Articles     Next Articles

Geochemical characteristics and genesis of mid-to-shallow natural gas on the periphery of Shawan Sag, Junggar Basin

Erting Li1,2(), Jun Jin1,2, Jian Wang1,2, Wanyun Ma1,2, Shijia Chen3, Cuimin Liu1,2, Haijing Wang1,2   

  1. 1.Key Laboratory of Conglomerate Reservoir Exploration and Development, CNPC, Karamay, Xinjiang 834000, China
    2.Research Institute of Experiment and Detection of Xinjiang Oilfield Company, PetroChina, Karamay, Xinjiang 834000, China
    3.Resource and Environmental Engineering College, Southwest Petroleum University, Chengdu, Sichuan 610500, China
  • Received:2021-08-17 Revised:2021-12-02 Online:2022-02-01 Published:2022-01-28

Abstract:

Natural gas on the periphery of Shawan Sag, is generally derived from mixed sources. A lack of holistic study on this area in the early stage has restricted the understanding of natural gas accumulation therein. In this regard, a systematic analysis of geochemical characteristics of the natural gas is carried out by means of source-rock thermal simulation, by which the genetic types of mid-to-shallow gas on the periphery of the Shawan Sag are identified. The research results show that the natural gas in the study area is dominated by methane, with a drying coefficient ranging from 0.73 to 1.00, and a δ13C1 value from -56.0 ‰ to -31.5 ‰, indicating coexistence of mature and high-to-over-mature natural gas therein. A δ13C2 value varying between -30.4 ‰ and -22.8 ‰ reflects a mixed distribution of coaliferous, petroliferous and mixed gas. In addition, the analysis of carbon isotopic characteristics of source-rock pyrolysis gas, shows that the natural gas in the study area is of 4 types in genesis: Type Ⅰ natural gas, featuring a very high δ13C2 value greater than -25.5 ‰ and a methylcyclohexane content greater than 50 % in lighter hydrocarbon C7, is derived from source rocks of the Jiamuhe Formation, and mainly distributed in the Cretaceous in the middle of Hongche fault belt; Type Ⅱ natural gas of sparse distribution is mainly secondary biogas from crude oil degradation, with an abnormally negative δ13C1 value and an extremely high drying coefficient; Type Ⅲ natural gas is originated from the Xiawuerhe Formation source rocks and mainly distributed in the Jurassic Formation in Xiaoguai area and southern Hongche fault belt, with a δ13C2 value varying from -27.9 ‰ to -26.4 ‰, a characteristic of mixed source rocks; and Type Ⅳ natural gas is mainly derived from source rocks of both Xiawuerhe Formation and Fengcheng Formation, and mainly distributed in the southern Hongche fault belt and northern Jinlong area. Among others, the natural gas mainly derived from the Xiawuerhe Formation features a δ13C2 value greater than -29 ‰, and that from the Fengcheng Formation features a δ13C2 value less than -29 ‰.

Key words: carbon isotope, thermal simulation, petroliferous gas, coaliferous gas, biogas from crude oil degradation, natural gas, Shawan Sag, Junggar Basin

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