Abstract:

The origin of natural gas in No.2 structural belt of Nanpu depression is still unclear. Geological and geochemical approaches are utilized to reveal the characteristics and genetic mechanisms of deep and shallow natural gas. The results show that the components of natural gas in No.2 structural belt are dominated by hydrocarbon gas with non-hydrocarbon mainly composed of CO₂, N₂ and H₂S, which are mainly found in buried hill reservoirs. The dry coefficient of natural gas varies from 0.68 to 0.94, indicating mainly a wet gas. Carbon isotopes of natural gas are relatively heavy, showing a tendency of normal order, but some samples display a partially reversed isotopic order with δ¹³C₃>δ¹³C₄. There are three types of natural gas in No.2 structural belt with the analysis of organic matter type and maturity: the mixture gas generated during early phase of mature stage (gas within Ed₁  and Ng), the mixture gas generated during late phase of mature stage (gas within LPN1 fault block) and coal-based pyrolysis gas generated during high mature stage (gas within NP280 fault block). The comprehensive analysis indicates that the natural gas within Ed₁ and Ng formations of Tertiary is mainly generated from Ed₃ and E_s1 source rocks, while natural gas within buried hill is mainly derived from humic type organic matter in E_s3 Member. Faults and unconformities are the main paths for hydrocarbon migration. This research should be helpful to further exploration of deep and shallow natural gas in Nanpu depression.

Key words: Nanpu depression, No.2 structural belt, natural gas origin, carbon isotope