Geochemical characteristics and genesis of Ordovician condensate gas in Shunbei area, Tarim Basin

To provide a theoretical basis for deep oil and gas exploration in the Shunbei area of the Tarim Basin, gas component and carbon and hydrogen isotope analyses were used to systematically explore the chemical composition and isotopic characteristics of Ordovician condensate gas in the Shunbei area. These data were compared with those of Ordovician condensate gas from the eastern Lungu, Yuke, and eastern and western Tazhong areas to investigate the genesis and formation process of condensate gas in the Shunbei area. The Ordovician condensate gas in the Shunbei area was mainly composed of hydrocarbon gases. The dryness coefficients of condensate gas in the southern section of No.5 fault zone (F5), F4, and F8 were mostly below 0.95, showing wet gas characteristics, while the natural gas in F12, F14, and the Shunnan area was predominantly dry gases. The non-hydrocarbon gases of the condensate gas in the Shunbei area mainly include minor amounts of carbon dioxide, nitrogen, and trace amounts of hydrogen sulfide. The alkane carbon and hydrogen isotope sequences generally exhibited a positive sequence distribution, with isotope reversal occurring in some samples. From the southern section of F5 to the Shunnan area, the maturity of natural gas gradually increased. The Ordovician condensate gas in the Shunbei area was primarily thermogenic oil-type gas. The natural gas in the southern section of F5, F4, and F8 was a mixed source of kerogen-cracking gas and crude oil-cracking gas, with crude oil-cracking gas dominated by wet gas formed in the early stage of cracking. In contrast, the natural gas in F12, F14, and the Shunnan area was mainly crude oil-cracking gas with high maturity, showing dry gas characteristics. Through geochemical analysis of the natural gas, the regional differences in condensate gas genesis in the Shunbei area are revealed, providing a basis for the evaluation and exploration deployment of deep oil and gas resources in the Tarim Basin.