Earth Science Frontiers ›› 2023, Vol. 30 ›› Issue (6): 232-246.DOI: 10.13745/j.esf.sf.2023.2.14

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The hydrogen isotopic composition of methane from Lower Paleozoic natural gases, cratonic platform areas, Tarim Basin and its geological significance

CHEN Zeya1,2(), CHEN Jianfa2,*(), LI Maowen3, FU Rao2, SHI Xiaofei2,4, XU Xuemin5, WU Jianjun6   

  1. 1. PetroChina Research Institute of Petroleum Exploration and Development, Beijing 100083, China
    2. State Key Laboratory of Petroleum Resources and Prospecting, China University of Petroleum (Beijing), Beijing 102249, China
    3. Petroleum Exploration and Production Research Institute, SINOPEC, Beijing 102206, China
    4. CNOOC Central Laboratory (Shanghai), CNOOC EnerTech-Drilling and Production Co., Shanghai 200941, China
    5. National Research Center for Geoanalysis, Beijing 100037, China
    6. PetroChina Tarim Oilfield Company, Korla 841000, China
  • Received:2023-01-07 Revised:2023-02-10 Online:2023-11-25 Published:2023-11-25

Abstract:

The Lower Paleozoic marine carbonate strata of the cratonic platform area, Tarim Basin are a deep oil and gas exploration hotspot in China. However, the origin of deep and ultra-deep natural gases and their accumulation process are still difficult problems in natural gas geoscience. In this study, the chemical and isotopic compositions of natural gases in the Lower Paleozoic were determined. Combined with previous results and regional geology we believe the hydrogen isotopic composition of methane (δ2H1) is a more reliable maturity indicator for source rocks in the study area, and a new way of discussing the origin, generation mechanism and secondary alterations of natural gas reservoirs is established using δ2H1 value, δ13C values of alkane gases and gas dryness coefficient. The Lower Paleozoic accumulated gaseous hydrocarbons generated from source rocks of different maturities, genetic types and geochemical characteristics. Moreover, most of the natural gas reservoirs were formed under relatively open environments, with low contribution of oil cracking gases. Nevertheless, there are also some natural gas reservoirs formed under relatively closed environments that received large amounts of oil-cracking gases due to high heterogeneity in carbonate reservoirs and transport systems, where natural gases of complex, diverse origins have undergone secondary transformations, mainly mixing and oxidative alteration, during the accumulation process.

Key words: Tarim Basin, Lower Paleozoic, hydrogen isotope, genetic identification, secondary alteration

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