Geochemical Characteristics and Gas Source Analysis of Natural Gas from the Lower Jurassic in Northeastern Sichuan Basin

The northeastern Sichuan Basin is rich in natural gas resources, with exploration breakthroughs achieved by multiple oil and gas companies in the Lower Jurassic strata, including the Ziliujing and Lianggaoshan formations. However, the genetic mechanisms and enrichment patterns of these gas reservoirs remain unclear. Here we employ experimental techniques including natural gas compositional analysis, carbon and hydrogen isotopic analyses of natural gas, and maceral analysis of kerogen, combined with geological data from the Lower Jurassic and Upper Triassic strata in the northeastern Sichuan Basin to investigate the gas reservoir characteristics and accumulation patterns in this region. The results indicate that the Lower Jurassic strata (Lianggaoshan and Ziliujing formations) in northeastern Sichuan Basin primarily produce wet gas, with a dryness coefficient ranging from 0.75 to 0.99 and an average value of 0.90. The natural gas exhibits a normal carbon isotopic distribution, the natural gas from Lianggaoshan Formation showing δ¹³C₂ values consistently below −28.0‰, indicative of typical oil-type gas characteristics. The natural gas from the Ziliujing Formation has δ¹³C₂ values ranging from −21.4‰ to −33.4‰, while δD1 values range from −144.9‰ to −218.7‰ (average: −179.3‰), suggesting the natural gas origin from a brackish-freshwater depositional environment. Comparing the isotope data distribution and regression analysis of gas samples from various areas, it is concluded that the natural gas from the Ziliujing Formation in northeastern Sichuan (e.g., Yingshan-Dazhou area) is a mixed-gas consisting of oil-type gas from its own source rocks and coal-type gas from the underlying Xujiahe Formation. In contrast, the gas reservoirs in the Ziliujing Formation of the Yuanba area exhibit an autochthonous self-sourcing feature. The natural gas in the Lianggaoshan Formation primarily originates from its own immature source rocks. Based on geochemical characteristics of natural gas and geological data, it is concluded that the Lower Jurassic gas reservoirs in the northeastern Sichuan Basin predominantly exhibit autochthonous self-reservoired hydrocarbon accumulation characteristics, with multiple-gas source characteristics as auxiliary. The migration pathway system of the Lower Jurassic gas reservoirs in the research area can be characterized by an accumulation model.