Quantitative characterization of gas invasion intensity in oil and gas reservoirs using methane carbon isotopes: Example from Tahedong Area in the Tarim Basin

The late-stage gas charging diversifies the material composition and chemical characteristics of the early reservoir, while the differences in the range and intensity of gas invasion lead to the formation of complex distribution pattern of oil and gas, which restricts a detailed understanding of the mechanisms of oil and gas accumulation. The research focuses on the Tahedong area of the Tarim Basin, utilizing geological background and integrating techniques such as crude oil geochemical analysis, fluid inclusion observation, scanning electron microscopy, and methane carbon isotope analysis to quantitatively characterize the intensity of gas invasion. The results indicate that: (1) The loss of n-alkanes in the research area is significant, with a loss rate ranging from 60.11% to 80.58%, while aromatics are relatively enriched, and the reservoir rocks develop gas inclusions with the presence of gas pores in the asphalt. (2) The gas charging ratio in condensate oil reservoirs and natural gas reservoirs ranges from 48% to 92%, with a high degree of gas invasion; in light oil reservoirs, the ratio ranges from 25% to 34%, with a moderate degree of gas invasion; in normal oil reservoirs, gas invasion is not significant. (3) The gas generation range of the source rocks in the Himalayan period matches the range of gas charging ratio greater than 35%, and the drying coefficient of natural gas decreases gradually from southeast to northwest, and the relative density increases, reflecting the decrease of natural gas charging ratio.It is therefore believed that the study area has developed varying intensities of gas invasion, with the southeastern region experiencing the strongest gas invasion, resulting in the formation of condensate oil and natural gas reservoirs. As the distance increases towards the northwest, the gas invasion weakens and overlaps with earlier oil reservoirs, transitioning in phase to light oil and medium oil. This understanding is of significant guiding importance for the exploration of late high-maturity oil and gas in the Tahedong area.