塔里木盆地塔中天然气的地球化学特征及其成因

塔里木盆地塔中天然气的成因十分复杂,长期以来存在不同的认识。对塔中天然气组分和碳同位素的地球化学特征研究表明:塔中的天然气以烃类气体为主,塔中东、西部天然气的组分和碳同位素存在显著差异,塔中东部和I号构造带的天然气具有较高的干燥系数、较重的碳同位素比值和较高的氮含量。研究认为这些差异除了一定程度上受有机质类型和次生变化的影响外,主要取决于有机质热演化程度的差异。塔中东部下古生界有机质成熟度的定型时间较早,紧邻满加尔凹陷的寒武系烃源岩,因此主要聚集了高-过成熟的寒武系烃源岩生成的天然气;塔中西部主要发育中—上奥陶统有机质,且有机质成熟度的定型时间较晚,因此主要聚集了中等成熟的中—上奥陶统有机质生成的天然气。综合分析表明天然气主要生成于不同热演化阶段的干酪根,原油裂解生气在本区不显著。 

GEOCHEMICAL CHARACTERISTICS AND GENESIS OF THE NATURAL GAS IN THE TAZHONG UPLIFT, THE TARIM BASIN

The genesis of the natural gas in the Tazhong Uplift is very complicated and there have been existing a variety of opinions for a along period.Based on the systematic research results of the composition and carbon isotope characteristics of the natural gas in the Tazhong Uplift,the natural gas in the Tazhong Uplift is mainly composed of hydrocarbon gases,but the composition and the carbon isotope in the west and east parts of the uplift differ apparently form each other.The natural gases in the eastern Tazhong Uplift and the No.1 structural belt have higher values of dry coefficients,higher nitrogen contents and heavier carbon isotopic ratios.It is regarded that these differences are mainly determined by the different thermal evolution degrees of the organic matter,in addition to the organic matter type and the secondary alteration.The maturity of the Lower Paleozoic organic matter in the east part was fina-lized earlier,and the organic matter distributed immediately near the Cambrian source rock layer of the Manjiaer Sag.Therefore,the gas accumulated in the eastern Tazhong Uplift mainly came from the high to over matured Cambrian source rocks.As a contrast,the maturity of the Middle-Upper Ordovician organic matter in the west part was finalized later,and the gas accumulated in the western Tazhong Uplift mainly came from the middle matured Middle-Upper Ordovician source rocks.These results suggest that the natural gas in the Tazhong Uplift is mainly generated from the kerogens of various thermal evolution degrees,and the amount of the oil-pyrolysis gas is not significant.