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四川盆地东北部三叠系飞仙关组天然气具有较高的H2S和CO2含量,这是川东北发育热化学硫酸盐还原作用(TSR)的重要依据之一.然而天然气中的CO2显著少于TSR所应有的数量,与H2S相比,CO2显著亏损;同时天然气中CO2在碳同位素组成上并不具有与TSR有关的有机碳的特征,这是一个长期困惑人们的地质现象.根据川东北飞仙关组天然气中CO2的含量及同位素组成,定量计算了TSR来源的有机碳和海相碳酸盐来源的无机碳对天然气中CO2的贡献值,以及这两种来源碳对深埋藏过程从高温流体中沉淀的方解石中碳的贡献值,表明与TSR有关的有机碳源对天然气CO2的贡献值只有约2%,而有机碳源对从高温热流体中沉淀的自生方解石的贡献值高达43%,说明川东北飞仙关组与TSR有关的有机来源的CO2主要进入了自生方解石中,目前天然气中的CO2主要来源于海相碳酸盐的溶解.与之有关的解释是,在TSR发生的相对高温和高PCO2分压的环境中,方解石以沉淀为主,而在TSR发生过后的盆地快速抬升过程中,由于温度大幅度降低,流体对碳酸盐欠饱和而导致溶解作用发生和次生孔隙形成,这是倒退溶解模式的结果.因此,燕山期的构造抬升是川东北飞仙关组碳酸盐在埋藏过程中发生溶解的重要地质基础之一,抬升幅度较大或相应的燕山构造层高点值得我们在寻找优质碳酸盐储层时关注.
The natural gas in the Feixianguan Formation of the Triassic in the northeastern Sichuan Basin has high H2S and CO2 contents, which is one of the important bases for the development of thermochemical sulfate reduction (TSR) in northeastern Sichuan Province. However, the CO2 content in natural gas is significantly lower than that of TSR At the same time, there is a significant loss of CO2 compared with that of H2S. At the same time, CO2 in natural gas does not have the characteristics of TSR-related organic carbon in the carbon isotope composition, which is a long-term confused people’s geological phenomenon. The CO2 content and isotopic composition of natural gas in Xianguan Formation were calculated quantitatively and the contribution of TSR-derived inorganic carbon and marine carbonate-derived inorganic carbon to natural gas was calculated and the contribution of these two sources of carbon to deep burial from high temperature Contributions of carbon from calcite precipitated in fluids indicate that TSR-related organic carbon sources contributed only about 2% of the contribution of natural gas CO2, whereas organic carbon sources contributed as much as 43% to the intrinsic calcite precipitated from high-temperature hydrothermal fluids , Indicating that the organic source CO2 related to TSR in Feixianguan Formation of northeastern Sichuan mainly came into the authigenic calcite. At present, the main source of CO2 in natural gas is the dissolution of marine carbonate, which is related to the fact that TSR Occurrence of relatively high temperature and high partial pressure of PCO2 environment, calcite precipitation mainly, and in the basin after TSR rapid uplift process, due to a substantial decrease in temperature, the fluid undercomplete saturation of the carbonate caused dissolution and Secondary pore formation is the result of retrograde dissolution, so the tectonic uplift of Yanshanian period is one of the important geological bases for the dissolution of Feixianguan formation in northeastern Sichuan during the burial process, with a large increase or corresponding The Yanshan tectonic height is worth our attention when looking for high-quality carbonate reservoirs.