四川盆地安岳气田龙王庙组气藏天然气有水溶气贡献的迹象

通过有机地球化学手段对四川盆地安岳大气田龙王庙组天然气和储层沥青进行了系统研究，发现天然气甲烷碳同位素较储层沥青明显偏重。这用目前大家普遍认为的油裂解气的观点难以解释，因根据歧化反应碳同位素分馏原理，油裂解生成的甲烷和沥青，甲烷碳同位素会明显轻于沥青。另外，甲烷碳同位素值与H₂S含量也没有明显的相关性，用TSR作用也很难解释气藏中甲烷碳同位素偏重的现象。从气田水中释放出的水溶气甲烷碳同位素较气藏游离天然气的甲烷碳同位素偏重或者重，推断可能是由于地层水水溶相天然气的脱溶，造成龙王庙组气藏中的天然气甲烷碳同位素较储层沥青碳同位素偏重。同时，经激光拉曼测试分析发现大部分烃类包裹体具有液相甲烷特征峰，进一步证实水溶相天然气的存在。通过对气源、地层水、温压环境、构造演化、盖层等地质条件进行分析，提出龙王庙组气藏具有良好的水溶气形成、保存以及晚期随构造抬升水溶气脱气成藏的条件。对比龙王庙组埋深最大时与现今气藏条件下甲烷在地层水中的溶解度，估算出龙王庙组1m³的地层水可释放出6m³的天然气。

The evidence of water soluble gas contribution to gas reservoir ofLongwangmiao Formation in Anyue Gasfield,Sichuan Basin

Natural gas and reservoir solid bitumen of Lower Cambrian Longwangmiao Formation in Anyue Gasfield have been systematically analyzed using organic geochemical methods.It is found that the methane carbon isotope value is less negative than that of reservoir bitumen.It is difficult to explain this kind of phenomenon using the generally accepted idea that the gas is from oil cracking.According to the principle of the disproportionation reaction of carbon isotopic fractionation,if both methane and bitumen are from crude oil cracking,the carbon isotope of oil cracked methane should be significantly more negative than that of bitumen.In addition,the δ¹³C₁ and the content of H₂S have no obvious correlation,so it is impossible to think that the less negative value of δ¹³C₁ is caused by thermochemical sulfate reduction.The carbon isotope of methane released from the gas field water is heavier than that of free gas reservoir.Water soluble gas adds to gas reservoir resulting in the carbon isotope value of methane carbon isotope bigger than that of reservoir bitumen from Longwangmiao Formation.At the same time,the majority of hydrocarbon inclusions with liquid phase methane characteristic peak are found by laser Raman measurement,which further confirms the existence of water soluble gas.Through the analysis of the geological conditions including the gas source,formation water,temperature and pressure environment,tectonic evolution and cover,the Longwangmiao Formation gas reservoir has good conditions of water soluble gas formation,preservation as well as degassing accumulation with the late tectonic uplift.Comparison of formation water solubility under the condition of Longwangmiao Formation deepest burial and present condition,estimating that 1m³ formation water of Longwangmiao Formation can release6m³ natural gas.