储层流体特征在天然气运移中的示踪意义探讨——以川西坳陷中段龙门山前中侏罗统上、下沙溪庙组气藏为例

以川西坳陷中段龙门山前中侏罗统上沙溪庙组和下沙溪庙组气藏为研究实例,通过分析地层水化学参数、方解石胶结物碳氧同位素组成、Fe–Mn元素含量差异明确了储层流体特征,并结合孔隙类型差异对天然气的运移方式和路径进行探讨。结果表明,下沙溪庙组地层水表现出高矿化度、高Na+/K+、高Ca2+、高HCO3–的特征,方解石具有相对较低碳氧同位素值,相对较高Fe、Mn元素含量,储层孔隙类型以方解石溶蚀孔为主;上沙溪庙组地层水表现出低矿化度、低Na+/K+、低Ca2+、低HCO3–的特征,方解石具有相对较高碳氧同位素值,相对较低Fe、Mn元素含量,储层孔隙类型以原生孔隙为主。储层流体特征差异表明,断层是龙门山前上沙溪庙组和下沙溪庙组天然气的主要运移通道,其中,下沙溪庙组气藏受下部热流影响较大,天然气以水溶相运移为主;上沙溪庙组气藏受下部热流影响较小,天然气以游离相和扩散运移为主。

Tracer significance of reservoir fluid characteristics in natural gas migration--a case study of upper and lower Shaximiao gas reservoirs in Longmen mountain of western Sichuan depression

By taking the upper and lower Shaximiao gas reservoirs of middle Jurassic in front of Longmen mountain in the middle part of western Sichuan depression as an example,the characteristics of reservoir fluid are clarified by analyzing the differences of formation water chemical parameters,carbon and oxygen isotope composition of calcite cement and Fe-Mn content,and the migration mode and path of natural gas are studied in combination with the differences of pore types.The results reveal that lower Shaximiao formation is characterized with formation water abundant in Ca2+,HCO3–,high Na+/K+ratio,high salinity,calcite cement relatively low carbon and oxygen isotopes and high content of Fe-Mn elements and well-developed secondary pores,while upper Shaximiao is relatively poor in Ca2+,HCO3–,low Na+/K+ratio,low salinity,relatively high carbon and oxygen isotopes and low content of Fe-Mn elements.These differences of reservoir fluid characteristics indicate that fault is the main migration pathway of natural gas in Longmen mountain,among which the gas reservoir from lower Shaximiao formation is greatly affected by the lower heat flow,and the water-soluble phase migration is the dominant,while the gas reservoir from upper Shaximiao formation is less affected by the lower heat flow,and the natural gas is mainly of free phase and diffusion migration.