川西南地区侏罗系沙溪庙组致密砂岩成藏特征及主控因素

Accumulation characteristics and main controlling factors of tight sandstone of Jurassic Shaximiao Formation in southwestern Sichuan Basin

综合利用地质构造、有机地球化学及储层物性、岩性等资料,从烃源岩、储层物性、有效盖层和输导体系等4个方面分析了川西南地区侏罗系沙溪庙组致密砂岩气的成藏条件及主控因素,并对气藏有利区进行了预测。研究结果表明:①川西南地区侏罗系沙溪庙组致密气具备良好的成藏条件,三叠系须家河组五段煤系烃源岩有机质以Ⅲ型干酪根为主,平均TOC为3.31%,Ro为0.70%~1.75%,为沙溪庙组煤型气的规模聚集奠定了良好的物质基础;上覆遂宁组泥岩厚度大,泥地比大于0.8,突破压力高,是良好的封盖层;区内广泛发育的气源断裂为油气运移提供了良好的输导通道。②研究区致密砂岩气的成藏规模受控于源储距离和储层物性,气藏地质储量与储层物性呈正相关,与源储距离呈抛物线关系;致密气的富集层位受控于源-储-盖配置类型,当源储分离、有区域盖层遮挡时,致密气富集于区域盖层下伏储集层,当源储分离、断穿区域盖层时,优势储层多层系富集;烃源岩生烃潜力与输导通道控制了天然气充注方式,区内整体充注程度高,烃源岩侧向运移排烃的成藏条件要好于垂向运移排烃。③研究区地质储量与成藏指数呈线性正相关,相关系数大于0.8;成藏指数大于0.4、泥地比大于0.5、断层封闭性良好的区域为气藏有利分布区,沙溪庙组一段致密气主要分布在名山—邛崃—新津一带、天府含气区;沙溪庙组二段致密气主要围绕浦江、眉山、成都等地分布。

The data of geological structures,organic geochemistry,reservoir properties and lithologies were used to analyze the accumulation conditions and main controlling factors of tight sandstone gas of Jurassic Shaximiao Formation in southwestern Sichuan Basin from four aspects such as source rocks,reservoir properties,effective cap rocks and migration pathways,and the favorable areas for tight gas reservoirs were predicted.The results show that:(1)Jurassic Shaximiao Formation in southwestern Sichuan Basin has good accumulation conditions for tight gas reservoirs.The organic matters in the coal measure source rocks of the fifth member of Triassic Xuji‐ahe Formation are mainly of typeⅢkerogen,with an average TOC value of 3.31%and Ro of 0.70%-1.75%,which provides a good material basis for the accumulation of coal-bed methane in Shaximiao Formation.The overlying Suining Formation developed thick mudstone,with a mudstone-formation ratio greater than 0.8 and high breakdown pressure,providing a good cap rock.The widely developed gas source faults in the area provide good migration pathways for oil and gas.(2)The size of the tight sandstone gas reservoirs in the study area is controlled by the distance between the source and reservoir and reservoir properties.The geological reserves of the gas reservoir are positively correlated with reservoir properties and have a parabolic relationship with the distance between the source and reservoir.The enrichment of tight gas is controlled by the source-reservoir-cap assemblage.When the source and reservoir are separated and the regional cap is covered,the tight gas is enriched under the regional cap below the reservoir.When the source and reservoir are separated and the regional cap is cut,the dominant reservoirs are multilayered and enriched.The hydrocarbon generation potential of source rocks and migration pathways control the natural gas charging mode.The overall charging degree in the area is high,and the accumulation conditions for lateral migration and expulsion of source rocks are better than those for vertical migration and expulsion.(3)Geological reserves are linearly and positively correlated with accumulation index,with a correlation coefficient greater than 0.8.Areas with accumulation index greater than 0.4,mudstone-forma‐tion ratio greater than 0.5 and good fault sealing,are favorable for gas reservoir distribution.Tight gas reservoirs of the first member of Shaximiao Formation are mainly distributed in Minshan-Qionglai-Xinjin areas,Tianfu gas-bearing areas,while the tight gas reservoirs of the second member of Shaximiao Formation are mainly distributed around Pujiang,Meishan,and Chengdu.