川南地区二叠系龙潭组页岩储层特征及勘探潜力

Characteristics and exploration potential of shale reservoirs of Permian Longtan Formation in southern Sichuan Basin

二叠系龙潭组是目前四川盆地页岩气勘探开发的新热点层位。通过偏光显微镜鉴定、场发射扫描电镜分析、X射线衍射测试、有机碳含量测定及高压压汞分析、现场解析气实验等手段和方法,对川南地区二叠系龙潭组页岩储层进行了综合研究。研究结果表明:(1)川南地区二叠系龙潭组为一套陆相曲流河沉积,河漫沉积的页岩发育,其矿物成分主要为石英和黏土矿物,含少量碳酸盐矿物。(2)研究区页岩储层主要发育粒间(缘)孔与微裂缝,局部发育粒内孔与溶蚀孔,偶见有机质孔隙,其中黏土矿物层间微裂缝最为发育。孔隙结构表现为以微孔为主,介孔(孔径为2~50 nm)次之,孔径普遍较小,分选较差,孔隙和喉道半径差异较大,非均质性强。(3)研究区页岩储层中煤岩含气量最高,炭质泥岩与泥岩次之,泥质粉砂岩、粉砂岩与细砂岩含气量最低。与龙马溪组页岩储层相比,龙潭组泥岩孔隙度与总含气量更好,渗透率较差,其中炭质泥岩的孔隙度和总含气量最佳。(4)研究区二叠系龙潭组勘探潜力较大,烃源岩中有机质类型主要为Ⅲ型,其次为Ⅱ_(2)型;其R_(o)值平均约为2.8%,达到高熟阶段;TOC含量变化范围大,炭质泥岩中的TOC一般大于15%。(5)川南地区二叠系龙潭组发育河漫平原沉积,富有机质泥页岩分布稳定,具备良好的页岩气形成条件,估算其天然气资源量约为2.4×10^(12)m^(3),有利勘探区主要位于川南古蔺—叙永地区。

The Longtan Formation is a new important layer for shale gas exploration and development in Sichuan Basin.The shale reservoirs of Permian Longtan Formation in southern Sichuan Basin was comprehensively studied by means of polarizing microscope identification,field emission scanning electron microscope analysis,X-ray diffraction test,organic carbon content determination,high-pressure mercury injection analysis and on-site analytical gas experiment.The results show that:(1)The Permian Longtan Formation in southern Sichuan Basin is a set of continental meandering river deposits,and shale is developed in fluvial deposits.Its minerals are mainly quartz and clay minerals,with a small amount of carbonate minerals.(2)There are mainly intergranular(marginal)pores and microfractures developed in the shale reservoirs in the study area,intragranular pores and dissolved pores are locally developed,and organic pores are rarely developed,among which microfractures between clay minerals are the most developed.The pore structures are mainly microporous,followed by mesoporous(with pore size of 2-50 nm).The pore size is generally small,with poor sorting,the radius difference between pore and throat is large,and the heterogeneity is strong.(3)In the shale reservoirs in the study area,the gas content in coal rock is the highest,followed by carbonaceous mudstone and mudstone,and the gas content in argillaceous siltstone,siltstone and fine sandstone is the lowest.Compared with the shale reservoir of Longmaxi Formation,the mudstone of Longtan Formation has better porosity and total gas content and poor permeability,and the porosity and total gas content of carbonaceous mudstone are the best.(4)The Permian Longtan Formation in the study area has great exploration potential.The organic matter in the source rocks is mainly typeⅢ,followed by typeⅡ_(2).The average value of R_(o) is about 2.8%,reaching the stage of high maturity.TOC content varies widely,and TOC content in carbonaceous mudstone is generally greater than 15%.(5)Alluvial plain is developed in Permian Longtan Formation in southern Sichuan Basin,being rich in organic shale,with good shale gas formation conditions.The estimated natural gas resources are about 2.4×10^(12)m^(3),and the favorable exploration area is mainly located in Gulin-Xuyong area in southern Sichuan Basin.