四川盆地龙马溪组与大安寨段页岩储层特征对比研究:海相与湖相的差异分析

Comparative study of shale reservoir characteristics between the Longmaxi Formation and the Daanzhai Member in the Sichuan Basin:Highlighting facies differences between marine and lacustrine depositional environments

储集特征是湖相页岩研究的重点及难点。为了对比分析海相、陆相页岩储层差异,为下阶段湖相页岩的勘探开发提供理论依据及借鉴,对川东南龙马溪组一亚段页岩及川北地区大安寨二亚段湖相页岩开展有机地化实验、全岩X射线衍射、氩离子抛光扫描电镜、低温氮气吸附等实验,对比分析了湖相、海相页岩有机地化、储层特征参数、孔隙结构及主控因素等的差异。结果表明:与龙马溪组页岩相比,湖相大安寨段页岩有机质含量较低,有机质干酪根类型以Ⅱ型为主,少量Ⅲ型,处于高-过成熟阶段;矿物组成上具有更低的硅质矿物与更高的黏土矿物含量;黏土矿物类型以伊利石及伊/蒙混层为主,介壳化石造成钙质成分较高;孔隙组成上龙马溪组以有机孔发育为主,大安寨段以黏土矿物间孔发育为主,占孔隙的70%以上,有机孔次之;微裂缝主要为黏土层间缝及矿物边缘缝,孔径分布以2~50 nm的介孔为主,在4 nm及20 nm两个区间形成“双峰态”。大安寨段湖相页岩优势储集空间主要受控于黏土矿物含量成分及有机质含量。有机质与黏土矿物组成的有效空间耦合为油气黏土间孔赋存提供了良好条件。湖相页岩纹层发育带具有高孔隙度、高TOC、高含气性及微裂缝发育的特点,有利于压裂造缝,是优质储层发育层段。

Reservoir characteristic is the key focus and challenge in lacustrine shale research.To provide a theoretical basis and reference for future exploration and development of lacustrine shale gas,this study compared and analyzed the differences between marine and lacustrine shale reservoirs.Organic geochemistry experiments,whole-rock X-ray diffraction(XRD)analysis,scanning electron microscopy(SEM)with argon ion polishing,and low-temperature nitrogen adsorption experiments were conducted on lacustrine shale from the 1st Member of the Longmaxi Formation in Southeast Sichuan and the 2nd sub-member of the Daanzhai Member in North Sichuan.This research aims to compare and analyze the organic geochemistry,reservoir characteristics,pore structure differences,and controlling factors between lacustrine and marine shales.The results show that,compared to the Longmaxi Formation shale,the Daanzhai shale exhibits lower organic matter content.The organic matter kerogen in Daanzhai shale is primarily typeⅡ,with a small amount of typeⅢ,and is in a high-over mature stage.Mineral composition in the Daanzhai shale is characterized by lower siliceous minerals and higher clay mineral content,primarily illite and mixed-layer illite.The presence of shell fossils contributes to a higher calcium content.In terms of pore composition,the Longmaxi Formation is dominated by organic pores.In contrast,the Daanzhai shale is primarily characterized by clay mineral interpore,accounting for over 70%of the total pores,followed by organic pores.Micro-fractures are mainly clay interlayer fractures and mineral edge fractures.The pore size distribution is dominated by mesopores ranging from 2 to 50 nm,forming a“double peak”at intervals of 4 nm and 20 nm,respectively.The dominant reservoir space in the Daanzhai shale is primarily controlled by clay mineral and organic matter content.The effective spatial coupling of organic matter and clay mineral composition facilitates the formation of pores between oil and gas-bearing clays,creating favorable conditions for reservoir development.The striated zone of the lacustrine shale exhibits high porosity,high total organic carbon(TOC)content,high gas content,and well-developed micro-fractures,making it conducive to fracturing and fracture formation.This zone represents a high-quality reservoir development interval,conducive to oil and gas accumulation.