剪切作用对页岩有机质孔发育特征和吸附能力的影响

Effect of shear on the development and adsorption capacity of organic pores in shale

有机质孔是高成熟页岩储层中吸附气和游离气赋存的主要储集空间类型。中国南方海相页岩地层经历了多期构造改造,滑脱构造广泛发育。为了认识剪切作用对页岩有机质孔微观结构和吸附能力的影响,以张家界三岔地区下寒武统牛蹄塘组页岩为例,通过大量扫描电镜图像观测统计,对比分析了滑脱带页岩、邻近滑脱带页岩和远离滑脱带页岩有机质孔的发育特征,同时对这三类样品进行了甲烷等温吸附测试。研究结果表明,有机质内孔发育在有机质内部,孔径一般<20 nm;位于有机质与矿物接触边缘的复合孔孔径整体大于有机质内孔,主要发育在滑脱带页岩中包裹有矿物碎片的有机质中。受剪切作用影响,这两类有机质孔均沿一定优势方向发生形变,形态更趋于狭长且定向性增强;同时页岩甲烷吸附能力变差,从滑脱带向远离滑脱带方向这种影响逐渐减弱。剪切作用对页岩储集性能和含气性具有重要控制作用,对认识复杂构造区页岩气保存条件和富集规律具有重要意义。

Organic pore is the most important type of storage space for adsorbed gas and free gas in highly mature shale reservoirs.The marine shale formations in South China experienced complex structural deformations and the detachment structures widely developed.To investigate the effect of shear-induced deformation on the microscopic structure and adsorption capacity of organic pores in shale,the lower Cambrian Niutitang shales in the Sancha outcrop in Zhangjiajie were taken as an example.Based on scanning electron microscope observations and statistical parameter analysis of pore images,the development characteristics of organic pores from the detachment belt,the near-by detachment belt and the far-away detachment belt were compared.The methane isothermal adsorption tests were also carried out on the three types of samples.The results show that the pores developed within the organic matters have dominant pore sizes less than 20 nm and the organic matter-mineral related pores have larger pore sizes and mainly developed in the organic matter encapsulating mineral fragments in the detachment belt.The organic pores experienced directional elongation and flattening under shear.The adsorption capacity of shales decreased under shear and the degree of such influence gradually decreased as the distance from the detachment belt increased.The shear has an important controlling effect on shale storage and gas-bearing capacity,which is significant to the understandings on shale gas preservation conditions and accumulation patterns.