渝东南‒黔北地区下古生界页岩非构造裂缝发育特征与主控因素

Non-Tectonic Fracture Characteristics of Lower Paleozoic Shale in Southeast Chongqing and North Guizhou Area(South China)and Its Main Controlling Factors

天然非构造裂缝是页岩气的重要储集空间,但目前针对非构造缝形成机理、控制因素及发育特征的研究不足.对研究区下志留统龙马溪组和下寒武统牛蹄塘组页岩岩心非构造裂缝进行观察描述,通过扫描电镜分析识别页岩非构造裂缝并研究其结构特征,结合沉积环境、有机质丰度和类型、热演化程度、生烃史、粘土矿物含量、水体古盐度和成岩作用,分析各主控因素对非构造裂缝发育特征和分布规律的影响作用.结果表明,渝东南‒黔北地区下古生界页岩非构造裂缝发育程度较高,主要类型包括成岩收缩缝、溶蚀缝和异常高压缝,其在纵向上切穿深度较浅,形态不规则,微观结构呈丝缕状、卷曲片状,缝宽一般10~500 nm,最大可超过1μm,延伸性和连通性较好,可改善页岩的储渗性能.牛蹄塘组下部和龙马溪组下部的深水陆棚相发育大量的水平层理,是非构造缝发育的有利相带.欠压实增压和生烃增压可产生大规模超压裂缝,埋藏早期欠压实为地层超压的主因,生烃增压与热演化深度有很好的对应关系,并且可释放有机酸促进次生溶蚀缝发育;当构造运动的破坏调整作用使异常超压释放,超压裂缝随之萎缩甚至闭合.下古生界页岩在中等古盐度水体环境中发育,其高粘土含量有利于成岩收缩缝的形成.龙马溪组页岩处于中成岩晚期,随着蒙脱石不断向伊利石转化,成岩收缩缝已接近最大值.牛蹄塘组页岩属于晚成岩期,成岩收缩缝形成速率减缓,其体积已接近或达到最大值.

Natural non-tectonic fracture is an important reservoir space for shale gas.However,there are currently insufficient researches on the formation mechanism,control factors,and development characteristics of non-tectonic fractures.It characterizes natural non-tectonic fractures of Lower Silurian Longmaxi Shale and Lower Cambrian Niutitang Shale in study area.The non-tectonic micro-fractures of Lower Palaeozoic Shale were identified and their interior structures were characterized by analyzing SME images.The main controlling factors on the characteristics and distribution of non-tectonic fractures were analyzed by researching sedimentary environment,organic matter abundance and types,thermal evolution degree,hydrocarbon generation history,clay minerals,paleosalinity and diagenesis.The results show that natural non-tectonic fractures,which produced because of diagenetic shrinkage,dissolution and abnormal high pressure,are abundant in both Longmaxi Shale and Niutitang Shale.Natural non-tectonic fractures,of which the microstructure is in the irregular shape of silk-thread and curly sheet,cut shallowly in longitudinal direction,with width ranging from 10 nm to 50 nm,even more than 1μm.The porosity and permeability of shale could be improved due to excellent extensibility and connectivity of non-tectonic fractures.The lower part of Niutitang Shale and Longmaxi Shale is deep-water shelf facies with numerous horizontal bedding,therefore,it is a favorable facies for the development of non-tectonic fractures.Undercompaction and hydrocarbon generation can produce large-scale overpressure fractures.In the early stage of burial,undercompaction was the main cause of formation overpressure.Hydrocarbon generation pressurization,that has a good corresponding relationship with thermal evolution,could release organic acids to promote the development of secondary dissolution fractures.While the destruction and adjustment of tectonic movement released the abnormal overpressure,the overpressure fractures shrunk or even closed.The lower Paleozoic shale was deposited in the water environment with medium paleosalinity,in addition,high clay content is conducive to the generation of diagenetic shrinkage fractures.Longmaxi Shale was in middle-late or late diagenesis stage,while the smectite transformed to illite gradually,the volume of diagenetic shrinkage fractures was close to the maximum.Niutitang Shale was in late diagenesis stage,the generation rate of shrinkage fractures became slow,moreover,the volume of diagenetic shrinkage fractures had approached or reached the maximum.