四川盆地东南缘海相页岩气保存条件及其主控因素

中国南方构造运动复杂,海相泥页岩热演化程度高,保存条件的好坏是该区页岩气能否富集高产的关键因素。为此,根据近年来四川盆地东南缘志留系页岩气的勘探实践,分析了页岩气与常规气在保存条件上的差异性:优越的顶底板条件是页岩气保存的基础;页岩气具有的自封闭性和纵、横向渗透率2~8倍的差异性,决定了侧向扩散是页岩气的基本散失通道,下寒武统底部的不整合面使其保存条件变差,断层的切割会加速页岩气的散失。同时分析了该区页岩气保存条件的主控因素及判别指标,研究认为:①构造改造强度是页岩气层保存条件的主控因素,构造样式宽缓、盖层连续、有背斜背景的构造更有利于页岩气的富集,而抬升时间较晚的封闭演化环境则有利于页岩气的保存;②在断层不发育或断层封闭性较好或断层封挡的断下盘区域、相对远离露头区或地层缺失带,页岩气能较为有效的被保存;③压力系数是保存条件的综合判别指标,该区地层的压力系数与页岩气的产量呈对数正相关关系,储层高压—超高压是页岩气保存条件好的表征;④川东南地区盆内区域,除邻近剥蚀区和地层缺失线的区域外,上奥陶统五峰组—下志留统龙马溪组压力系数总体高(最高达2.25),保存条件较好,而向盆缘和盆外方向压力系数逐渐降低,保存条件变差。

Main controlling factors for gas preservation conditions of marine shales in southeastern margins of the Sichuan Basin

South China has experienced complex tectonic movements during geologic history and the marine shales there are high in thermal maturity, thus the preservation condition is one of the major factors controlling shale gas enrichment and production. Based on the exploration practices of the Jurassic shale gas on southeastern margins of the Sichuan Basin in recent years, a comparative study was performed on the preservation conditions of conventional gas reservoirs and shale gas reservoirs and revealed that top and bottom seals of high quality are the indispensable conditions for shale gas preservation. The self sealing ability and the huge gap up to 2 8 times between vertical and lateral permeability of shale gas reservoirs determine that the lateral diffusion is the basic pattern of shale gas migration. The major controlling factors for shale gas preservation and their criteria of discrimination were further studied. The following conclusions were obtained. The strength of tectonic modification is the major controlling factor for shale gas preservation. Broad and gentle structures with continuous seals and an anticlinal setting are more favorable for the enrichment of shale gas, and a closed evolutionary environment with late uplifting is more favorable for the preservation of shale gas. Shale gas preservation is more effective in areas with undeveloped faults, areas with a good fault sealing ability, downthrown blocks with fault barriers, and areas far away from outcrops or zones with hiatus. The pressure coefficient is a comprehensive discrimination criterion for sealing condition, so a high or super high pressure of a reservoir is a signal of good preservation condition for a shale gas play. The pressure coefficient is in positive correlation with shale gas production in the study area. In the areas within the southeastern Sichuan Basin except for those with strong erosion or hiatus, the Wufeng and Longmaxi Fms are high in pressure coefficient as a whole and have good preservation conditions for shale gas. In contrast, the pressure coefficient lowers progressively toward southern and eastern margins of the basin and the overall shale gas preservation conditions are quite unfavorable.