滇黔北坳陷威信凹陷页岩气成藏条件分析与有利区优选

摘要

根据页岩气区域地质调查研究、钻探和地震勘探成果,研究滇黔北坳陷威信凹陷页岩气成藏条件,优选页岩气勘探“甜点”.寒武系牛蹄塘组和志留系龙马溪组黑色页岩是威信凹陷页岩气两套重点勘探评价层系,单层厚度达50 m以上,有机质丰度高,具有低孔、低渗和富硅质特征,具备形成页岩气藏的有利地质条件并易于后期储集层压裂改造.上覆碎屑岩盖层、无通天断层、浅层整体封闭3大因素并存利于页岩气藏保存.岩心解吸显示页岩气成分以甲烷为主,页岩气含量随保存条件变好、页岩TOC丰度增大和地层压力加大而增加.页岩气勘探甜点综合评价优选认为,凹陷北部的筠连—罗布地区为龙马溪组和牛蹄塘组页岩气目的层叠合的最有利甜点勘探区,中部的花坭—威信地区为牛蹄塘组页岩气有利甜点区.建议重视下石炭统旧司组、上二叠统乐平组的页岩气勘探评价.%Forming conditions of the shale gas reservoir of the Weixin Sag in the Dianqianbei Depression are studied and preferable sweet spots for shale gas exploration are proposed based on the results of geologic survey, drilling and seismic exploration in the shale gas region. The black shales of Cambrian Niutitang Formation and Silurian Longmaxi Formation are the two principal exploration targets in the Weixin Sag, having the characteristics of big single layer thickness (>50 m), abundant organic matter, low porosity and permeability, and rich silicon. Those two formations have favorable conditions for the formation of shale gas reservoirs and also can be easily hydraulic fractured in the later stages of production. The coexistence of the clastic rock layer on the top, shallow entire seal, and the absence of exposed faults are the three desirable factors for the preservation of shale gas reservoir. The shale core analysis shows that the major composition is methane, the content of which increases as the preservation conditions become better and the shale TOC abundance and formation pressure increase. After integrated evaluation and optimized selection of shale gas exploration zones, it is determined that the Junlian-Luobu zone located in the north of the sag is the most favorable sweet spot for shale gas exploration in the overlapping target layer of the Longmaxi Formation and Niutitang Formation, and the Huani-Weixin zone in the middle of the sag is proposed to be the sweet spot for shale gas exploration in the Niutitang Formation. It is recommended that importance be attached to the evaluation of shale gas exploration in the Lower Carboniferous Jiusi Formation and Upper Permian Leping formation.