米仓山—大巴山地区赫兰特期岩相古地理及其页岩气地质意义

Hirantian lithofacies palaeogeography and their geological significance for shale gas in Micangshan-Dabashan area

上扬子地区赫兰特期发育一套含赫兰特贝和达尔曼虫化石的独特的沉积单元,其沉积环境长期以来存在争议,还缺乏更为精细的沉积相时空变化研究。本文以米仓山—大巴山地区赫兰特期地层为研究目标,通过系统资料收集,野外详细观测和室内镜下分析等手段,采用“点-线-面”的岩相古地理分析方法,重建了研究区赫兰特期岩相古地理。剖面沉积相分析表明,观音桥段发育于不同沉积环境之中,因而具有不同的岩相特征,总体上为赫兰特期海平面下降的产物,但经历了不只一次的海平面升降变化,海退过程的沉积记录相对明显,海侵记录仅在盆地边缘或水下高地边缘有所响应。复原的岩相古地理显示,该时期呈现北西高、北东和南西低,中间多个水下高地的沉积古地理格局,这种格局有利于扬子克拉通盆地内形成优质的富有机质页岩;研究区内观音桥段沉积相主要以浅海陆棚相(浅水陆棚亚相)为主,局部发育孤立台地和碳酸盐缓坡,滨海相局限于盆地西北古陆边缘。页岩气勘探开发中,区内粉砂质泥岩、碳质泥岩发育的浅海陆棚相区为页岩气优质储集层和勘探有利区,但应尽量远离物源区和水下隆起区;白云岩、泥质灰岩发育的孤立台地和碳酸盐缓坡区,压裂过程中,应选择合适层位,尽可能压穿白云岩、泥质灰岩,连通五峰组—龙马溪组含气页岩段,提高页岩气产量。

A unique sedimentary unit with Hirantina-Dalmanitina fauna develops in the upper Yangtze block.Its sedimentary environment is controversial and the detailed study of its space-time changes of sedimentary facies is deficient.In this paper,by the ways of systematic data collection,detailed observation of outcrops,detailed microscopic studies and analyses of thin sections,and the“point-line-plane”analytical method of lithofacies palaeogeography,the Hirantian lithofacies and palaeogeography in Micangshan-Dabashan area have been rebuilt.Analyses of sedimentary facies show that the Guanyinqiao Member in the study area developed in different sedimentary environments with different lithofacies.It was the product of Hirantian sealevel fall,and more than one sea level fluctuation happened during Hirantian period.The sedimentary records of the marine regression are relatively obvious,with the transgression records responding only at the edge or the underwater highlands of the Yangtze basin.The recovered paleoeography shows a sedimentary paleogeographical pattern high in the north,low in the north and the west,and a number of underwater highlands in the middle,which is conducive to the formation of high-quality organic-rich shales in the Yangtze craton basin.Sedimentary facies of Guanyinqiao Member in the study area are dominated by continental shelf facies,especially the shallow shelf sub-facies,with some local isolated platforms and carbonate ramps,and the littoral facies are only distributed in Yangtze craton basin along the northwestern margin of the old land in the region at that time.The continental shelf facies developing silty mudstones and carbonificious mudstones,which is far away from sedimentary provenances and submarine highs,are the high-quality reservoir rocks for shale gas.It is suggested that in order to promote the production of shale gas in the isolated platforms and carbonate ramps,dolostone and muddy limestone layers of the Wufeng-Longmaxi Formation should be fractured throughout.