中扬子宜昌地区沉积—构造演化与寒武系页岩气富集规律

Analysis of sedimentary-tectonic evolution characteristics and shale gas enrichment in Yichang area,Middle Yangtze

鄂西宜昌斜坡是四川盆地外寒武系页岩气勘探新区,与盆内勘探不同,宜昌斜坡发育于黄陵隆起之上,页岩气富集地质条件研究薄弱。利用最新的地震、钻探和测试资料,分析认为研究区沉积、构造演化具有特殊性,先后经历了印支期隆起雏形、燕山期快速抬升剥蚀、喜马拉雅期充填改造3个关键沉积—构造阶段。宜昌地区相对稳定的构造条件,对页岩气富集成藏有4个方面的影响:①桐湾运动末期形成早寒武世隆、凹相间的古地理格局直接影响宜昌地区寒武系水井沱组富有机质页岩的展布,对页岩气的形成分布有重要的源控作用,水井沱组页岩含气量与TOC含量具有较好正相关性,指示有机质丰度是影响页岩气富集的关键因素。②印支期开始隆升、燕山期迅速隆升的黄陵岩体控制宜昌斜坡区构造沉降,使下古生界页岩层埋藏深度适中、抬升早、有机质热成熟度相对低,在中、下扬子区下古生界页岩过成熟演化背景下尤为特殊。适宜的热演化条件下,水井沱组页岩有机质纳米孔发育,构成页岩气重要储集空间。③受黄陵隆起的砥柱作用,宜昌斜坡在南方中、新生代多期次强烈构造活动中免遭构造破坏和强改造,表现为隆升剥蚀为主、局限断裂活动为辅、整体构造变形弱的特点,构造保存条件较临区优越,加之页岩渗透性极低,使宜昌单斜构造上水井沱组页岩气仍有效保存,是当前该区勘探的主体。④寒武系页岩裂缝发育,早期构造作用形成的高角度裂缝、晚期由于重力顺层滑脱作用形成的低角度剪切缝,共同构成页岩气运移、储集的重要空间。宜昌地区页岩气的勘探突破表明,古隆起、古斜坡等稳定古构造周缘是中下扬子区古生界页岩气勘探的潜在有利区。

The Yichang slope,located in western Hubei Province,is a new area for shale gas exploration of the Cambrian strata outside the Sichuan Basin.Few studies have focused on the shale gas enrichment characteristics of the Yichang slope,which developed on the ancient Huangling anticline.Based on the analysis of sedimenta⁃ry,structural and the latest seismic and drilling data,as well as laboratory test,the sedimentary,tectonic and hydrocarbon accumulation characteristics are analyzed.Compared to other region of the Middle Yangtze,it is conformed that the study area has experienced three important key tectonic stages,namely the embryonic uplift in the Indosinian Period,the rapid uplift and denudation in the Yanshan Period,and the filling and reconstruc⁃tion in the Himalayan Period,respectively.The relatively stable tectonic conditions in Yichang area have impor⁃tant influences on shale gas accumulation:(1)At the end of the Tongwan tectonic movement,the paleogeo⁃graphic pattern of early Cambrian had a direct influence on the distribution of organic shale in Yichang area,and played an important role in controlling the formation and distribution of shale gas.A positive correlation between gas content and TOC content indicate that the abundance of organic matter is the key factor influencing shale gas enrichment.(2)The Huangling anticline,which began to rise in the Indosinian tectonic movement and rose rap⁃idly in the Yanshan movement,controlled the structural subsidence of the Yichang area.The Lower Paleozoic shale strata is characterized by suitable burial depth,earlier tectonic uplifting than the west Hubei area.This rel⁃atively lower thermal maturity of the organic matters in the Sinian-Early Paleozoic shale is particularly special among the high maturity background in the south China.Organic nanopores formed an important shale gas stor⁃age space.(3)This ancient anticline played an important role in protecting the Yichang slope from suffering structural alteration and destruction during the multi-stage strong tectonic activities of the Mesozoic and Cenozo⁃ic in the South China.Tectonic activities are characterized by structural uplift with limited fracture activity and weaker tectonic deformation.Although significant uplift and erosion of overburden rocks occurred in the Late In⁃dosinian orogeny,the Yichang area was not significantly influenced during the tectonic movement comparing to the surrounding area.The Shuijingtuo shale in Yichang area is characterized by suitable structure preservation and low permeability.Shale gas is preserved effectively in this monoclinal structure and becomes the main explo⁃ration target in this area.(4)Bedding slip joint related to stratum slip shear joint during thrusting was an impor⁃tant reservoir space for shale gas.Shale gas exploration of the Yichang area confirmed that the paleo-uplift,the periphery of the paleo-tectonic slope,and the fault footwall of the thrust nappe structure are favorable explora⁃tion areas for shale gas.