川东地区寒武系源--储对接型天然气成藏模式探究

Gas reservoir-forming pattern of the Cambrian source-reservoir connection,eastern Sichuan Basin

川东地区处于四川盆地油气富集带,其下组合为重点勘探领域,虽历经多年勘探却未取得战略性突破。近期,川东地区平桥构造寒武系盐上领域测试获工业气流,证实了该领域具备油气勘探潜力。为了明确其油气成藏类型,以地震、钻井、露头等资料为研究对象,分析了上寒武统洗象池群沉积储层特征、源储配置关系及油气动态成藏过程,指出了川东地区洗象池群源—储对接型天然气成藏模式及主控因素。研究结果表明:①川东地区上奥陶统五峰组—下志留统龙马溪组优质页岩段有机碳含量一般大于3%,气源条件优异;②川东地区洗象池群微古地貌高点周缘发育台内浅滩储层,储层受到裂缝和溶蚀作用改造;③川东地区中构造层内发育逆冲断层,使得下盘五峰组—龙马溪组“源”与上盘洗象池群“储”形成侧向对接关系,洗象池群气藏与五峰组—龙马溪组页岩气同源证实了侧向供烃的有效性。结论认为:①研究区存在以五峰组—龙马溪组为有效烃源岩,洗象池群白云岩为储层,以高陡构造带断层相关背斜圈闭为主要勘探目标的“新生古储、源—储对接、侧向供烃”的晚期天然气成藏模式;②良好的源储条件是天然气成藏的基础,高源—储对置程度提高了成藏效率,页岩气藏晚期抬升泄压提供气源充注。

In eastern Sichuan Basin,also a prolific hydrocarbon zone in Sichuan Basin,the lower assemblage as the key exploration domain has not been made strategic breakthrough after many years of exploration practice.However,some industrial gas flow recently from the Cambrian postsalt,Pingqiao structure in eastern basin,may prove the postsalt potential.In order to figure out its reservoir-forming type,the Upper Cambrian Xixiangchi Group was taken as an example to analyze the reservoir characteristics,source-reservoir association,and dynamic reservoir-forming process by using seismic,drilling,and outcrop data.In addition,the reservoir-forming pattern of the source-reservoir connection in the study area and the main factors affecting this pattern were pointed out.Results show that(1)in the high-quality shales from Upper Ordovician Wufeng Formation to Lower Ordovician Longmaxi Formation,eastern Sichuan Basin,the total organic carbon(TOC)is generally higher than 3%,indicat⁃ing an excellent gas source condition;(2)certain intraplatform shallow shoal reservoirs of Xixiangchi Group,which are developed in the periphery of micropalaeogeomorphic high of eastern basin,may be reworked by frac⁃tures and dissolution;and(3)a thrust fault developed in the middle assemblage in eastern basin may lead to a direct lateral connection between the source of Wufeng to Longmaxi formations at the fault's footwall and the res⁃ervoir of Xixiangchi Group at the hanging wall.And the Xixiangchi gas reservoir and the shale gas in Wufeng to Longmaxi formations are of the same source,which confirms the effectiveness of lateral hydrocarbon supply.In conclusion,a kind of late gas reservoir-forming pattern of"old gas source and new trap,source-reservoir con⁃nection,and lateral hydrocarbon supply"is represented,which takes the Wufeng to Longmaxi formations as effec⁃tive source rocks,the dolomites of Xixiangchi Group as reservoirs,and the fault-related anticlinal trap in the high and steep structural belt of eastern Sichuan Basin as an exploration target.What's more,the better sourcereservoir condition is the base to form gas reservoirs,the useful source-reservoir connection increases the reser⁃voir-forming efficiency,and the late uplift and pressure relief of shale gas open gas charging.