四川盆地东北部元坝地区中三叠统雷口坡组四段古岩溶特征及其油气地质意义

Paleokarstification in carbonates of 4th Member of Middle Triassic Leikoupo Formation in northeastern Sichuan basin and its petroleum geological significance

四川盆地东北部元坝地区雷口坡组四段中的油气突破显示了其重要的油气地质意义。通过对野外露头、岩心、薄片、阴极发光、钻井、录井、测井和地震资料的综合分析,认为研究区发育地表残积相、洞穴垮塌堆积相、地下暗河搬运充填相、基岩溶蚀破裂相、基岩溶蚀相和基岩溶蚀交代相6种岩溶相类型。根据不同岩溶带的水动力作用特征,由上而下将其分为地表岩溶带、垂直渗流带、水平潜流带和深部岩溶带。以上部地层印模法为主并结合雷口坡组的残余地层分布、风化壳厚度发育情况恢复古岩溶地貌,进一步刻画元坝地区的微地貌情况,认为岩溶斜坡为最有利的储层发育地貌单元,其中以中上斜坡更优,岩溶残丘为最有利的储层发育微地貌单元。古岩溶对雷口坡组四段储层的形成具有重要的作用,有利储层的平面和纵向发育、分布受古岩溶作用特征的控制。

The hydrocarbon breakthrough in the 4th member of the Leikoupo Formation shows important geological significance for petroleum exploration.Based on comprehensive analyses composed of outcrop and drill-core observations,thin-section petrographic examinations,well log analysis,fluid inclusion studies,and carbon,oxygen,strontium isotopic composition measurements,the authors divided paleokarst faces of the study area into six types,i.e.,surface eluvium,speleothem composed of break-down clasts,fillings from underground rivers,crack of base rock,solution of base rock,and dissolution and replacement of base rock.According to hydrodynamic conditions,the paleokarstification can be divided in descending order into surface-karst zone,vertical vadose zone,horizontal phreatic zone and deep-karst zone.Using the impression method and based on the distribution characteristics of relict stratum,the authors restored palaeokarst geomorphology for the 4th Member of the Leikoupo Formation and presented more details of micro-geomorphology for Yuanba area.It is held that karst slope is the most advantageous geomorphic unit for the formation of reservoirs,with the most favorable place being middle-upper slope.Karst hammock is the most advantageous micro-geomorphic units for the formation of reservoirs.Paleokarstification plays an important role in the reservoir formation of 4th Member of Leikoupo Formation and its features control the distribution of favorable reservoirs in both vertical and planar directions.