四川盆地南部地区茅口组古岩溶缝洞充填演化特征

四川盆地南部地区中二叠统茅口组碳酸盐岩经历了复杂的孔隙流体活动,目前从微观地球化学的角度精细分析多期成岩流体与储层之间关系的研究尚不多见。为此,通过对蜀南地区茅口组野外采集样品和钻井岩心中岩溶缝洞充填物的形态、碳氧同位素、包裹体及稀土元素进行分析,明确了岩溶缝洞充填环境,厘定了主要成岩矿物充填期次及时间序列。整体来讲,茅口组古岩溶缝洞有四个主要形成期,对应有4期主要充填期,导致储层破坏:①准同生暴露顺层岩溶孔洞形成—充填期;②表生暴露岩溶裂缝充填钙泥质期;③表生暴露后龙潭组碎屑充填期;④埋藏裂缝或孔洞充填方解石期。现今而言,准同生期形成顺层孔洞后期埋藏充填,保存可能性很小;表生暴露期径流带岩溶洞穴储层尚可能存在;茅口组钻遇大量的裂缝型储层多属于第④种情况,是目前勘探的有利方向。

Evolution characteristics of paleo-karst fracture and vug filling in Maokou Formation,southern Sichuan Basin

In southern Sichuan Basin,most carbonate rocks in the Middle Permian Maokou Formation experience complex pore-fluid activities.So far,however,the relationship between multi-period diagenetic fluid and reservoirs has been less analyzed precisely from the perspective of microscopic geochemistry.So,the filling environment of karst fracture and vug was clarified and the filling period and time series of the main diagenetic minerals were determined by analyzing the morphology,C/O isotope,inclusion and rare earth elements of the karst fracture and vug filling in both samples and drilling cores from the Maokou Formation,southern Sichuan Basin.On the whole,to form the paleo-karst fracture and vug can be mainly divided into four periods,namely the forming and filling period of the bedding karst pore and vug during penecontemporaneous exposure(Period I),the filling period of the karst fracture by calcareous and argillaceous during supergene exposure(Period II),the filling period of the clastics in Longtan Formation after supergene exposure(Period III),and the fracture-buried and pore-vug-filling period by calcite(Period IV).And correspondingly,there are four filling periods,which may lead to reservoir damage.At present,the bedding pore and vug formed in the penecontemporaneous period are buried and filled in the late period,so the probability of their preservation is rare.However,the karst vuggy reservoirs formed in the runoff zone during the supergene explosion may be preserved.And the drilled abundant fractured reservoirs of Maokou Formation which chiefly belong to Period IV are currently favorable exploration targets.