Study on the development model of deep carbonate paleokarst reservoirs in the Tarim Basin

ZHANG Qingyu; NIE Guoquan; JI Shaocong; DONG Hongqi; LI Jingrui; BA Junjie; MO Guochen; ZHANG Meng

doi:10.11932/karst20250612

Abstract：As a critical hydrocarbon-bearing basin in China,the Tarim Basin contains exceptionally abundant oil and gas resources within its Paleozoic marine carbonate formations,particularly in the deep subsurface where it exhibits significant exploration potential.However,the complex development patterns of paleokarst reservoirs in carbonate rocks,along with their extreme heterogeneity,pose severe challenges for hydrocarbon exploration and development.This study focuses on the paleokarst reservoirs of representative carbonate oil and gas fields in the Tarim Basin.By integrating multidisciplinary and multitechnique approaches-including core analysis,drilling and logging data,well logging data,high-resolution seismic data,and comprehensive regional geological background materials-this study conducts an in-depth and systematic identification and analysis of paleokarst reservoirs. The findings reveal that the paleokarst development in the Tarim Basin has undergone multiple superimposed transformations.The combined effects of uplift,erosion,and burial have resulted in highly complex heterogeneity characterized by the types,scales,and distribution of reservoir spaces.Paleokarst reservoir spaces primarily include dissolution pores,caves,and fractures;these diverse secondary pore types collectively constitute the main reservoir spaces in carbonate rock formations.Based on the dominant reservoir space categories,the reservoirs can be further subdivided into fracture-dominated,pore-dominated,fracture-pore-dominated,and cave-dominated reservoir types.Paleokarst development is controlled by factors such as tectonic evolution,paleogeomorphology,paleohydrodynamic conditions,and lithology.To identify paleokarst fractures and caves,this study employs a multi-source data fusion approach.Core data provide the most direct and detailed evidence,clearly revealing the macroscopic morphology of dissolution pores,dissolution stylolites,and caves,along with their internal filling characteristics.The filling materials are diverse,including mechanical infillings such as mudstone,breccia,and black organic matter,as well as chemical precipitates like calcite,silica,fluorite,and pyrite,reflecting a multi-stage geological filling process.Drilling,logging,and well logging data indicate the presence of large-scale caves or fractures underground through anomalies such as lost circulation of drilling fluid,increased drilling speed,kick events,and borehole enlargement.In terms of well logging curves,abrupt changes in conventional parameters-gamma values,resistivity,and acoustic transit time-along with dark black features observed in imaging logs,can effectively indicate the presence of underground paleokarst fractures and caves.Seismic data,as a macroscopic detection method,reveals the spatial distribution and geometric morphology of large-scale fracture-cavity bodies through seismic wave characteristics such as frequency reduction,amplitude attenuation,chaotic reflections,weak reflections,bead-like reflections,and low velocity.Morphologically,ancient karst fracture-cavity bodies often exhibit various seismic facies patterns,including"multiple bead-like","single bead-like",and"continuous sheet-like"forms. Additionally,ancient surface rivers,large karst skylights,sinkholes,entrances and exits of ancient underground rivers,and other karst landform features can be clearly identified on seismic profiles.In summary,guided by modern karst theory and based on the specific karst geological conditions of the Tarim Basin,this study conducted a comprehensive analysis of the types of karst strata groups,the developmental scale of paleokarst fractures and caves,the morphological combinations of paleokarst,the flow characteristics of ancient groundwater,as well as karst features revealed by multi-source data such as drilling,logging,and well testing.It innovatively divided the developmental characteristics of paleokarst fractures and caves in the piedmont area into four vertical zones:the shallow karst zone,the vertical percolation dissolution zone,the runoff dissolution zone,and the subflow dissolution zone.Additionally,lateral comparisons of the karst developmental characteristics across these vertical zones were performed.The shallow karst zone is closely linked to surface water systems,where groundwater primarily undergoes dissolution through horizontal runoff and vertical percolation,making it prone to mechanical filling.The vertical percolation dissolution zone is dominated by vertical percolation with weak lateral connectivity and is primarily characterized by mechanical filling.The runoff dissolution zone is dominated by horizontal runoff,featuring large-scale underground river systems,strong karst action,and mechanical filling,often accompanied by mechanical collapse deposits.The subflow dissolution zone is characterized by deep,slow flow dominated by chemical filling,with frequent occurrences of carbonate minerals such as calcite and dolomite. Building on this foundation,the study comprehensively investigated and summarized the developmental patterns and geological models of paleokarst reservoirs in typical carbonate rock oil and gas fields,including piedmont-type paleokarst,shallow cover-type paleokarst,shore-island-type paleokarst,and fault-controlled paleokarst.By exploring these models,the study elucidated the formation mechanisms of different types of paleokarst and their impacts on reservoir properties,thereby establishing a geological model for the development of paleokarst reservoirs in typical carbonate rock oil and gas fields.This provides important theoretical support and practical guidance for the exploration and development of paleokarst-type oil and gas reservoirs in the Tarim Basin and analogous geological settings worldwide.