Characteristics and dominant controlling factors of karst reservoirs in the middle Permian Maokou Formation of the eastern Sichuan region

YANG Rong; YANG Xiyan; FAN Cunhui; LI Yang; LI Yue; HUANG Zisang

doi:10.11932/karst20250216

Abstract：The middle Permian Maokou Formation in the eastern Sichuan region is characterized by the widespread development of karst reservoirs.However,systematic analysis of the identification criteria and controlling factors of these karst reservoirs remains insufficient.This study integrates extensive field outcrops,drilling cores,thin sections,well logging and well logging interpretation,and seismic data to comprehensively analyze the response characteristics of karst reservoirs in the Maokou Formation of the eastern Sichuan region.Furthermore,this paper elucidates the fundamental characteristics and distribution patterns of these reservoirs while conducting a comprehensive evaluation of their primary controlling factors.The key findings are as follows. (1)Karst reservoirs exhibit a variety of identification markers.The petrological response characteristics are as follows:residual dissolution pores,ranging from 2 mm to 70 mm in diameter,and fractures-mainly high-angle structural fractures-are generally developed in both cores and outcrops.Additionally,the caves are mostly half-filled with calcite,dolomite,or asphalt,while some remain unfilled.Bauxitic clay rocks and karst breccia containing limonite are visible at the top of the stratum,indicating the existence of ancient weathering crust.The logging response characteristics are as follows,the acoustic time difference,compensated neutron curves and compensated density curves are"box-shaped"in the karst cave section.The phenomena of pronounced acoustic time difference leap are observed,accompanied by a reduction in the positive difference of deep and shallow dual lateral resistivity.Imaging logging shows the black patchy karst cave characterized by low resistance and high conductivity.Natural gamma values in the karst cave section is generally lower than 40 API,with low content of radioactive elements.The logging response is characterized by frequent well leakage and venting during drilling,mainly occurring in the range of 0 m to 100 m from the top of the Maokou Formation.High-yield gas wells are mostly located in the second and third members of the Maokou Formation,reflecting the development of fracture-cavity reservoirs.The seismic response characteristics are marked by"low-frequency,high-amplitude,and wide-crest"reflections at the top of the Maokou Formation.In the dense area of karst caves,"bright spots"anomalies appear.The development of karst caves reduces wave impedance difference,resulting in downward-dragged and chaotic reflections. (2)The rocks of karst reservoirs are mainly composed of grain limestone.The main reservoir spaces consist of dissolved pores and caves,with caves being the most significant reservoir space,accounting for more than 65％.These caves are predominantly distributed in isolated patterns,exhibiting poor connectivity.Dissolved pores,including intragranular,intergranular,and intercrystal types are predominantly filled.The measured porosity of the karst reservoir ranges from 2.0％to 8.7％,with an average of 3.4％.The permeability varies from 0.008 mD to 65.263 mD,averaging 18.3 mD,showing the characteristics of low porosity and low permeability.Among the three types of karst reservoirs,fracture-cavity reservoirs exhibit the most favorable physical properties,with a porosity of 3.3％,and a permeability of 23.3 mD.In contrast,the cavity-type reservoirs demonstrate inferior physical properties due to the lack of filtration channels.Karst landforms are categorized into karst highlands,steep karst slopes,gentle karst slopes,and karst basins.The porosity of steep karst slopes(3.96％)is significantly higher than that of gentle karst slopes(3.27％)and karst highlands(2.10％).Vertically,the karst reservoirs are mainly distributed within 100 m from the top of the Maokou Formation,particularly in the Mao 2a sub-member and the Mao 3 member.These reservoirs exhibit poor lateral ductility,presenting as isolated point-like or belt-like distributions.Horizontally,there is a NW-SE zonal distribution in the Linshui-Fengdu area,with reservoir thicknesses ranging from 7.68 m to 11.04 m. (3)Karst reservoirs are influenced by the combined effects of sedimentation,karstification and tectonism.Specifically,the grain shoals(intra-platform shoals and platform margin shoals)are mainly developed in the Mao 2 and the Mao 3 members.The thickness of the granular limestone is positively correlated with the thickness of the reservoir(R2=0.82),which indicates that the grain shoals provide a high-quality material foundation for the karst reservoirs and offer percolation channels for diagenetic fluids in later stages.The karst paleogeomorphology controls the distribution of karst reservoirs.High-yield wells are mostly located in the geomorphic units of residual mounds on steep karst slopes and gentle karst slopes.The transitional zones between karst highlands and karst steep karst slopes are also favorable areas for the development of karst reservoirs.Multiple stages of fracturing improve the reservoir storage performance.Specially,three stages of fractures came into being during the Dongwu Movement(early high-angle fractures),the Indosinian-Yanshan Movement(mid-term X-shaped fractures),and the Himalayan Movement(late horizontal fractures).The late horizontal fractures intersected the earlier fractures and connected with karst caves,significantly enhancing the permeability.For instance,the fracture density of Well W78 reached 11 fractures/m,resulting in a substantial increase in daily gas production.These fractures provide channels for oil and gas infiltration,facilitating the accumulation of large oil and gas reservoirs. (4)Based on the analysis of the dominant controlling factors,two favorable areas have been identified.The first is the Linshui-Fengdu steep karst slope zone,which is characterized by thick grain shoals reaching up to 23 m in the Mao 3 Member,the well-developed residual mound landform,dense karst caves and fractures,all of which contribute to optimal reservoir physical properties.The second is the Dazhou-Kaijiang gentle karst slope zone.This zone features the superposition of multi-stage fractures,particularly med-term fractures,which significantly enhance permeability.Although karstification is relatively weak,this area has the potential of high-yield production.