| The karstification characteristics, distribution and type in Middle Permian Maokou Formation in southern Sichuan Basin were studied using cores, thin sections and well logging data. In study area, Maokou Formation develops open sea platform deposits. Effected by polycyclic sea-level drops, it develops polycyclic superimposed, development layer stable, good lateral continuity and unexposed grain shoal in micro-geomorphic highland controlled by Leshan-longnvsi inherited palaeohigh and Luzhou palaeohigh. Then, Dongwu movement made seawater out of Yangtze platform in end of Middle Permian, developing7-8Ma sedimentary discontinuity and exposed karst. Through10coring wells’cores and microscopic observation, find out the karst is controlled by grain shoal facies vertically and horizontally, low-develops in other facies, which presents the characteristics of stratified karst controlled by polycyclic superimposed grain shoal facies. Further observation and analysis, indicating Emei taphrogeny generally achieves climax, whole basin develops tensile fracture and its associated fracture system under the influence of vigorous tensile fracture and normal fault activities. Meanwhile, matrix has experienced eogenetic stage, after the original sediment experiences eogenetic different diagenesis, micritic rocks consolidate into dense lithological water-resisting layer due to compaction and cementation. High-energy grainstones have already consolidated into rocks, but they still reserve a large amount of matrix pore,thus form polycyclic superimposed characteristics of permeability layer and dense water-resisting layer vertically, early permeability layer and fracture system provide the karst water conducting system. Inside dense layer, karst water flows and dissolves along fissure and beding plane, mainly develops vertical karren and saccular cave, what’s more, the boundaries between karst system and wall rocks are clear. In permeability layer formed by high-energy grainstones, karst water diffuse flows and dissolves, developing cave system due to particles discrete in the region of faster flow, meanwhile, cave system can be filled with autochthonous, upper discrete particles and carbonaceous mud of Longtan stage, the edge of cave system still can be mixed filled, thus makes the boundaries between cave system and wall rocks unclear, forming "Friable Halo" with transitional characteristics. The results indicates, after experienced eogenetic different diagenesis, polycyclic superimposed permeability layer and dense water-resisting layer, crack and fracture system formed by tensile fracture activities jointly control conducting system of karst water during karstification, thus forming "same period multilayer" karst system controlled by grain shoal facies, in addition, karst reservoir can be developed both in karst platform and karst slope equally, which is different from the characteristics of tranditional karst apparently. This research achievement is of great significance to oil-gas exploration of Maokou Formation in southern Sichuan Basin, at the same time, provides new reference to karst study in the future. |
PDF Full Text Request