Paleokarst Genesis And Distribution Prediction In Ordovician Carbonate Rocks,in The Tarim Basin

The Tarim Basin is one composite basin,superimposed by Paleozoic craton basin,Mesozoic and Cenozoic foreland basins.Paleozoic Ordovician carbonate rocks are well developed and the paleokarst reservoirs in them have the characteristics of large amount and scale.They are representative.Based on previous researches on the characteristics and reformation of Ordovician carbonate reservoirs in the Tarim Basin,the evaluation on paleokarst reservoirs could be furtherly enhanced by the understanding on foregoing study results and the utilization on geological,core,logging,drilling,electric logging,seismic and geochemical testing data and information.Three principal areas(Northern,Central and Southwest Tarim Areas),in which Ordovician paleokarsts occurred,have been studied,discussed and summarized,respectively,on the aspects of deposition,tectonics,karst development and later reformation.The regularities of paleokarst development could be concluded by different paleokarst types,eposides,processes and later reformation in diverse areas.In the Tazhong area,the reefal flat bodies of Ordovician Lianglitag Formation,near No.I faulted zone underwent multiple syngenetic karst phases;the weathering karst mainly occurred in Late Caledonian Period,which has certain reformation on the Lianglitag Formation reefal flat bodies;there are three burial karst development phases,in corresponding with the hydrocarbon generation phases of source rocks.In the North Tarim Basin,there are mainly weathering crust paleokarst reservoirs formed from Late Caledonian Period to Early Hercynian Period,accompanying with the reformation of burial karst.In the SW Tarim Basin,there are three weathering crust karst development phases in Early Caledonian Periond,Late Caledonian Period and Early Hercynian Period with multiple burial karst phases.The development of effective reservoirs in Ordovician carbonate rocks of the Tarim Basin is constained and controlled by favorable lithofacies types,multiple types and phases of karsts and tectonic movements.The type of sedimentary facies determines the rock fabrics and then the diagenetic types in later stages.The complicated characteristics of paleokarst superimposition and reformation are defined by multiple phases and types.The stress of tectonic movement controls the fracture occurrence and distribution,in combination with favorable lithofacies types and the types and phases of karst development.The development of Orodovician karst carbonate reservoirs in the Yubei Area mainly depends on the paleogeomorgraphy and paleohydrology of weathering karst occurrence,the formation of fractures in tectonic movements and their matching to the episodes of weathering karst and the reformation of late burial karst.The paleogeomorgraphy of weathering karst is a critical factor to directly determine whether the storage space by dissolution was denuded or filled by later cements or not.