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Palaeogeomorphology and Its Controlling Effect on Karst Reservoirs in Tahe Oilfield, Tarim Basin

Abstract

Tahe Oilfield experienced many times of tectonic uplift from Caledonian to Hercynian, Ordovician carbonate formation uplifted, exposed and weathering, formed a complex karst landform and karst reservoir. Palaeogeomorphology is the main controlling factor of the development and distribution of karst weathering in reservoir. Large data set from drilled well core, thin-section, well cuttings, wireline logging and 3D seismic have been employed to identify the palaeokarst at the top of Ordovician in Tahe oilfield. The palaeogeomorphological map was compiled on basis of impressions and residual-thickness and unconformity. Types of the palaeogeomorphology were subdivided and their relationship to the reservoirs were analyzed. The palaeogeomorphology of Ordovician in Tahe oilfield can be subdivided into four second-order landform units: 1- karst highland, 2- karst slight slope, 3- karst steep slope and 4- karst basin. The main type is karst slight slope andkarst steep slope, while the karst highland and karst basin are localized. The second-order landform units can be further divided into four third-order micotopography: monadnock, tableland, level terrace and cheuch. The karst slight slope of the karst reservoirs depth and karstification is significantly greater than the karst steep slope. the favorable area of karst reservoirs development is monadnock, tableland and tableland edge, near the branch cheuch end in the karst slight slope. level terrace and cheuch is relatively weak, while tableland and level terrace is the favorable landform units in karst steep slope.