Paleokarst Reservoir Features and Their Influence on Production in the Tahe Oilfield, Tarim Basin, China
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This study analyzed the characteristics of cave heights and fillings, influences on cave distribution by unconformity and geomorphology, and production control trend for geological factors. The heights and numbers of caves in different geomorphologies are basically similar. The cave heights are greater near the unconformity surface. On an average, there are higher degrees of cave development and larger cave heights in the karst slopes than in the karst highlands and karst depression. Based on core observations, four types of reservoirs were identified: unfilled caves, filled caves, open fractures and filled fractures. The formation of caves is mainly controlled by faults and underground water surfaces. Faults works obviously in the karst highland, while underground water surfaces obviously in the karst slope. Caves controlled by faults are mostly unfilled, while controlled by underground water surfaces are mostly filled with clastic sediments. According to the results from both first-day and cumulative production, production is not highly correlated with cave heights, but rather storage and connectivity of caves and fractures. Generally, wells with unfilled caves or open fractures have higher first-day and cumulative production than filled caves or fractures. The distribution of high first-day and cumulative production is consistent overall.