Carbonate karst reservoirs of the Tarim Basin, northwest China: Types, features, origins, and implications for hydrocarbon exploration

Karst reservoirs are important exploration targets in marine carbonates of hydrocarbon-bearing basins around the world. In the Tarim Basin of northwest China, four types of carbonate karst reservoirs were delineated using data from more than 200 wells, including cores, thin sections, wireline logs, drilling logs, and production data; these were supplemented with seismic and other data. The four types of karst reservoirs described were all formed by meteoric water circulation but not limited in the vadose or phreatic zones, including buried tower karst, interlayer karst, overlying-aquiclude confining karst, and fault-related karst. Buried tower karst occurred in the carbonate successions underneath a regional unconformity, resembling the active karsting in the Guilin Region of South China. Interlayer karst developed during a relatively shorter exposure period compared with buried tower karst but is also situated on a paleouplift. Overlying-aquiclude confining karst is essentially a by-product and on the fringes of buried tower karst, and though it appears to be distributed evenly below a disconformity, it is not related to the disconformity in origin. Fault-related karst occurred in the carbonate succession of a high steep anticline with faults cutting through aquicludes and into the carbonates; dissolution is concentrated along faults and the crest of the anticline. We studied three previously undocumented types of karst and proved that karst reservoirs in the Tarim Basin occur not only in the carbonate succession under a regional unconformity, but also where no significant unconformity is recognized. Similar examples have been found in other basins of China and are expected to occur in other basins worldwide, particularly in areas of complex geologic history.