Tahe Oilfield is a carbonate bottom-water reservoir with fractures and caves. The producing wells penetrate only part of the pay-zone. So the formation under the penetration can not be recognized clearly. In order to understand the geology of the whole reservoir, the production data are used and analyzed. Based on the characteristics of the production rate decline, the wells in Tahe Oilfield are classified into three types: rapidly declined well, medially declined well and slowly declined well. The cave system also can be classified into three types correspondingly, including closed cave system, semi-closed cave systemand open cave system. Closed cave systemis lack of natural drive energy, while open cave system has abundant natural drive energy, and semi-closed cave system behaves between the closed cave system and open cave system. According to the closure property of cave system, production procedures of oilfield can be directed properly.