Abstract:

Paleocave carbonate reservoirs are characterized by their heterogeneity,and they are very difficult to predict. For the sake of building a realistic geological model of the paleocave carbonate reservoir, correlation has been made with the paleocaves occurred in Wudaoban outcrop sections in Bachu area. The shape, size, structure, cave deposit, genesis and reservoir properties of the Ordovician paleocaves in Tahe oilfield, Tarim basin,have been studied on the basis of the theory of karst and the paleocave marks identified by means of drilling, logging, well logging, testing and seismic methods. The vertical and lateral heterogeneities of paleocave carbonate reservoir have also been discussed. Vertically, paleocave sediments can be divided into the top facies, filling facies and the lower sloughing facies. The top facies and the lower sloughing facies are quite favorable for hydrocarbon entrapment. But the development of multiple paleocaves have mainly been controlled by tectonic uplift and sea-level fluctuation. Ancestral structures, such as paleofault, paleodrainage system and paleokarst topography etc. have mainly been influencing the lateral heterogeneity of paleocave carbonate reservoirs, among which the paleokarst topography has the closest relation to lateral heterogeneity of paleocave carbonate reservoir. The study indicates that the relatively good paleocave carbonate reservoirs include the unfilled paleocave reservoirs and large paleocave-filling deposits—porous sandstone reservoirs.

Key words: paleocave, carbonate reservoir, reservoir characteristics, Ordovician, Tahe oilfield