Characterization of paleo-karst reservoir and faulted karst reservoir in Tahe Oilfield, Tarim Basin, China

Abstract

The Ordovician carbonate reservoir is the most productive deep-earth reservoir in Tahe Oilfield and other oilfields in Tarim Basin. Exploration and production successes in recent years reveal a new reservoir type, namely faulted karst reservoirs, which is closely related to regional strike-slip faults and very different from the well-recognized paleo-karst reservoir. The paleo-karst reservoirs distribute mainly in weathering crust regions in the northern Tahe Oilfield. Their primary reservoir spaces are meter-scale caves and the fluid conduits are predominantly the unconformable surfaces. In production, paleo-karst reservoirs always have sufficient energy, therefore high productivity. The faulted karst reservoirs mainly develop in southern Tahe Oilfield, controlled by the different ordered strike slip faults and related dissolutions. Their reservoir space is smaller than which of paleo-karst reservoirs. The predominant fluid conduits in these reservoirs are the faults. In production, reservoirs along major strike-slip faults have sufficient energy, high productivity and slow watercut increase like paleo-karst reservoirs. While in areas with less strong energy, faulted karst reservoir exhibits weak productivityand rapid watercut increase, implying a rule of “big fault big reservoir, small fault small reservoir, no fault no reservoir. A comprehensive understanding of the geophysical features, distribution characteristics, reservoir property, and production behaviors of the two reservoir types will assist further exploration and production in Tahe Oilfield and other basins containing such reservoirs.