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The Ordovician paleokarst reservoirs in the Tahe oilfield, with burial depths of over 5300 m, experienced multiple phases of geologic processes and exhibit strong heterogeneity. Core testing can be used to analyse the characteristics of typical points at the centimetre scale, and seismic datasets can reveal the macroscopic outlines of reservoirs at the >10-m scale. However, neither method can identify caves, cave fills and fractures at the meter scale. Guided by outcrop investigations and calibrations based on core sample observations, this paper describes the interpretation of high longitudinal resolution borehole images, the identification of the characteristics of caves, cave fills (sedimentary, breccia and chemical fills) and fractures in single wells, and the identification of structures and fill characteristics at the meter scale in the strongly heterogeneous paleokarst reservoirs. The paleogeomorphology, a major controlling factor in the distribution of paleokarst reservoirs, was also analysed. The results show that one well can penetrate multiple cave layers of various sizes and that the caves are filled with multiple types of fill. The paleogeomorphology can be divided into highlands, slopes and depressions, which controlled the structure and fill characteristics of the paleokarst reservoirs. The results of this study can provide fundamental meter-scale datasets for interpreting detailed geologic features of deeply buried paleocaves, can be used to connect core- and seismic-scale interpretations, and can provide support for the recognition and development of these strongly heterogeneous reservoirs.

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.

Nitrogen gas flooding and foam flooding have been widely used for oil and gas development in fracture-vuggy carbonate reservoirs.

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