Characteristics of imaging logging response and indication of paleokarst development in Lungu area

The Ordovician carbonate reservoir in the Lungu area of Tarim basin has been reformed by the superimposition of multi-stage tectonic movement and karstification. Primary oil and gas storage space can be found in the secondary pore structures, such as caves and fractures, formed by paleokarstification. Evaluation of karstification is an important area of research for evaluation of carbonate reservoirs. Based on processing of a large set of imaging logging data from the Lungu area, and mainly through analysis of the imaging logging response of caves and fractures, karstification rules are determined for the area. In imaging logging, a high conductivity fracture often manifests as a brown-black sine curve, generally with higher continuity; a corrosion fracture generally corresponds to a sine curve with irregular edges; a unfilled cave appears as dark clumps with local light colored clumps; additionally, fully filled caves, bedding and other sedimentary phenomenon can be observed in the resulting imaging logging map. The different levels of karstification can be identified by fractures and breccias in caves, which are interpreted by imaging logging. In western Lungu,the breccias in some large caves have been completely corroded and were filled with mud and sand, indicating a high level of karstification in this area. In central Lungu region, where some caves have greater amounts of breccia, the size of the cave produced by the corrosion of a single strike fracture is clearly smaller than that in western Lungu. This indicates weaker karstification in central Lungu compared with the western region. In eastern Lungu, small caves have not been produced by the corrosion of single strike fractures, only proto-caves or cross cracks and karst fissures can be observed by imaging logging. Statistical analysis of the number of caves in Lungu area indicates that the distribution of cave reservoirs is mainly affected by epigenetic karst zones.