Karst types and characteristics of the Ordovician fracture-cavity type carbonate reservoirs in Halahatang oilfield, Tarim Basin

In recent years, fracture-cavity type carbonate rocks in Tarim Basin have attracted much attention because of the continuous discovery of ultra-deep resources. However, for this type of reservoir, there are still problems such as unclear genesis and characteristics and lack of effective guidance for exploration directions. Based on seismic data interpretation, in combination with data investigation, investigation of field outcrop, core and thin section observation, geochemical characteristics analysis and production characteristics analysis, the authors have conducted an in-depth research into the karst types and characteristics of Ordovician fracture-cavity type carbonate reservoirs in Halahatang oilfield. It is believed that two types of karst reservoirs, epigenetic karst and hydrothermal karst, are developed in the Halahatang area; the both have obviously different features. Epigenetic karst reservoirs are mainly developed in Lianglitage Formation. Typical karst landforms are formed on the top of Lianglitage Formation, and a horizontal tubular cave system is formed inside; the caves are filled, collapsed and strongly compacted, resulting in tight reservoirs and relatively poor reservoir properties. The hydrothermal karst reservoirs are mainly developed in the upper part of Yijianfang Formation and Yingshan Formation, and vertical caves are formed along faults. This type of caves and their collapsed bodies keep plenty of reservoir space, shown as the beads-like strong amplitude on the seismic profile, causing frequent drilling break and leakage; they are the main reservoir bodies of Ordovician in Halahatang oilfield. Hydrothermal karst reservoirs are the main target of exploration and development in the study area at this stage, and epigenetic karst reservoirs should be avoided reasonably. © 2021, Editorial Office of ACTA PETROLEI SINICA. All right reserved.