Features and Potential of the Ordovician Fracture-Cavity Carbonate Reservoirs in the Northern Tarim Basin, China

Abstract

Summary Epigenic karst and deep hydrothermal dissolution both resulted in fracture-cavity reservoirs in the northern Tarim Basin, China. Through the combination of seismic interpretation (spectral decomposition and attribute fusion), outcrop observation, core and thin sections description, well logging analysis, the characteristics and distribution patterns of the fracture-cavity carbonate reservoirs with the two genetic types (epigenic karst and hydrothermal dissolution) were fully studied. Epigenic karst was mainly developed in the Ordovician Lianglitage formation and resulted in typical karst landform and subsurface reservoirs. Evidence in cores, thin sections, seismic attributes and oil production shows that this type of reservoir is of poor quality due to collapse, filling and strong compaction at depth. Hydrothermal karst reservoirs were formed by hydrothermal dissolution and subsequent collapse during burial. They mainly exist in Ordovician Yijianfang Formation and Yingshan Formation, and have quite different features from the epigenic karst reservoirs in cores, thin sections, seismic attributes and oil production, which means they are favourable reservoirs. The main targets of exploration and development in this area are the hydrothermal karst reservoirs and the epigenic karst reservoirs are suggested to be avoided after effectively identified and distinguished from hydrothermal karst reservoirs.