Fracture Characteristics and Its Influence on Oilfield Development of M Formation Carbonate Reservoir in A Oilfield, Southeast Iraq

The Cretaceous M formation carbonate reservoir is widely recognized as the primary productive reservoir in the southeastern oilfield of Iraq, with numerous studies considering the M Formation an exemplary porous carbonate reservoir. The M carbonate reservoir in the A oilfield in southeast Iraq was investigated through core observation, cast thin section analysis, physical property analysis, 3D seismic data, and production dynamics evaluation. The result shows that the M Formation in the A oilfield exhibits a significant abundance of high-angle structural compression fractures oriented from NW to SE, with a predominant occurrence of effectively opened fractures. These fractures serve as primary conduits for oil and gas migration. There are variations in the development of fractures, influenced by sedimentary rock types and tectonic compression, between plane and longitudinal orientations. The stress intensity is higher on the western side of the structure compared to the eastern side, resulting in significantly greater fracture development intensity and longitudinal depth on the western side. Sedimentary rocks with low water energy during their formation primarily consist of wackestone and micrite. These rocks exhibit low porosity and weak diagenetic transformation during later stages, making them prone to developing tectonic fractures under stress. The development of fractures significantly impacts the selection of production wells and water injection capacity in A oilfield. Similarly, horizontal production wells drilled into fractures exhibit 3–5 times higher productivity compared to directional wells, while water injection wells drilled into fractures possess much greater injection capacity than non-fractured wells. The above understanding is of great significance for the new well deployment and development plans in the study area, as well as playing a crucial guiding role for this type of carbonate reservoir.