Classification and Main Controlling Factors of Low Permeability Carbonate in the Lower Part of Cretaceous Mishrif Formation, W Oilfield, Iraq

The lower part of Mishrif Formation of W oilfield in Iraq is a typical low-permeability carbonate reservoir with large oil reserves, but there are few related researches. Based on the core, logging and laboratory analysis data, their reservoir characteristics, main control factors, and distribution have been studied. The results show that reservoirs in the lower mB2L and mC layers have low permeability with an average of 6.7 mD. Three reservoir types can be divided, of which reservoir I has the best reservoir quality, mainly including bioclastic grainstone and bioclastic limestone with intergranular pores and mold holes, mostly developed in the margin of the platform. The reservoir quality of reservoir II is the second, mainly including bioclastic packstone and mud-lean packstone with intergranular pores and biological cavity pores. Reservoir III has the worst reservoir properties, mainly including wackestone and packstone with intraparticle pores and micropores, which are mostly developed in shallow water shelf. The reservoir quality of low permeability reservoirs is mainly controlled by the sea level and diagenesis. During their formation, the sea level was always at a high position, and the water energy was maintained at a low level, which made it difficult to form high-quality reservoirs. When the sea level dropped, the water energy near the platform edge increased, which was conducive to the formation of grainstone. At the same time, the adjacent reservoirs underwent bedding dissolution, forming a large number of dissolution holes. Reservoir III distributes most in the lower Mishrif Formation, while reservoir I and II are mainly distributed in strips, gradually increasing from bottom to top. Meanwhile, the distribution of reservoir I in the north is wide and the thickness is large. This study can provide theoretical support for further exploitation and high-quality reservoirs prediction of low-permeability carbonate reservoirs.