Geological features of grain bank reservoirs and the main controlling factors: A case study on Cretaceous Mishrif Formation, Halfaya Oilfield, Iraq

The geological features and distribution regularity of the grain shoal reservoirs of the Cretaceous Mishrif Formation in Halfaya Oilfield and their main controlling factors were analyzed based on cored data, 302 pieces of thin sections and 2507 experiment data points. Various types and multiple phases of grain shoal reservoirs are developed in Mishrif Formation, which are dominated by grainstone, followed by wackestone packstone. The formation and distribution of grain shoal reservoirs are mainly controlled by relative sea level change, paleogeomorpholoy and contemporaneous-penecontemporaneous dissolution. Relative sea level change controls the types and development phases of grain shoals. When the sea level falls, rudists, coquina, arene and pelletoids are extensively deposited, among which, rudists clastic shoal and coquina shoal that are deposited at the lowest sea level have the best reservoir properties, with a maximum porosity of 33.94%, maximum permeability of 764.571×10−3 μm2 and an average permeability of 45.81×10−3 μm2. Paleogeomorpholoy controls high-energy facies belt and the dissolution of meteoric water. The submarine paleohigh controls the formation and distribution of high-energy facies belt during depositional period. However, during uplifting and denudation period, the reservoirs in paleohigh are intensively dissolved and represent good reservoir properties, with an average porosity of 14.78% and permeability of 7.849×10−3 μm2. Contemporaneous-penecontemporaneous dissolution results in the formation of most effective pores, which is the key factor for the formation of grain shoal reservoirs in the study area.