Sedimentary diagenesis of rudist shoal and its control on reservoirs: A case study of Cretaceous Mishrif Formation, H Oilfield, Iraq

Based on the core, cast thin section, whole rock analysis, conventional physical properties and high pressure mercury intrusion test, the sedimentary diagenesis characteristics of rudist shoal in Cretaceous Mishrif Formation of H Oilfield, Iraq and its control on the reservoir were studied. The rudist shoal of the Mishrif Formation develops in the high-stand systems tract and is distributed in the high places of paleogeomorphology on the edge of platform with strong hydrodynamic force. According to the relative sea level changes, lithologic evolution and sedimentary structure characteristics of the rudist shoal, the single rudist shoal is divided into four lithologic sections: A, B, C and D, that is, low-angle cross-bedding pelletoids-rudist packstone, low-angle cross-bedding and parallel bedding arene-rudist grainstone, parallel bedding rudist gravel limestone, and horizontal bedding carbonaceous mudstone. The complete sedimentary sequence of a single rudist shoal is often disrupted. Several rudist shoals superimpose to form thick rudist shoal sediment. The single rudist shoal thickness and lithologic sections assemblage change regularly in vertical direction. The rudist shoal has the characteristics of “strong dissolution, weak cementation and strong compaction”, forming pore-type reservoir with intergranular pores, intergranular dissolved pores, mold pores, and dissolved pores. With mainly coarse pore throats larger than 5 μm, the reservoir is of medium-high porosity and high permeability. There is lithological reverse cycles inside single shoals and between single shoals, with content of mud crystals decreasing from the bottom to the top, dissolution increasing, cementation decreasing in strength, pore throats getting larger, and physical properties turning better. The rudist shoal of MB2-1 at the top of the high-stand systems tract has the largest thickness, moreover, subject to the strongest atmospheric freshwater leaching, this layer has the most significant dissolution and the largest pore throat, so it is the best reservoir of the Mishrif Formation. Key words: Iraq, Cretaceous, rudist shoal, sedimentary process, diagenetic evolution, reservoir characteristics