Reservoir Quality Related to Diagenetic Development in the Carbonate Mishrif Formation: A Case Study from the X Oilfield, Southern Iraq

Abstract

The Middle Cretaceous Mishrif Formation is considered as the main productive reservoir, southern of Iraq. The rudist-bearing productive zones are characterized by high heterogeneity due to the effects of diagenesis and the depositional environment. The Mishrif Formation represents a carbonate succession which deposited through the Middle Cretaceous (Cenomanian-Early Turonian) within the main retrogressive depositional period succession. This research is identifying the effect of each diagenetic process on the reservoir properties by using the utility of core data and thin sections collaborated with the well logs data along the wells X-46, X-41 and X-42 in the X oilfield. The Mishrif Formation have been subjected into various diagenetic processes such as micritization, neomorphism, dissolution, cementation, dolomitization and pressure solution or stylolitization. Effect of eight diagenetic processes have been recognized in the Mishrif Formation, these processes have affected constructively and destructively on the reservoir quality of the Mishrif Formation; dissolution and fractures have been affected positively, while micritization, cementation, dolomitization, compaction, bioturbation and neomorphism have a negative affect. The porosity logs (Neutron, Density and Sonic) and Gamma ray log are interpreted to find the porosity and clay content to determine the reservoir quality. The porosity results show a wide variation, the primary porosity is decreased in the well X-46 at the depth of 2300-2450 due to the cementation while the secondary porosity is enhanced due to the dissolution and fractures. In well X-42, at the depth of 2225-2400 m there is an increasing in the porosity due to the increasing of grains to clay ratio, while in the X-42 at the middle to the upper parts, the porosity is very low due to cementation, compaction and micritization processes as well as the high content of clay. In general, the porosity is enhanced in the reefal zones due to the geometry nature of the pore throat which increasing both of the effective porosity and permeability.