Reservoir Characteristics of Ratawi Formation in Southern Iraq

The Lower Cretaceous Ratawi Formation in southern Iraq and throughout the Arabian Plate region comprises sandstone and limestone units with important hydrocarbon reservoir potential. Three wells in the Mesopotamian Basin in south Iraq have been selected to study the key differences in some petrophysical properties of sandstone, argillaceous sandstone, and limestone units. The Density, Neutron, and Gamma Ray logs are applied to identify the lithology of the Ratawi Formation. The Density-Neutron logs cross plot shows the association of sandstone, limestone, and shale lithologies. The presence of the shale lowers the quality of reservoir properties of the Ratawi Formation, as demonstrated by the inverse relationship between effective porosity and volume shale. The evaluation of well logs shows the best reservoir quality in clean sandstone units, which are characterized by the lowest values of shale volume and maximum effective porosity. Although limestone units have low shale volume, they may have low total and effective porosity due to the effects of compaction and cementation. The shale volume and effective porosity values overlap for argillaceous sandstone and limestone units. However, the former shows better reservoir quality due to higher average values of effective porosity. The geological analysis of porosity-permeability cross plots for the Ratawi Formation shows trends for different textures and diagenetic overprints that affect sandstone and carbonate reservoirs. Coarse-grained and grain-supported textures with negligible compaction and cementation effects characterize the best reservoirs.