Stratigraphic Reservoir Characterization of Ratawi Formation in Southern Iraq

The Early Cretaceous Ratawi Formation in the Arabian Plate consists of sandstone and carbonate reservoirs. In Iraq, the Ratawi Formation is productive in several oilfields of the Mesopotamian Basin in southern Iraq. Selected wells are studied by using well logs of the formation and using a sequence stratigraphic approach to predict the distribution of reservoirs in the Mesopotamian Basin. The combination use of Gamma-ray, Neutron, Density, and Sonic logs shows that the major lithologies of the Ratawi Formation are shale, limestone, and sandstone. The calculated effective porosity and volume of shale indicate that the best reservoir quality occurs in clean sandstone units, characterized by high effective porosity and low shale volume. Limestone units have lower reservoir quality as low effective porosity and high shale volume values are recorded. The effect of shale is recognizable in reducing reservoir quality as the shale volume decreases effective porosity. The sequence stratigraphic hierarchy of the Ratawi Formation includes three transgressive cycles. The regressive cycle (R1) hosts the main reservoirs that occur as prograding sandstone channels or bars. The cap units for these reservoirs consist of transgressive limestone or shale units. Due to the accommodation increase and weak sand influx of the middle and upper regressive cycles (R2 and R3), reservoir progradation shifted westward of the study area. The occurrence of isolated or stacked sandstone reservoirs as channels or bars increases the possibility of finding stratigraphic traps.