Facies Analysis and Diagenetic Effects on the Nahr Umr Reservoir in Ratawi Oil Field, Southern Iraq

One of the primary producing reservoirs in southern Iraq is the Nahr Umr Formation, considered one of the most important geological formations during the Cretaceous period. Based on well log interpretation (GR, RHOB, NPHI, Rt, and Rxo), three main reservoir units were identified: the upper Nahr Umr member, which is composed of rocks that are mostly shale; the middle Nahr Umr member, which consists of interbedded sandstone and shale; and the lower part of Nahr Umr Formation, which is characterized by rocks composed mainly of sandstone. This lower section represents a major reservoir unit in the formation with excellent reservoir parameters and high sand thickness. Five wells in the Ratawi oil field (Rt-2, Rt-14, Rt-17, Rt-20, and Rt-23) were selected for core and thin section studies. Petrography and lithofacies analyses were used to determine the sedimentary environments of the Nahr Umr Formation. Compaction and cementation (calcite, silica, and iron cement) decrease the primary porosity, while dissolution and replacement processes make the rocks more porous and permeable by dissolving the mineral components and creating secondary porosity. Five facies are distinguished in the Nahr Umr Formation: cross-bedded sandstone, flaser-bedded sandstone-siltstone, interbedded bioturbated silty sandstone-fissile shale, interlaminated shale and siltstone, and laminated heterolithic siltstone with shale. These were determined to be representative of the various subenvironments found in different depositional settings (shallow marine, delta, and fluvial).