Facies Analysis and Depositional Environments of the Nahr Umr Formation in Rumaila Oil Field, Southern Iraq

The Nahr Umr Formation (Albian) was studied in the Rumaila oil field, southern Iraq. It consists of sandstone with high porosity and permeability, and contains a few shale overlaps that permeate the sandstone in the lower part of it, while it consists of shale and a few limestone rocks from the electrification in the upper part. The upper contact surface of this formation is conformable and graded with the Mauddud Formation. The limestone at the base of the Mauddud Formation may be on top of the black shale of the Nahr Umr Formation, and this formation is bounded from below by the Shuaiba Formation in an unconformable, which corresponds when the black shale of the Nahr Umr Formation sits on the limestone to form the Shuaiba Formation. The monocrystalline Quartz mineral is the major component of the Sandstone, with small percentages of polycrystalline quartz, feldspar, and rock fragments, according to the petrographical investigation. Mineral and textural maturity were present in the formation of rocks. Compaction, and cementation, were the most important processes that influenced the rock characteristics of Nahr Umr's sandstone rocks. Six lithofacies shale, lenticular bedded sandstone–mudstone, false bedded sandstone–mudstone, siltstone–shale, cross-bedded sandstone, and parallel and cross-lamination sandstone were identified as representing the different sub environments within the different parts of transitional environments (fluvial and deltaic depositional environments), which represented oscillations in sea level rise and stillstands of the relative sea level controlled by tectonics as sediments of the basin transitioned from freshwater to marine conditions.