Microfacies Analysis and Depositional Environments of Lower Sa’adi Formation, Southern Iraq

Abstract

The Sa’adi Formation is a part of the late Cretaceous period in the Santonian-Campanian stage that represents a potential hydrocarbon-bearing reservoir across many oilfields in the Mesopotamian Basin, South of Iraq. The Formation was divided into two main parts as a stratigraphy sequence. It consists of chalky limestone with argillaceous limestone in the upper part and limestone with marly limestone in the lower part. The lower part is considered an important stratigraphic unit marked by petroleum shows. Thus, current research constructs the depositional environment, evaluates the reservoir, and predicts the best zones with good reservoir quality. The microfacies analysis was carried out on thirty-five thin sections to reveal the primary depositional environment, and well logs data were used to evaluate the petrophysical properties of the lower Sa’adi Formation. Four microfacies appeared related to the carbonate ramp, which identified the depositional system track from mid to inner ramp. These are; mudstone, wackestone, packstone, and grainstone. Twelve sub-microfacies were identified and interpreted in the lower Sa’adi Formation. These are pelagic lime mudstone to benthic foraminiferal-argillaceous wackestone in middle ramp experienced burial diagenesis and syngenetic diagenesis with intra-fossil pores. The results characterize the bioclast echinoderms, bivalves, and algae packstone to grainstone in inner ramp (open marine and shoal environments) experienced marine pore-water diagenesis, meteoric freshwater dissolution, and burial diagenesis. Shoal facies with open marine facies are the best favorable microfacies in the lower Sa’adi Formation. Diagenesis processes were represented by dissolution that improved the porosity and permeability with higher reservoir quality in the inner ramp; besides that, it was recognized that cementation and micritization reduced the reservoir quality in the middle ramp.