Microfacies and Depositional Model of the Shuaiba Formation (Lower Cretaceous), in Selected Wells, Southern Iraq

Abstract

The Shuaiba Formation (Aptian age) is an important formation due to its wide extent in Iraq and Arabian Gulf. In northern Iraq, it is called Qamchuqa Formation and has an important role in contracting the Arabian plate. The study focused on several wells, which are: ZB-290, WQ1-353, RU-358, R-624, and R-5 from several selected fields, such as Zubair, Rumaila, and West Qurna in southern Iraq. The purpose of the study is to determine the sedimentary and paleontological components of the Shuaiba Formation. The lithology of the Shuaiba Formation consists of very fine crystalline massive limestone that gradually converts to chalky limestone with a large percentage of dolomites, especially in the middle part of the formation. The thickness of the studied formation is about 50 – 110 meters and its main microfacies are two types, which are carbonate and dolomite. The carbonate is subdivided into five secondary types, which are burrowed bioclastic lime mudstone, planktonic foraminiferal lime mudstone, larger foraminifera wackestone, algal wackestone and mixed planktonic, and benthonic Foraminifera wackestone, in addition, to dolostone microfacies. The effect of the diagenesis in the formation is distinguished and most of these processes are destructive, such as dissolution and stylolite. The first depositional stage of the investigated area began with sea level rise following the deposition of the Zubair Formation from still stand. Shuaiba Formation deposited within the ramp setting as the water level rose. During the late Aptian, tectonic and sea levels were the active factors that controlled the deposition of the formation. According to data collected from microfacies and diagenesis processes, the Shuaiba Formation has several depositional environments, which are from the shallowest: restricted, open marine (inner ramp), mid ramp, and finally outer ramp. Generally, toward the upper sequence, the formation reflects deep deposits and then starts to be less deep until it is a shallow basin (Nahr Umr).