Microfacies analysis and depositional development of Shuaiba formation in the West Qurna oil field, Southern Iraq

Abstract

The Aptian carbonate succession in this study is represented by the Shuaiba Formation which was deposited during the Early Cretaceous. The present study is including petrography and microfacies analysis for four boreholes WQ-12, WQ-13, WQ-15, and WQ-178 within the West Qurna oil field. There are four major microfacies recognized in the succession of the Shuaiba Formation, bioclastic mudstones to wackestone, Orbitolina wackestone to packstone, Peloidal wackestone to packstone, Miliolids wackestone, and Planktonic and small benthic mudstone to wackestone. The four depositional environments deposited in the Shuaiba Formation are; shallow open marine, Semi-restricted, restricted, and outer ramp environments. The Shuaiba Formation was deposited during two stages of transgression and a high stand of sea level. The first during the change of deltaic—fluvial environment of the Zubair Formation which became open shallow marine environment of the lower part of Shuaiba Formation. The presence of restricted facies overlying the shallow open marine refers to still stand sea level and deposition of high stand deposits and marked the first maximum flooding surface (mfs1). After the high stand deposits and within the upper part the Shuaiba Formation, the facies appeared further deepening upward with observed outer ramp facies to mark the transgressive stage. The second time, overlying the semi-restricted facies for the deeper facies refers to the second stage of still standing sea level (high stand deposits) within the Shuaiba Formation and marks maximum flooding surface (mfs2). The high stand deposits continued shallowing upward until the sea level and fell in all parts of southern Iraq to deposit the deltaic-fluvial deposits of Nahr Umr Formation. This marks the regression stage and sequence boundary type which separates the Shuaiba Formation and Nahr Umr Formation.