High-resolution sequence stratigraphy and microfacies analysis of the lower part of the Sa'di Formation in selected wells from the West Qurna oilfield, southern Iraq

Abstract

Tectonic and eustatic changes have had major impacts on the Cretaceous stratigraphy and its hydrocarbon potential in southern Iraq, and the Sa'di Formation is one of several Cretaceous carbonate plays that reflect these types of changes. To further examine the influence of these impacts, thin sections from cores, as well as wireline and image logs, from the lower part of the Sa'di Formation in four wells from the West Qurna Oilfield were examined. Wackestone, packstone, grainstone and floatstone microfacies groups were identified and subdivided into ten submicrofacies. The microfacies were deposited at several environments such as open marine, restricted and outer ramp, as well as to position in transgressive and highstand sequence tracts in two fourth-order parasequences. Moreover, microfacies were compared with Formation Microresistivity image (FMI) logs and show that image logs can be used to recognize microfacies and their likely environments. Integration of these analyses with paleogeography and regional tectonics suggests that the lower Sa'di Formation reflects early deposition in a newly formed foreland basin that represents ongoing subsidence accompanying collision and closure of the Neo-Tethys Ocean. The carbonate-rich nature of the unit is linked to deposition on the distal margin of the foreland basin in an arid climate, while cyclicity in the unit represents both tectonic and eustatic fluctuations.