High resolution sequence stratigraphy of the Mishrif Formation (Cenomanian-Early Turonian) at zubair oilfield (al-rafdhiah dome), southern Iraq

Abstract

The Mishrif Formation (Cenomanian -E Turonian) is one of the most important geological formations in the Middle East and Iraq because it contains enormous petroleum accumulations. It is considered to be the first reservoir in the region, and is still being studied because of its economic significance. The carbonate of the Mishrif Formation derives from a variety of depositional settings, including mid-ramp, shoal, lagoon, and intertidal. The five main microfacies discussed in this paper are wackestone, packstone, grainstone, floatstone, and bindstone. The most frequent fossilised components found in the Mishrif Formation are rudists, benthic foraminifera, echinoderms, burrows molluscs, and algae. According to the microfacies and analysed wireline log data, the sequence stratigraphy of the studied formation is composed of two regression cycles. Five parasequences of transgressive–regressive cycles make up the depositional sequence of the Mishrif Formation. The standard depositional environments seem to demonstrate a gradual regression, beginning with a short period of the outer ramp, then a steady period of the mid-ramp, and ending in the intertidal environment. Additionally, the study recorded two regional maximum flooding surfaces: K-135 and K-140. The former is present in the lowermost part of the formation, while the other lies in the middle. This study shows a close relationship between facies (environments) and hydrocarbon accumulation. The increased accumulation focuses on the lower part of the studied formation, and seems to be lower in the upper part of the formation as a result of changes in the environment of deposition.