Sedimentology and Diagenesis and Their influence on the Reservoir Quality: A Case Study of the Late Cenomanian Mishrif Formation, Western Region, Abu Dhabi

Abstract

The Cenomanian-early Turonian Mishrif Formation is an important reservoir rock throughout the Middle East. Although extensive studies have been carried out on the Mishrif Formation across the Middle East, very limited information is available about the Mishrif Formation from the western region of Abu Dhabi. The primary objective of this study is to review the sedimentology of the Mishrif Formation in the oilfields of the western region of Abu Dhabi and to define the petrology, the depositional and diagenetic history, and the reservoir quality. The Mishrif Formation is sedimentologically characterized using an industry-standard lithofacies classification scheme. Subsequent sedimentological interpretations define the depositional evolution of the sediments. Petrologic assessment highlights the depositional texture and successive diagenetic alterations that have affected the sediments, thereby modifying the reservoir properties. The Mishrif sediments represent deposition on a carbonate ramp system modified by a potential barrier shoal, along with localized relief developing shoal-like geobodies. A broad range of lithofacies represent the range from wackestone to boundstones, with the sediments interpreted to represent deposition in a shoal complex through backshoal and rudist buildups to a distal mid-ramp to an outer ramp setting. A petrographic study reveals multiple diagenetic phases have affected the sediments. Porosity is enhanced considerably due to multiple leaching phases, which are followed by subsequent cementing phases comprised of calcite and dolomite that degrades the porosity in places. Fracture events do not seem to have any significant impact towards enhancing the reservoir properties as they remain dominantly filled by cements but locally, a few fractures, which are open, contribute to enhancing the connectivity. A moderate to very good reservoir quality is inferred for the sediments of the Mishrif Formation. The porosity values range between 7% to 34% and permeability values from 0.06mD to >10 D. Interrogation of the core analysis data indicates a depositional control along with a diagenetic influence on the reservoir quality. This study helps to resolve the complexities associated with the sedimentological makeup and diagenesis of the Mishrif Formation in the western region of Abu Dhabi. The understanding from this study will help in further works related to reservoir geomodelling and pattern prediction, which may also be extrapolated to uncored intervals for representativeness and regional correlation of the Mishrif Formation.