MID‐CRETACEOUS RUDIST‐BEARING CARBONATES OF THE MISHRIF FORMATION: AN IMPORTANT RESERVOIR SEQUENCE IN THE MESOPOTAMIAN BASIN, IRAQ

Abstract

This paper reports on the sedimentology and stratigraphy of the mid‐Cretaceous Mishrif Formation, one of the principal carbonate reservoirs in Central and Southern Iraq. The Cenomanian Mahilban, Maotsi and Fahad Carbonate Formations of Central Iraq are the lateral chronostratigraphical equivalents of the Mishrif and underlying Rumaila Formations of Southern Iraq. Together, these units represent a single mid‐Cretaceous carbonate succession in the Mesopotamian Basin.The Mishrif Formation in Central Iraq reflects the continuous deposition of shallow‐shelf carbonates; periodic rises in sea level led to episodes of deeper‐water sedimentation, during which the outer‐shelf and basinal deposits of the Rumaila Formation were laid down. A ramped platform was the principal depositional setting for the entire Cenomanianearly Turonian carbonate succession.The best reservoir conditions in the Mishrif Formation occur in rudist‐bearing facies, such as rudstones and rudistid packstone/grainstones. Reservoir units are characterised by porosities of >20% and by permeabilities of 100 mD to 1 Darcy. Other carbonate facies, such as pelagic mudstone/ wackestones, bioclastic wackestones and peloidal packstones, are less significant as reservoir rocks. All the carbonates were affected by a range of diagenetic processes, among which dissolution and dolomitization led to the formation of secondary porosity; porosity was reduced by compaction, stylolitization, micritisation, neomorphism and cementation.The Mishrif Formation is divisible by a prominent unconformity into two large‐scale regressive sequences, which are particularly distinguishable in the east of the Mesopotamian Basin. Multiple reservoir units are present in both sequences. The west of the basin is dominated by the lower sequence, which has relatively few reservoir intervals. The shallow‐water reservoir units in the east of the basin are thick, reflecting relatively high subsidence rates throughout the Cenomanian (e.g in the Amara oilfield and nearby areas). Subsidence rates in the western side of the basin were lower, and reservoir units are thinner and more limited. The Mishrif Formation carbonates wedge‐out in the western and SW deserts of Iraq.