--> Rudists Reservoir Characterization in Middle Cretaceous Mishrif Formation of Halfaya Oilfield, Iraq

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Rudists Reservoir Characterization in Middle Cretaceous Mishrif Formation of Halfaya Oilfield, Iraq

Abstract

The extinct rudists sedimentary was one of the most typical characteristics of Cretaceous, which is the main reservoir of Middle Cretaceous Mishrif Formation in Halfaya Oil Field, Iraq. The principle sedimentary facies of Mishrif Formation was a rimmed platform, in which the platform margin rudists shoal/reef were the favorable sedimentary settings. The main lithofacies in Mishrif Formation are: rudists reef limestone, coexisted coral reef limestone, sponge reef limestone, algal reef limestone and reef breccias. 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. Topmost of the Mishrif Formation (same as the boundary of Middle to Late Cretaceous) is an obvious unconformity, which caused the fabric-selective meteoric dissolution in rudists of Mishrif Formation. 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. Because the aragonite shells of rudists are easy to dissolve, there is positive correlation between the aragonite contents and the porosity of rudists in Halfaya Oil Field. 5 kinds of main pore types related to rudists were distinguished: (a) inter-grain/inter-breccia pores, (b) framework pores, (c) vuggy intraskeletal and biomoldic pores, (d)dissolution pores and (e) caverns.