Shallow marine carbonate sediments of the Mishrif Formation (Mid-Cenomanian to Early Turonian)<br>were deposited on a low energy ramp, before a Mid-Turonian relative sea-level fall. Depositional<br>environments vary from inner ramp to open mid-ramp, with very shallow rudist biostromes. In the<br>predominant mud-supported sediments (mudstones, wackestones…), the heterogeneity of reservoir<br>properties (e.g. porosity, permeability, pore access radii distribution…) is closely related to microtextures<br>of the micritic matrix. Microporosity is relatively constant, high (up to 35%) and represents up<br>to 98% of the total porosity. Permeability is low (below 1mD) to moderate (up to 100mD).<br>Using cathodoluminescence (CL), scanning electron microscopy and isotopic analyses, 240 samples<br>coming from seven cored wells of a Mishrif oil field have been studied to characterize the sedimentary<br>and diagenetic factors that have controlled reservoir properties.<br>Micritic facies with the best permeability (up to 100mD) and the higher pore threshold radius (PTR - up<br>than 0.5μm) generally show coarse, badly sorted and poorly luminescent micrites. These micrites are<br>spatially and chronologically associated with eogenetic phases indicating the development of an<br>important oxidizing vadose interval (up to 30m thick) below the Mid-Turonian exposure surface: (1)<br>endokarstic cavities; (2) rare poorly luminescent sparry low magnesium calcite (LMC) with low δ18O<br>and low δ13C; (3) corrosion gulfs on early spars. In this vadose zone, the development of coarse<br>(crystallometry > 2μm), poorly luminescent micrites with similar geochemical signature is explained by<br>the early dissolution of fine aragonite and HMC particles leading to a simultaneous overgrowth of LMC particles.<br>Below the vadose zone most of micritic facies are associated with low permeability and PTR (less than<br>10mD and 0.5μm, respectively). Micrites are finer (crystallometry less than 2μm), well sorted and<br>luminescent under CL. This micritic pole is explained by a mineralogical stabilization of micritic particles<br>that ends later, in poorly oxygenated waters, probably after the deposition of the Laffan shales that<br>sealing the Mishrif reservoir.


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