Geological Factors Controlling Reservoir Properties in Microporous Carbonates - Example of the Mishrif Formation (Upper Cenomanian) of the Middle East

Matthieu, Deville de Periere¹; Christophe, Durlet ¹; Emmanuelle, Vennin ¹; Caline, Bruno ²; Laurent, Lambert ²; Raphael, Bourillot ¹; Carine, Maza ²; Poli, Emmanuelle ²; Pabian-Goyheneche, Cecile ²
(1)Biogeosciences, University of Burgundy, Dijon, France. (2) Total E&P, Pau, France.

Shallow marine carbonate sediments of the Mishrif Formation (Mid-Cenomanian) were deposited on a low energy ramp, before a Mid-Turonian relative sea-level fall. Depositional environments vary from inner platform to very shallow bioclastic rudist-rich shoals.Using cathodoluminescence, scanning electron microscopy and isotopic analyses, 240 samples coming from seven cored wells of a Mishrif oil field have been studied to characterize the sedimentary and diagenetic factors that have controlled reservoir properties of microporous sediments.

In the predominant mud-supported sediments (mudstone and wackestone), the heterogeneity of reservoir properties (porosity, permeability, pore access radius distribution) is closely related to micro-textures of the micritic matrix (morphology and crystallometry of particles). Microporosity ranges from 15 to 35% and represents up to 98% of the total porosity. Permeability is low (below 1 mD) to moderate (up to 100 mD).

In the current oil zone, micritic facies with the best permeability (up to 100 mD) and the higher pore threshold radius (PTR - up than 0.5µm) generally show coarse (crystallometry > 2 µm) micritic particles with poorly luminescence. These micrites are spatially and chronologically associated with eogenetic phases. These phases include endokarstic cavities, poorly luminescent early spars with an initial low Mg content (LMC), as well as low δ18O, and δ13C and corrosion gulfs. These features indicated the development of an important oxidizing leaching interval (up to 45m thick) below the Mid-Turonian exposure surface. In this zone, the development of coarse, poorly luminescent micrites with similar geochemical signature is explained by the early dissolution of fine aragonite and high magnesium calcite particles leading to a simultaneous overgrowth on LMC particles.

Below the leaching zone, most of the micritic facies are associated with low permeability and PTR (less than 10 mD and 0.5 µm, respectively). Micritic particles are fine with homogeneous crystallometry, and generally display bright luminescence. This is explained by a latter mineralogical stabilization of micritic particles, which proceeds during burial in poorly oxygenated waters. Coarse micrites are also observed in this lower zone, but only in association with bioclastic rudist-rich shoals and back-shoals facies, suggesting an influence of depositional environments through initial crystallometry or initial mineralogy.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.