Quantitative Characterisation of Microporous Micritic Matrix. Examples from Cretaceous of the Middle East
Matthieu Deville de Periere
Microporosity may account for more than 90% of the total porosity of hydrocarbon and water reservoirs in Cretaceous limestones of the Middle East. In these microporous facies porosity is moderate to excellent (up to 35%) while permeability is poor to rarely excellent (up to 190 mD). Conversely, microporous facies may form dense inter-reservoir or cap rock layers with very low porosity and permeability values (2–8% and 0.01–2mD, respectively). For this study, samples were collected from three oil-bearing reservoir within two Cretaceous Formations (the Berriasian/Valanginian Habshan Fm. and the Cenomanian/Mid-Turonian Mishrif Fm.) to examine the vertical and lateral discrepancies in their petrophysical parameters. Scanning Electron Microscopy (SEM) was used to investigate two potential controls of reservoir properties: (1) micrite particle morphology (shape and inter-crystal contacts); and (2) micrite crystallometry, defined as the median size of micrite particles measured on SEM photomicrographs. These data are compared with three petrophysical parameters (porosity, permeability and pore threshold radius distribution) and the results reveal that micrite matrixes can be subdivided into three petrophysical classes, each with its own distinctive crystallometry, morphology and reservoir properties.
Class C is made up of coarse (crystallometry >2μm) polyhedral to rounded micritic fabrics, which have good to excellent porosity (8–28%), generally poor to moderate permeability (0.2–190mD) and a mean pore threshold radius of more than 0.5μm. The class C is usually observed in rudist shoal facies where relatively high hydrodynamic energy disfavoured deposition of the finer micritic crystals. It also developed within meteoric leaching intervals below major exposure surfaces and ar ethus likely to be related to the early input of meteoric fluids.
Class F is composed of fine (crystallometry <2μm) polyhedral to rounded micritic microtextures with poor to excellent porosity (3–35%), but permeability values of less than 10 mD and a mean pore threshold radius of less than 0.5μm. Aquifer intervals generally fall into this class. It is mostly observed in sediments deposited in a low energy muddy inner platform setting. For classes C and F alike, there is a close correlation between increased crystallometry, permeability and pore threshold radius dispersion.
Class D (strictly microporous mud-dominated facies with compact anhedral to fused dense micrites) comprises subhedral to anhedral crystals with sutured contacts forming a dense matrix. It has very low porosity and permeability. Class D is only found in low energy muddy inner platform facies and forms inter-reservoir or caps rock layers in close association with stylolites and clay contents.
Deville de Periere, M, Durlet, C, Vennin, E, Lambert, L and Caline, B. (2013). Quantitative Characterisation of Microporous Micritic Matrix. Examples from Cretaceous of the Middle East. Paper presented at the SPE Advanced Carbonate Reservoir Characterisation Workshop, Dubai, UAE February 12-14th, 2013
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