‘Matthieu DEVILLE de PERIERE soutiendra sa thèse de Doctorat de l’Université de Dijon sur l’ :
« Origine sédimento-diagénétique des reservoirs carbonates microporeux : Exemple de la Formation Mishrif (Cénomanien) du Moyen-Orient ».
le 30 Juin 2011 à 14h00 à l’Université de Bourgogne
(6, Bd Gabriel – Dijon – Amphithéâtre d’Orbigny, RdC Aile Sud).
Devant un jury composé de :
Prof. E. VENNIN (Directrice de Thèse – Université de Bourgogne)
Dr. C. DURLET (Co-Directeur de Thèse – Université de Bourgogne)
Prof. M. FLOQUET (Rapporteur – Université de Provence)
Dr. J.P. GIRARD (Rapporteur – Total)
Dr. B. CALINE (Examinateur – Total)
Prof. J.F. DECONINCK (Examinateur – Université de Bourgogne)
Dr. J. RICHARD (Invité – Université de Franche-Comté)
Résumé et mots clefs :
Microporosity may account for as much as 95% of the total porosity of hydrocarbon and water reservoirs in Cretaceous limestones of the Arabian Gulf. In these microporous facies porosity is moderate to excellent (up to 35%) while permeability is poor to moderate (up to 190mD). 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 mainly collected from the Cenomanian Mishrif Formation, but also from the Berriasian-Valanginian Habshan Formation, so as to examine the wide vertical and lateral discrepancies in their petrophysical parameters. Scanning Electron Microscopy 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. The morphometric data are
compared with three petrophysical parameters (porosity, permeability and pore threshold radius distribution). Results reveal that micrite matrixes can be subdivided into three petrophysical classes each with its own distinctive crystallometry, morphology and reservoir properties. Class C (strictly microporous limestones with coarse punctic-to-partially coalescent micrites) is made up of coarse (>2µm) polyhedral to rounded micritic crystals, it has good to excellent porosity (8–28%), 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-rich bioclastic shoal facies where several sedimentary factors (hydrodynamism, bioproduction …) would disfavour deposition of the finer micritic crystals. Diagenetic study shows that the development of coarse micrites (Class C) must also be explained by the early dissolution of fine aragonite and high magnesium calcite particles in oxygenated meteoric fluids lead
ing to a simultaneous in-situ overgrowth on LMC particles at the top of the meteoric phreatic lens. These processes induce an increase of the crystallometry of micritic particles, an early lithification of the carbonate mud, and so the mineralogical stabilization of coarse Class C micrites. Class F (strictly microporous limestones with fine punctic-to-partially coalescent micrites) is composed of fine (<2µm) polyhedral to rounded micrites with poor to excellent porosity (3–35%), but permeability values of less than 10mD and a mean pore threshold radius of less than 0.5µm. It is mostly observed in sediments deposited in a low energy muddy inner platform setting. The formation of fine micrites (Class F) is also explained by an early mineralogical stabilization of micritic particles in confined meteoric waters, favoring neomorphism processes, which may proceeds during burial. Later, during burial, reservoir properties of classes C and D strictly microporous samples where locall
y enhanced by mesogenetic dissolution (probably due to organic acids) affecting the microporous matrix during the oil emplacement. Class D (strictly microporous mud-dominated facies with compact anhedral to fused dense micrites) comprises subhedral to anhedral crystals with sutured to fused 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 usually in association with stylolites and clay contents that exceed 10%. Regardless of how they formed, though, the three classes can be usefully incorporated into future rock-typing of the microporous carbonate reservoirs of the Middle East.
Key words: Micrite, Microporous, Crystallometry, Morphometry, Diagenesis