Online Journal for E&P Geoscientists

Al Hajri, Aisha A.^*1; Hollis, Cathy ¹; Amthor, Joachim ²; Warrlich, Georg ²
(1) School of Earth Atmospheric and Environmental Sciences, The University of Manchester, Manchester, United Kingdom. (2) Petroleum Development Oman, Muscat, Oman.

Understanding the nature of microporosity in micritic limestone hydrocarbon reservoirs is important as it contributes to reservoir heterogeneity. The microporous nature of the Aptian Shuaiba Formation has been reported and well studied in eastern United Arab Emirates, but in Oman very few studies have been undertaken and as a result the origin and the genesis of the limestone-hosted microporosity are not well established. This paper focuses on characterization of microporosity in the uppermost strata of the Shuaiba Formation (Aptian), below the Aptian-Albian unconformity in several oil fields in Northern Oman.

This regional, core-based study aims to qualitatively and quantitatively characterize the microporosity within rudist, Lithocodium-Bacinella and Orbitolina micritic-lithofacies using integrated thin sections petrography, point counting, geochemistry and core analysis.

Four micropore types have been defined. These include matrix micropores, intraparticle (i.e. micro-pelomouldic and intraskeletal) micropores, and boring micropores. The point counting results show that the micropore types within the micritic limestones at the top of the Shuaiba Formation account for 60 %-100 % of the total pore space for samples with a porosity range between 14 %- 41 %. Back-scattered electron microscopy (BSEM) observations demonstrate that the matrix is composed of subhedral to euhedral crystals, which are less than 5µm in size. The majority of these crystals tend to have a granular texture and corroded edges. The occurrence of the latter is common adjacent to solution-enlarged microfructures and stylolites, suggesting their development in the burial realm. Key observations which support the development of micropores in the burial realm include: (1) petrographic relationships that demonstrate development of microporosity which is paragenetically younger than several phases of phreatic calcite cement (2) preservation of abundant micropores directly adjacent to pressure solution seams (3) microporosity development which postdates mechanical compaction and the generation and cementation of microfractures and tensional gashes within the reservoir facies.