Online Journal for E&P Geoscientists

Al Hajri, Aisha A.^*1; Hollis, Cathy ¹; Van Boxel, Steven ²; Amthor, Joachim ³; Warrlich, Georg ³
(1) School of Earth, Atmospheric and Environmental Sciences, The University of Manchester, Manchester, United Kingdom. (2) HMXIF School of Materials, The University of Manchester, Manchester, United Kingdom. (3) Petroleum Development Oman, Muscat, Oman.

Description of carbonate pore systems has traditionally utilized two-dimensional (2D) images generated from thin-sections and back-scattered electron microscopy (BSEM). Although these methods can be used to quantify pore size and shape in 2D, the pore connectivity cannot be constrained. X-ray computed tomography (CT) scanners provide a powerful technology for multi-scale, non-destructive visualization of pore systems in three dimensions (3D). A range of parameters such as pore volume, pore shape, pore throat size, and pore connectivity (co-ordination number) can be computed from digitized images of the scanned samples. This technology is used to understand the complex and heterogeneous pore networks encountered in the uppermost strata of the Shuaiba Formation in several oil fields in Northern Oman below the Aptian-Albain unconformity.

As part of the effort to establish a relationship between different pore types and permeability within three lithofacies (i.e. rudist, Lithocodium-Bacinella, and Orbitolina-dominated lithofacies), X-ray CT scanning of core samples was undertaken at macro, micro and nano scales. The output of the digital image analysis of these CT scans has been integrated and calibrated with thin sections petrography, BSEM, and core-based porosity, permeability, and mercury-injected capillary pressure measurements.

Helium porosity and permeability values for all the samples that were obtained from the top 15m of the Shuaiba reservoir range between (2- 45) % and (0.01-1738) mD, respectively, with the best reservoir quality within the rudist lithofacies. Thin-sections petrography, BSEM and CT- images reveal a complex pore system in the grain-dominated rudistid lithofacies, involving moulds, vugs, matrix micropores, secondary intraparticle pores, solution-enlarged microfractures, and primary intraparticle and interparticle pore spaces. This complexity can be related to different phases of dissolution, with significant porosity generation probably in the burial realm. The matrix-dominated Lithocodium-Bacinella and Orbitolina lithofacies, on the other hand, show relatively less complexity driven mainly by patchy neomorphism of the matrix with comparatively higher occurrence of micropore types. The 3D pore network model constructed from digital images show poor connectivity between macropores but relatively higher connectivity between micropores; the majority of the pore network is within the microporous pore spaces.