Quantification of cm-Scale Heterogeneities in Tight-Oil Intervals of the Cardium Formation at Pembina, WCSB, Alberta, Canada.

Abstract

Abstract A detailed, fine-scale, reservoir characterization study of (light) tight oil rock intervals in the Cardium Formation is performed using an innovative combination of X-Ray CT Scan technology, hardness and profile (probe) permeability measurements. This study aims to quantify the geometry, degree of heterogeneity, and several other key properties, of individual microlithofacies within the highly bioturbated sandstone lithofacies. Contrasting bulk density values between these microlithofacies domains is identified using X-ray CT-Scan imagery reconstruction. Statistical analysis yields numerical information regarding geometry and connectivity of individual bodies for a given microlithofacies within the analyzed volume. Permeability, as estimated from a profile (probe) permeameter, and hardness of the samples, as quantified using a hardness index (from Leeb Hardness measurements), is investigated for a sub-centimeter 2D grid on slabbed surfaces. This detailed mapping allows the analysis of mm- to cm-scale correlations between rock quality and CT images. Identified microlithofacies include SS1 and SS2 (sandstones), SH1 (shale), and PB (pyrite-filled burrows). Permeability can exceed 50 mD in SS1 microlithofacies, but generally is less than 1 mD for SS2 and 0.01 mD for SH1. Volumetric reconstruction of these microlithofacies yields approximately 85% total sandstone content for an average sample from tight-oil Cardium lithofacies (which is higher than visual assessment of 50-60% sandstone). The sandstone ratio of SS2/SS1 is between 3 and 5; microlithofacies SH1 and PB have a patchy distribution through the core. SS1 appears as highly disturbed relics of very thinly bedded patches, and sandstone-filled, subhorizontally oriented, sub-centimeter scale burrows. Relatively higher values of bulk density and hardness index characterize SS2 and SH1 microlithofacies. The overall low permeability usually measured in this lithofacies can be attributed to the complex interplay between biological and physical sedimentary structures at the cm-scale, generating more or less randomly connected volumes with contrasting flow and storage capacity. This study provides a roadmap for more accurate property measurements and modeling at the core scale for this complex lithofacies of the Cardium Formation; we believe that this will also be useful for other tight oil reservoirs.