Quantitative and Qualitative Evaluation of Micro-Porosity in Qusaiba Hot Shale, Saudi Arabia

Abstract

The Lower Silurian Qusaiba Hot Shale (QHS) has long been known as the major source rock in the Paleozoic petroleum system of Saudi Arabia. However, during the last few years, special focus has been directed toward this formation as a candidate for unconventional shale gas reservoir due to its high organic content, laterally extensive occurrence and reasonable vertical thickness. Yet, one of the main challenges is the lack of understanding of the size, distribution, and types of the micro-porosity systems in this organic-rich shale. This work provides an integrated analysis of the micro-porosity in QHS.Comprehensive sequential analysis includes thin section petrography, X-ray diffraction, scanning electron microscope (SEM) imaging, and nuclear magnetic resonance (NMR) analysis. In particular, these techniques were used to characterize the micro-porosity systems of six subsurface core samples of QHS from the Rub'al-Khali basin, Saudi Arabia.The investigated samples consist mainly of dark to light grey, silty, flaky-to-subflaky, micaceous, non-calcareous, highly-cemented shales. Three lithofacies have been identified from the petrographic analysis of QHS: silica-rich, clay-rich, and mica-rich mudstone facies. Micro-porosity in the studied samples reflects high diversity in both pore size and type. The micro-porosity fraction of the investigated shale comes in three major groups according to origin: (1) inorganic micro-porosity, which is commonly associated with the inorganic components of QHS (Silica-Mica -clay and pyrite mineral grains), particularly due to the random edge-face clay and mica flake orientation as well as the dissolution and degradation of pyrite framboids; (2) organic-related micro-porosity which mainly occurs inside the organic particles as a result of thermal maturity; and (3) microfracture-related micro-porosity.Low field NMR has been used to examine the T1 and T2 relaxation times. The total porosity measured by T1 for the three identified lithofacies ranges between 6.6 and 0.9 p.u., with the highest values corresponding to the mica-rich mudstone facies (6.6 and 6.5) and the lowest values corresponding to the silica-rich mudstone facies (0.9 and 1.4). As for the clay-rich mudstone facies, it had an intermediate porosity (3.2-3.3). The presence of bimodal porosity size in QHS is clearly reflected by the T1 results. NMR results also showed that organic-related porosity which represented by the primary relaxation peak is greater than the inorganic-related porosity, which is represented by the secondary relaxation peak.