An integrated characterization of the porosity in Qusaiba Shale, Saudi Arabia

A comprehensive sequential analysis including thin section petrography, Rock-Eval pyrolysis, X-ray diffraction, scanning electron microscope (SEM) imaging, and nuclear magnetic resonance (NMR) analysis has been used to characterize the porosity of the organic-rich Qusaiba Shale, Saudi Arabia. The studied samples consist mainly of dark to light grey, silty, flaky-to-subflaky, micaceous, non-calcareous, highly cemented shales. Three lithofacies identified from the petrographic analysis of the Qusaiba Shale include silty-clayey laminated mudstone, clay-rich mudstone, and mica-rich mudstone lithofacies. XRD analysis documented that the main minerals of the QS are: quartz, K-feldspar, muscovite, illite, chlorite, as well as minor amounts of pyrite and albite. The studied samples have a total organic content (TOC) ranging from 1.4% to 1.82% and Tmax ranges between 472 °C and 529 °C. SEM analysis showed that the QS hosts a wide range of pore sizes and types. The analysis allowed to identify three groups of porosity: inorganic-related, organic-related and microfracture-related porosity. Quartz, clay, mica and pyrite mineral grains are the main particles hosting the inorganic related porosity. The pore size of organic-related porosity type is typically less than 1 µm, with a scattered, isolated, pinpoint shape. NMR relaxation time T2 of the studied samples indicates the presence of a porosity range between 1.1 and 2.8 p.u. with the maximum value attributed to clay-rich mudstone lithofacies and the minimum to silty-clayey laminated mudstone facies. Additionally, the longitudinal relaxation time T1 gives a porosity range between 6.6 and 0.9 p.u. with the highest value corresponding to the clay-rich mudstone lithofacies and the lowest values to silty-clayey laminated mudstone lithofacies. The T1 spectrum exhibits a bimodal configuration with a dominant primary peak at less than 1 ms and a secondary peak located around 10 ms. Both T1 and T2 reveal the contribution of organic-related microporosity to the total porosity of QS as previously indicated by SEM analysis. However, inorganic-related porosity, particularly in the mica and clay particles, also contribute to QS porosity.