Silurian qusaiba shale: Petrophysical, mineralogical and geochemical analysis

Abstract

The organic rich Lower Silurian Qusaiba Shale is the main effective source rock for most of the Paleozoic hydrocarbon in Eastern and central Arabia. Outcrop samples were obtained from Tayma quadrangle, northwest Saudi Arabia and extensive analyses were performed including petrophysical, geochemical, mineralogical, elemental, paleontological and lithofacies identification. Visual observation of samples tested shows highly cemented, thinly laminated and silty micaceous shale, light to dark grey in color with wide range of grain size. Paleontological analysis indicates no trace of micro or macrofossils presence in the studied samples. Elemental XRF analysis indicates major oxides of silica, calcium and alumina, reflecting the composition of the shale samples detected by XRD analysis. Silica sulfur, potassium, calcium, magnesium, titanium and high iron content reflect the presence of the quartz, pyrite, biotite, anhydride, clay minerals and iron oxides. The chemical index of alteration (CIA) and the chemical index of weathering (CIW) indicate relatively deep marine paleoclimate environment with intermediate to strong weathering conditions. High iron contents and presence of traces of titanium oxides confirm the mafic source of these shales. The lithofacies variation and mineralogy were tested using SEM/EDS and different pore systems namely organic, non-organic, and micro-fractures were recognized. Image processing indicate the presence of large macro size organic pores and Mercury Intrusion Porosmetry supports that indicating one meso and three macro pore size ranges with average porosity and permeability of 7.8% and 1.56 nD respectively. Geochemical analysis shows TOC slightly over 2 mg/g indicating good hydrocarbon generation potential with low S2 and HI values, indicating mature kerogen with gas prone type III. The results obtained confirm the potential of the Qusaiba shale and provide more insight on the characteristics of this formation.