Lithofacies controls on mechanical properties and brittleness in Qusaiba Shale, Rub' Al-Khali Basin, Saudi Arabia

The lower Silurian Qusaiba Shale is the main source for Paleozoic hydrocarbon reservoirs in Saudi Arabia and is considered as potential shale gas resource. The study aims to evaluate the prospectivity of Qusaiba Shale through the integration of mechanical properties with geological characteristics. The main objective of the study is to improve the production potential of Qusaiba shale by defining the lithofacies and mineralogy as controlling factors for brittleness and other mechanical parameters. The continuous 30 feet subsurface cores and log data of Qusaiba Shale from Rub' Al-Khali Basin were utilized for the study. Geological characteristics on the core were fully described in terms of color, size, mineralogy, primary structures and diagenetic features to identify lithofacies. In addition, 30 thin sections were used to study micro scale geological characteristics. Mineralogical compositions were determined using the XRD technique. Surface morphology visualization were performed using the SEM. Furthermore, the energy dispersive spectroscope (SEM-EDS), was used for elemental analysis. Acoustic velocity measurements and compressive strength tests were performed on 15 core plugs (5 from each lithofacies). Based on the above mentioned analyses, three lithofacies were identified: (1) Micaceous laminated organic-rich mudstone facies (Lithofacies-I), (2) Laminated clay-rich mudstone facies (Lithofacies-II), and (3) Massive siliceous mudstone facies (Lithofacies-III). Mineralogical composition resulted in variable amounts of quartz ranging from 39 to 40, 45-55 and 60 to 78% for Lithofacies-I, II and III, respectively. Lithofacies-I having relatively lower quartz and higher clay contents exhibited low stiffness in terms of low Young's modulus (average 26 GPa) and high Poisson's ratio (average 0.34). Mineralogy- and elastic parameters- based brittleness indices exhibited ductile behavior of this lithifacies. Lithofacies-II with relatively higher quartz (45 to 55%) and lower clay contents than Lithofacies-I resulted in relatively higher stiffness and brittleness. The brittleness index exhibited brittle behavior for silica rich Lithofacies-III as reflected by Young's modulus (average 32 GPa) and low Poisson's ratio (average 0.25). Hence, it is concluded that mineralogy and geological characteristics including texture, lamination patterns, primary sedimentary structures, diagenesis, microfractures and porosity are the main controlling factors on mechanical properties and brittleness. The integration of three essential disciplines i.e. geology, mineralogy and geomechanics, plays the key role to better evaluate the production potential by highlighting the sweet spots within the heterogeneous shale gas reservoirs.