Multi-Scale Geological Characterization of the Lower Silurian Qusaiba Shale, the Rub' Al-Khali Basin, Saudi Arabia

Abstract

The Lower Silurian Qusaiba Shale is considered to be one of the main targets for unconventional shale gas in Saudi Arabia. This study evaluates the micro and macro-scale geological features of this shale in order to have clear understanding of heterogeneity. The study was conducted on 30 feet subsurface core samples representing the Qusaiba Shale from the Rub Al-Khali Basin. It includes lithofacies, sedimentary structures, mineralogy, micro images, and elemental composition determination which are essential to delineate the full reservoir potential. Geological characteristics on macro-scale were studied on core using binocular lens and fully described in terms of color, size, mineralogy, primary structures and diagenetic features to identify lithofacies, their variation. In addition, 30 thin sections were used to determine different components of shale, its mineralogy, natural fractures, porosity type as well as other micro-scale geological features. Mineralogical compositions and their relative abundances were determined for 10 samples using the XRD technique. Surface morphology visualization including identification of various types of minerals and the study of the porosity network were performed using the SEM. Furthermore, the SEM supplemented with energy dispersive spectroscope (SEM-EDS), for elemental analysis. Based on the above mentioned analyses, the Qusaiba Shale has been categorized in three different lithofacies: (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). Various sedimentary features including lamination, bioturbation, soft sediment deformation, and natural fractures (open and filled) were observed. Major minerals include quartz, illite, kaolinite, muscovite, albite and chlorite. All three lithofacies have variable amounts of quartz ranging from 39 to 40, 45–55 and 60 to 78% for lithofacies-I, II and III, respectively. High quartz content revealed brittle nature of the Qusaiba Shale indicating its good completion quality. The presence of nano-pores within organic matter (upto 15%) indicates maturity of hydrocarbons. It has very low inter-granular porosity of about 0–1%, however, fractures along laminations and few micro faults enhance porosity up to 5–6, 2–3, and less than 1 percent in Lithofacies-I, II and III respectively. These fractures are mainly filled with clay cement, organic matter and mica.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015