Lithofacies Controls on Natural Fracture Distribution in the Qusaiba Shale, Rub’ Al-Khali Basin, Saudi Arabia

Abstract

The Lower Silurian Qusaiba Shale is main hydrocarbon provider for the Paleozoic petroleum system of Saudi Arabia and considered one of the main targets for unconventional shale gas. The study focuses on the lithofacies and fractures on both macro and micro scale in the Qusaiba Shale in order to better understand the effect of reservoir heterogeneity on fractures generation. The study was conducted on 30 feet subsurface core samples representing the Qusaiba Shale from the Rub Al-Khali Basin. The type and intensity of fractures were integrated with lithofacies type and mineralogy. Lithofacies were categorized on the basis of geological characteristics on both macro and micro scale. Cores using binocular lens are fully described in terms of color, size, mineralogy, primary sedimentary structures, fractures type and orientation, and diagenetic features. In addition, 30 thin sections were used to determine micro geological features including components of shale, 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 XRD. 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 was supplemented with energy dispersive spectroscope (SEM-EDS), for elemental analysis. Fractures study has been carried out using cores and thin sections for each lithofacies. Based on the analyses and observed features, 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. The type and intensity of fractures are different for Lithofacies-I, II and III, based on the variable amounts of quartz ranging from 39 to 40, 45-55 and 60 to 78% respectively. It has very low inter-granular porosity of about 0-1%, however, fractures along laminations, at angle with 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.

AAPG Datapages/Search and Discovery Article #90332 © 2018 AAPG International Conference and Exhibition, Cape Town, South Africa, November 4-11, 2018