Understanding Reservoir Properties of the Organic-Rich Qusaiba Shale, a Potential Shale Gas Reservoir, NW Saudi Arabia: An Outcrop Approach

Abstract

The reservoir properties of the Lower Silurian Qusaiba Shale (QS), a potential unconventional shale gas reservoir in Saudi Arabia, are not completely understood. One way to better understand the vertical and lateral variations which significantly control the reservoir properties of QS is an outcrop study. Sedimentological and geochemical analyses were carried out on ~ 31 m thick outcrop section of QS at the northwestern region of Saudi Arabia to determine its lithofacies, paleo-redox conditions and to re-construct the depositional environment of QS. Based on the organic matter richness of QS, there are two broad units; the basal 20m thick, comprised of organic-rich dark grey to black shale and the overlying 10-12 m thick, consists of organic-lean variegated shale and siltstone to fine grained sandstone beds. Lithologically, QS is mainly a siliciclastic sediment with traces of carbonates (average 0.5 %). The main mineral components are; clay minerals (~ 55%) and Quartz (~ 24%). Common proportions of K-feldspar (~7%) and plagioclase (~4%). LECO TOC analysis showed high organic content in the basal unit (range: 1.09 - 6.1 wt. %, ~ 2.61 wt. %). The TOC content decreases drastically in the upper silty/sandy unit (range: 0.04-0.5 wt. %, ~0.2 wt. %). QS is predominantly characterized by type III (gas prone) kerogen with a considerable amount of type IV, which was likely sourced from reworked and oxidized materials with low H content and high O content. Tmax values with an average of 425 °C, vary between 414 °C and 452 °C. The studied section of QS presents a general trend of upward shifting from anoxic-euxinic, distal and deep marine during the deposition of the basal unit to oxic, high energy, proximal and shallow marine settings for the upper silty/sandy unit. The kerogen type and maturity level of the organic matter suggest that the QS can be a self-sourced shale gas reservoir. The depositional settings, sedimentary influx and the vertical lithological variation, were the main controls on the organic matter distribution and ultimately the quality of source rock in the Qusaiba Shale. Qualitative evaluation of porosity showed that there are three different types of porosity; inorganic porosity, organic porosity and natural fracture porosity. Quartz, feldspar and pyrite contents significantly contribute to the brittleness character of the basal unit which further control the reservoir quality of the QS.

AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017