Osman Abdullatif Earth Sciences Department, KFUPM, Saudi Arabia osmanabd@kfupm.edu.sa

1. Facies, Depositional Environments and Sandstone Composition of the Late Ordovician Glacio- fluvial Sanamah Member, Wajid Formation, South West Saudi Arabia Osman Abdullatif Earth Sciences Department, KFUPM, Saudi Arabia osmanabd@kfupm.edu.sa Tight Gas Reservoirs Tight Gas Reservoirs are defined as unconventional gas reservoirs that occur in tight rock with low porosity (<10%) and low permeability (<0.1millidarcy). Production from these reservoirs is uneconomical because of low natural flow rates. Tight gas is a prospect which to date remains largely unexploited not only in the Middle East but worldwide. Volume of gas in place in tight gas sand reservoirs is estimated to be several hundreds trillion cubic meters ( 300 to 500 TCM) distributed all over the world. Paleozoic belt and study area Wajid Formation in study area Sarah Formation Glacio-fluvial valleys in central and NW Saudi Arabia Introduction Late Ordovician Sarah Formation of glacial and glacio-fl uvial origin represent the first glacial event in Arabia (Vaslet, 1989;1990;2009; Senlap and Al-Laboun, 2000;McClure, 1978; Clark-Lowes,2005) . Sarah Formation is important reservoir target in the subsurface of Saudi Arabia, especially in north western part of Saudi Arabia and the Rub' Al-Khali Basin in the south western part of the country (Vaslet, 2009; Mcgillivray and Husseini, 1992, Zhang et al., 2009). The later Ordovician Sarah Formation of the glacio-fluvial sandstone is a primary reservoir targets in these two areas and specially as tight gas reservoirs. These reservoirs are characterized by low porosity and permeability and moderately over pressured. Late Ordovician outcrops of Sanamah member of Wajid Formation in south west Saudi Arabia, which is equivalent to the Sarah Formation, may provide an excellent outcrop equivalent to subsurface reservoirs in Rub al Khali Basin Previous studies showed that reservoir quality trends are complex and controlled by depth as well as strong controls of depositional facies and paleogeographic setting. Reservoir quality shows variation among glacial and non-glacial environments and among marine and non-marine environments (Al-Mahmoud and Al-Ghamdi, 2010; Briner et al., 2010). Commercial accumulation of tight gas is associated with enhanced reservoir quality, known as “Sweet Spots”. Sweet spots with enhanced porosities and permeabilities are controlled by depositional trends, enhanced fracturing or over-pressuring. Identification of tight gas heterogeneity and quality and determination of sweet spots can be facilitated by detailed reservoir characterization. However, exploration work indicated the difficulty of accurate sweet spotting of good tight gas reservoir was not possible given the reservoir heterogeneity, limitation of available well data and complexity of paleogeographic and stratigraphic setting of glacio-fluvial facies in the subsurface. Objectives To characterize the facies and depositional environments and to assess reservoir rocks heterogeneity and quality. Sanmah Glacio-fluvial and Upper Dibsiyah shallow marine sandstone

2. Sanamah Glacio - Fluvial Facies and Environments Braided river and glacial facies Acknowledgements: Thank are due to Earth Sciences Department and KFUPM for their support. QLF indicates quartz arenite to subarkose References Abdulkadir, I. T., A. Sahin, and O.M. Abdullatif, 2010, Distributions of petrophysical parameters in Dibsiyah Member of Wajid Sandstone, south-western Saudi Arabia, Journal of Petroleum Geology, 33(3): 269-280, July 2010. Al-Mahmoud and Al-Ghamdi, 2010. Overview of Tight Gas reservoirs in Saudi Arabia. 2nd Middle East Tight Gas Workshop, Bahrain. Briner et al., 2010. Regional Reservoir quality of Tight Gas plays, the Ordovician Sarah Formation in the Rub al Khali Basin of southern Saudi Arabia. 2nd Middle East Tight Gas Workshop, Bahrain. Clark-Lowes, D. D., 2005, Arabian glacial deposits: recognition of paleovalleys within the Upper Ordovician Sarah Formation, Al Qasim district, Saudi Arabia, Proceedings of Geologists Association, 116, 331-347. Hulver, M., A. Azzouni, and C. Harvey, 2009, Regional geology and reservoir quality of the Sarah Formation in the Rub' Al-Khali Basin of Southern Saudi Arabia, Paleozoic Clastic Gas Reservoirs of the Gulf, Workshop Abstracts, 30-31. Stacked glacio-fluvial facies with scoured channel bases, deformation and slumping features related to ice tectonics SEM analysis kaolinite occurs as grain coating and pore fill. Petrography shows variation in texture, grain size and porosity and diagenesis Summary &Conclusions Tight Gas has great potential energy resource in the Saudi Arabia The Late Ordovician Sanamah member of Wajid Sandstone (Sarah equivalent) is a good reservoir target in Rubal Khali Basin. Sanamah member is composed of glacio-fluvial facies reflecting periods of ice advance and retreat in the Late Ordovician. Lithofacies vary from massive to cross-bedded matrix-supported conglomerates, pebbly sandstone, and medium to very coarse grained trough cross-bedded, low-angle and horizontally-bedded sandstone facies. The facies show scoured and erosive channelized surfaces are common, locally interpreted by striation, deformation and slumping and shearing features within the facies. QFL analysis indicated quartz arenite to subarkose sandstone. Diagenetic features include dissolution and leaching and replacement of feldspars and rock fragment and clay cementation. Depositional and post depositional controls have affected reservoir quality and architecture at outcrop scale. Integrated outcrop analog study might provide better understanding of reservoir quality and information that can help to solve reservoir challenges in the subsurface.