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.
Paleozoic belt and study area
Objectives
To characterize the facies
and depositional environments
and to assess reservoir rocks
heterogeneity and quality.
Wajid Formation in study
area
Sarah Formation Glacio-fluvial valleys
in central and NW Saudi Arabia
Introduction
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.
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.
Sanmah Glacio-fluvial and Upper Dibsiyah
shallow marine sandstone
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
Stacked glacio-fluvial facies with scoured
channel bases, deformation and slumping
features related to ice tectonics
Summary &Conclusions
References
Petrography shows
variation in texture,
grain size and porosity
and diagenesis
SEM analysis
kaolinite occurs as
grain coating and
pore fill.
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.
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parameters in Dibsiyah Member of Wajid
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Formation in the Rub al Khali Basin of
southern Saudi Arabia. 2nd Middle East
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within the Upper Ordovician Sarah
Formation, Al Qasim district, Saudi
Arabia, Proceedings of Geologists
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