Facies, Depositional Environments and Sandstone Composition of the Late Ordovician Glacio- fluvial Sanamah Member, Wajid Formation, South West Saudi Arabia

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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_at_kfupm.edu.sa
Tight Gas Reservoirs Tight Gas Reservoirs are
defined as unconventional gas reservoirs that
occur in tight rock with low porosity (lt10) and
low permeability (lt0.1millidarcy). 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,
198919902009 Senlap and Al-Laboun,
2000McClure, 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
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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.