الملخص الإنجليزي
Carbonate reservoirs demand special attention due to their substantial contribution
to global oil and gas reserves. In fact, these reservoirs account for approximately 60% of
the world's remaining oil and gas reserves. Moreover, they naturally exhibit numerous
favorable attributes for carbon dioxide (CO2) storage. As such, the continued exploration
and development of these reservoirs are not just essential for meeting future global energy
demands, but they also play a crucial role in achieving net-zero carbon emissions by
utilizing depleted reservoirs as sinks for CO2 sequestration. The quality of these reservoirs,
particularly their porosity and permeability, is significantly influenced by a variety of
factors including their depositional environment and diagenetic evolution. Thus,
conducting research in these areas offers a unique opportunity to predict reservoir
properties and gain an understanding of their distribution and evolution. The Lower
Aptian Carbonates, namely the Lower Shu’aiba and its equivalent, the Upper Qishn, are
widely recognized in the Arabian Peninsula for their substantial hydrocarbon reserves
presenting an excellent opportunity for studying and evaluating the depositional
environment and the impact of diagenetic modifications, which can further the
understanding of reservoir quality.
Approximately 59 m of the outcropped Lower Shu’aiba in Wadi Bani Kharus and
22 m of the outcropped Upper Qishn in Wadi Baw were studied for facies analysis,
diagenesis, and reservoir quality. A total of 26 selected samples were collected from both
outcrops, all of which were used to prepare thin sections. These were then studied under
a microscope and subjected to point counting (300 points). Of these samples, 12 were
further analyzed using Scanning Electron Microscopy (SEM) and geochemical analyses,
including X-ray Fluorescence (XRF), X-ray Diffraction (XRD), and Energy-Dispersive
X-ray Spectroscopy (EDS). Additionally, 16 samples underwent coring and porosity and
permeability measurements.
In the investigated sites of Wadi Baw in East Central Oman and Wadi Bani Kharus
in North Oman, 17 different microfacies have been identified deposited in a diverse array
of depositional environments namely, lagoon, reef, open marine-algal platform, and open
marine-slope, underscoring a clear gradational transition from proximal to distal settings.
The deeper, distal part of the basin is represented by Wadi Bani Kharus with its distinctive
slope microfacies, while Wadi Baw, with its coarser-grained lithofacies, is indicative of
the proximal, high-energy environment.
Regional variations in diagenetic alterations have been noted, influencing the
reservoir quality of the Lower Aptian Carbonates. Specifically, these variations are
evident when comparing the Lower Shu’aiba of Northern Oman to the Upper Qishn in
Central Oman. The northern region is marked by pronounced fracturing and subsequent
calcite cementation that fills these fractures, while Central Oman shows a strong presence
of dolomitization and dissolution. Notably, the Upper Qishn in Wadi Baw is home to
unique dolomite microfacies. These microfacies present a classification challenge due to
their non-mimetic nature and suggest the presence of dolomitization mechanisms in
Central Oman that aren't found in the north. Such diagenetic changes play a crucial role
in determining the reservoir quality in both areas.
The microfacies, which are indicative of the depositional environment, play a role
in determining reservoir quality. In particular, the presence of interparticle microporosity
and intrafossil porosity, especially within microfacies containing foraminifera,
significantly boosts the overall reservoir quality. While the Lower Aptian Carbonates
display high porosity, their permeability remains low due to limited pore
interconnectivity. In Wadi Baw, these carbonates have higher porosity and permeability
values than in Wadi Bani Kharus, where the porosity is poor, and permeability is nil. This
disparity can be attributed to the different burial depths experienced by these regions;
Wadi Baw underwent shallow burial, whereas Wadi Bani Kharus was subjected to deep
burial during the ophiolite obduction.
The Upper Qishn emerged from preliminary assessments as a promising site for
Carbon dioxide (CO2) storage. Despite slightly below-threshold permeability, the
formation's favorable average porosity, thickness, and depth indicated potential for
efficient supercritical CO2 storage. Geochemical analyses further revealed the formation's
mineralogical suitability for stable, long-term CO2 storage via mineralization.
