الملخص الإنجليزي
Carbonate reservoirs store a large portion of world-known hydrocarbons, and
they have a lot of underexplored potentialities. Diagenetic processes affect carbonate
rocks immediately from the depositional stage and throughout the geological time,
producing a lot of alterations to the original textures and thus raising a lot of challenges
in predicting the reservoir quality. Buah Formation is a dolomitic carbonate unit in the
second clastic-carbonate cycle of the Nafun Group, which was accumulated during the
Late Neoproterozoic, Ediacaran age (635-541 Ma), with variation in thickness ranging
from 50m up to around 400m. Few published studies on the Buah Formation cover
mainly the lithofacies and depositional environment on the outcrops, with much less
focus on diagenesis. Thus, this project was conducted on the Buah Formation utilizing
subsurface core samples from east Central Oman to identify the depositional facies
distribution, to understand the diagenetic alterations, and to evaluate the controls in the
reservoir quality. Four cored wells from the study area covering different intervals from
the Buah Formation were selected for this study. Well X and Well Y cores covered
intervals from Middle and Lower Buah, Well Z cored intervals within Lower Buah, and
Well R had core intervals from Middle Buah. A total of eighty-two carbonate-stained thin
sections from these four wells have been used for the petrographic study. And six-core
chip samples have been collected from the Well R core for SEM and EDS analysis.
Eight lithofacies have been identified from the studied cores representing mainly
middle ramp and tidally influenced inner ramp facies: (1) crinkly laminated
dolomudstone, (2) microbial dolostone, (3) dolograinstone, (4) dolostone breccia, (5)
crystalline dolostone, (6) argillaceous mudstone with anhydrite, (7) red mudstone to
siltstone, and (8) red mudstone breccia. These lithofacies were interpreted to be
deposited on widespread carbonate ramps, with variable subsidence rates across Oman,
resulting in variations in the depositional facies. The main diagenetic alterations observed
in the studied samples: micritization, multiple phases of cementation (dolomite, silica,
vi
and anhydrite), at least two generations of dolomitization, dissolution, mechanical and
chemical compaction, and various fracturing phases. Therefore, the porosity and
permeability changed over time; in some cases, they got enhanced, and in other cases,
they got reduced. Buah Formation has primary porosity as interparticle in the
dolograinstone lithofacies, and different secondary porosities such as vuggy,
intraparticles, intercrystalline, moldic, and fracture enlargements.
Depositional facies, diagenetic alterations, and tectonic fracturing are controlling
the reservoir quality of the Buah Formation in the studied cores. The timing of each one
of these factors played a major rule in defining the reservoir quality. The dolograinstone
lithofacies have better reservoir quality if the primary porosity has been preserved by
early cementation or by early hydrocarbon migration. However, tectonic fracturing and
dissolution altered dolomitized fine grain lithofacies and created connected vuggy
porosity. Moreover, in some cases, the dolomitization of mudstone lithofacies resulted in the porosity of the intercrystallite matrix.
