الملخص الإنجليزي
The late Cambrian Al Bashair Formation is well known as one of the Haima Supergroup seals for
the underlying gas reservoirs in different fields of north central Oman, but upon the latest gas
shows discoveries it was considered as an economical valuable potential reservoir. This thesis
project aims to have a better understanding of the textural and compositional characteristics of Al
Bashair Formation outcrops along with the dominant diagenetic alterations and their subsequent
control on reservoir quality within Al Huqf area. Textural, compositional, petrographic description
and geochemical analysis (X-ray Diffraction Analysis and X-Ray Fluorescence) on samples collected
from nine outcrop sections within Al Bashair Formation were analyzed in this study. The studied Al
Bashair Formation samples from these various sections are relatively within a common range of
textural and compositional characteristics.
Al Bashair Formation is comprised of fine-grained, well-sorted, grain supported, subangular to
subrounded sandstones interbedded with siltstones and carbonate units. The samples of the Al
Bashair Formation are composed mainly of monocrystalline quartz, carbonate cement, feldspar,
allochems, polycrystalline quartz, mica and rock fragments. Those which are found in trace
amounts are heavy minerals, opaque minerals, silica cement and iron oxide cements.
This study revealed that Al Bashair Formation sandstones and have undergone various degrees of
diagenetic modifications during eo-and mesodiagenetic regimes and telodiagenetic regimes.
During eodiagenetic regime, Al Bashair Formation was affected by mechanical compaction,
feldspar overgrowth, pyrite and precipitation of early dolomite cement. For the mesodiagenetic
regime, the sandstone and carbonates were altered by chemical compaction and precipitation of
different types of cement including multiple phases of silica cement (quartz overgrowth,
concentration of silica grains and mega quartz filling fractures), subordinate amounts of illite,
calcite cement, calcite dolomitization and anhydrite cement. Oxides are not clear whether they
occurred in telodiagenetic regime or not.
Interpretation of Al Bashair rocks porosity concluded that it has a moderate reservoir quality with
average porosity in percentage (12%), where the average primary porosity is 10.8 %, and average
secondary porosity is 1.3 %. The modal composition confirmed that the average total cements was
found to be comprising 32%.The overall samples with low porosity values have higher amounts of
dolomite or calcite and anhydrite cements, whereas sandstones and carbonates with high values
have in general more secondary porosity. Loss of primary porosity is due to compaction, except
that a relation was found between compaction and the amount of quartz cement; the lower the
quartz cement the greater the compaction. The observed phases of silica cementation controlled
the reservoir quality of Al Bashair Formation sandstones in Mahattat Humaid area, as it lower the
porosity and permeability of the reservoir. Silica cement dominance prevents further compaction
from destroying the porosity. The porosity enhancement is attributed mainly to partial dissolution
of feldspar, and thus intergranular development of secondary porosity.
