English abstract
Clays are important in several geological and environmental processes, as well as
in many human activities. They are a complex phyllosilicate group of minerals that
are most often intergrown, consist of a wide variety of solid solutions, and form
through different processes. The analysis of clay minerals was investigated for
diagenesis and reservoir quality prediction, as well as environmental
determination, stratigraphic correlation, and hydrocarbon production. The process
of clay neoformation and transformation in buried sediments is known as clay
diagenesis, which is affected by several factors such as detrital sediment
composition, depositional environment, temperature, and burial history. Grain coating clays, amongst other types of clays, are a common observed diagenetic
alteration in the studied Barik Formation. Clay coatings have a significant impact
on reservoir quality and petrophysical properties in sandstone reservoirs across
the world.
The Upper Cambrian-Lower Ordovician Barik Formation is a considerable deep
tight-gas reservoir in Central Oman. It is part of the Mahatta Humaid area in the
Huqf-Haushi region, Central Oman. The Barik Formation represents a series of
regressive-transgressive braid delta systems with influences from a marine setting.
The outcrop-equivalents in Central Oman are a good example for studying and
understanding how the grain clay coating is formed, as well as their distribution
and control on the Barik reservoir quality.
Scanning Electron Microscope (SEM) equipped with a Dispersed Energy
Spectrometer (EDS), X-Ray Diffraction (XRD), X-Ray Fluorescence (XRF), and
Quantitative Electron Microscopy Scanning (QEMScan) was performed on the
representative collection of samples to determine the petrographical,
mineralogical, and geochemical examinations. Petrog Software was used to
measure the coverage and thickness of grain coating clays on a few samples.
There are four major types of clay minerals present in Barik sandstone. Barik
sandstone was observed to undergo several diagenetic alteration processes, such
as mechanical compaction, dissolution and kaolinization, clay coating grains, and
cementation by quartz and feldspar overgrowths. Diagenetic grain-coating clays
are mainly composed of chlorite and illite. Illite and chlorite are partially and
completely coating the sand grains. The average coat coverage in the
representative samples exceeds 30%, and the average thickness of clay coatings
varies from 1.7 μm to 9 μm. The chlorite and illite grain coatings are more abundant
within the mouth bar/shoreface and tidal channels, while being less common in the
tidal flat and delta distributary channels. Illite and chlorite are suggested to have
formed from smectite clay precursors infiltrated mechanically by tidal
hydrodynamic pumps. Thick and continuous diagenetic chlorite and illite grain
coatings inhibit the nucleation sites for quartz and feldspar overgrowth
cementation, thus preserving the reservoir porosity.
