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.