The early Jurassic, mixed siliciclastic-carbonate Mafraq Formation of the Sahtan Group, is distributed widely in northern Oman and missing in the south of Oman. The Upper Mafraq is considered a reservoir in central and eastern Interior Oman. The Mafraq Formation comprises four facies associations (i) fluvial (FA 1), (ii) coastal complex (FA 2), (iii) lagoon-backshoal (FA 3), and (iv) shoal (FA 4). This study is based on petrographic analysis of 110 thin sections using a conventional microscope, XRD, SEM, CL, and isotopes analysis to examine the texture, mineral composition, and diagenetic alterations. Texturally, the sandstone of the Mafraq Formation has an average of very fine-grained and moderately well-sorted. On the other hand, the sandstone of the Mafraq Formation is composed of fine and moderately well-sorted grains. The sandstones are dominantly quartz in composition. The carbonate grains in Mafraq Formation are dominantly intrabasinal grains, including echinoderms, bivalves, gastropods, brachiopods, and foraminifers. Petrographic and geochemical analysis of the Mafraq Formation has shown that the sandstones and carbonates have undergone various diagenetic alterations during eo-, meso-, and telodiagenesis. However, during eodiagenesis, mechanical compaction exists in FA 1 effected on quartz grains. In addition, anhydrite coated some quartz grains. It filled some pores in FA 1 by providing the needed SO4 from the seawater, possibly during sea-level rise and deposition of overlaying marine carbonates. It could also occur at eodiagenesis, evidenced by the absence of quartz overgrowth and/or telodiagenesis filling pores created by dissolution. Calcite cement is more dominant in FA 2, FA 3, and FA 4 than FA 1, which is attributed to the bioclasts' presence. Dolomite is another type of carbonate cement that may have occurred in eo- and mesodiagenesis, which mainly was formed by alteration of calcium carbonate and the seawater and/or meteoric water (under semi-arid conditions) providing the need for Mg2+. Although the dissolution of different bioclasts and cement is a significant attribute for creating secondary porosity, this is filled by calcite and dolomite cement at a later stage. Quartz overgrowth is more dominant in FA 1, which was controlled by the silica released from the chemical compaction of quartz grains. Siderite is a diagenetic process post-dating quartz overgrowth and can be caused at meso- or telodiagenesis stages.