Diagenesis and pore-water evolution in Permian sandstones, Gharif Formation, Sultanate of Oman

Shallow-marine, coastal plain, and fluvial sediments of the Lower Permian G harif Formation in the Interior Oman Sedimentary Basin presently range in d epth from outcrop in the Huqf-Haushi area to almost 5000 m in the northwest . This large depth range results from a varied subsidence history with only minor uplift during the Late Triassic to Early Jurassic. The degree of diagenetic overprint varies, depending on burial history, San dstone at shallow depth (<2600 m) shows intense dissolution of aluminosilic ate and carbonate minerals, combined with minor kaolinite precipitation and compaction, whereas deeply buried sandstone (>2600 m) is highly compacted and tightly cemented by quartz, carbonate, and sulfate minerals, This varie d diagenesis is caused by increasing temperature with burial, by surface wa ter infiltration during the Late Triassic to Early Jurassic, basin inversio n and uplift in the Late Cretaceous along the southeastern basin margin, an d obduction of the Oman Mountains in the Late Cretaceous. Depositional pore water was initially of low salinity (<50 gn) with delta(1 8)O(SMOW) values of -0.5 to -1.5 parts per thousand in the marine Lower Gha rif and 2 parts per thousand in the arid Middle and Upper Gharif, During sh allow burial (Late Triassic to Early Jurassic), pore water in the three Gha rif members changed to high-salinity brines (similar to 180 g/l) with delta (18)O(SMOW) values of -4 to -7 parts per thousand. Highly saline, deep-buri al pore water with a heavy delta(18)O signature (0.5-5 parts per thousand) is the result of rock-water interactions in a rock-dominated system. Presen t pore-water compositions mainly reflect subsurface dissolution of Ara Grou p evaporites, precipitation of late diagenetic carbonates and sulfates, and recharge of surface water along the uplifted southeastern basin margin.