Late Neoproterozoic Alkaline Intrusives in Huqf Sediments; a New Reservoir Target in Southern Oman

Recent drilling in Block 56 on the southern Oman coast north-east of Salalah has revealed the presence of intrusive rocks in Huqf sediments. The igneous bodies are found as both discrete and composite, decametre scale sills separated by screens of Huqf host rock. Compositionally they form a cogenetic suite of monzonites and syenites that can be related by fractional crystallization to a gabbro-diorite parental magma. Enrichment in alkalis and other large ion lithophile elements points to either an enriched mantle source or magma-crust interaction on ascent. Geochemical profiles match closely those of the latest Neoproterozoic alkaline eruptives now found as exotic volcanic blocks entrained in the salt diapirs of the Ghaba Salt Basin. Limited geochronological data indicate that the Block 56 intrusives are contemporaneous and derived from the same magma source. Porosities in excess of 10% have been recorded in the intrusives; it is a dissolution porosity created by volume loss during the conversion of primary ferromagnesian minerals to biotite and ferroan dolomite. Porosity creation took place during a short lived, fault-controlled Cretaceous hydrothermal event contemporaneous with uplift and alkaline volcanic activity in the Masirah Ophiolite. The intrusives contain a Huqf-sourced oil with light to medium viscosity appearance. Charging is believed to be coming from the cooler and shallower parts of the South Oman Salt Basin to the north and from beneath thick Cenozoic cover in the basin to the south.