Sedimentology and diagenesis of the Miocene Nutaysh member of the Burqan formation in the Midyan area (Northwestern Saudi Arabia)

Turbidite sandstones deposited in rift settings are currently among the main targets of hydrocarbon exploration. However, the impact of style of sedimentation, cyclicity and diagenesis on reservoir quality of such sandstones is relatively poorly explored in the literature. The sedimentology, stratigraphic architecture, and diagenetic alterations ofdeep marine sandstones of the Miocene Nutaysh Member of the Burqan Formation in the Midyan area (Saudi Arabia) are described based on number of measured sedimentologic sections, lithofacies identification in the field and laboratory studies. Three lithofacies types are here ident ified in the measured sect ions. These are from bottom to top: (1) "lithofacies A" consisting of massive to thickly-bedded, coarse-to-very coarse-grained sandstone and conglomerates; (2) "lithofacies B" consisting of well-bedded, coarse-to-medium-grained, well-sorted sandstone, and (3) "lithofacies C" consisting of thin-bedded, fine to very fine-grained, current-rippled sandstone, bioturbated shaley siltstone and marl. The main diagenetic processes in the sandstones include the formation of grain-coating smectite and rhombic dolomite. Small amounts of cements include the formation of authigenic kaolinite and calcite. The reservoir quality is anticipated to have been preserved under the transformation of smectite to deep burial illite, which is believed to prevent formation of quartz cements.