Facies, Petrography, Geochemistry and Reservoir Potential of the Late Cretaceous Adaffa Formation, Red Sea Region, Saudi Arabia

Abstract

The Late Cretaceous Adaffa Formation is the oldest sedimentary rock under Suqah Group in the Red Sea Region, Saudi Arabia, considered as one of the targeted reservoir rock in the subsurface. This Formation is nonconformably underlying by Proterozoic Basement and unconformably overlying by Usfan Formation in subsurface at Jeddah. The outcrop distribution of Adaffa Formation is poorly known, but seen at Midyan and Wadi Azlam basin along Red Sea coast. This work conducts outcrop based study of the formation in Midyan region integrating field sedimentological investigation, thin section petrography, X-ray powder diffraction(XRD), X-ray fluorescence(XRF) and Scanning Electron Microscope(SEM) methods. The study aimed at understanding reservoir properties and potentiality of the formation. The lithofacies comprises several meters of stacked (i) alluvial fan deposits with pebbly to coarse sand, (ii ) fluvial deposits of fine to medium grained cross bedded sandstone interbedded with thin shale and siltstone, and (iii) shallow marine mixed siliciclastic and bioclastic carbonates deposits. The sandstones are medium to coarse grained, subrounded to rounded, well sorted and somewhat fossiliferous. The carbonates contain ooids, peloid, bivalve, gastropod, brachiopod, foraminifera, other fossils and mineral grains. The carbonates are classified as packstone and grainstone with few wackstone. Mineralogically, the carbonates are dominating by calcite, dolomite and aragonite, whereas sandstone are dominating by quartz with subordinate feldspar and mica. The post depositional diagenetic features as grain alterations and dissolutions are also observed. The visual porosity assessed as 15% to 20% with secondary porosity as higher percentage. The major elements encountered as Ca(44.82-89.33%), Si(1-41%), Mg(1-18%), S(1-10%), Fe(1-5%), Al(1-5%), Mn(1-2%), K(0-2%), Na(1%) and trace elements Ba, La, Ni, Co, Rb, U, Pb, Zn, Cu & Ti as less than 1%. The major oxide found as CaO(31% to 84%) along with SiO2, MgO, SO3, Al2O3, Fe2O3 & MnO as higher than 1%. Others Na2O, P2O5, ZnO, BaO, La2O3, TiO2 etc as less than 1%. The diagenetic grain dissolutions might have played as an important factor for enhancing porosity and reservoir potentiality. This outcrop analogue study might provide understanding of reservoir heterogeneity and potentiality of the Adaffa Formation and also may help to provide new insight for prediction and guide for minimizing exploration challenges in subsurface.

AAPG Datapages/Search and Discovery Article #90364 © 2019 AAPG Middle East Geoscience Technology Workshop, Integrated Emerging Exploration Concepts: Challenges, Future Trends and Opportunities, Dhahran, Saudi Arabia, December 2-4, 2019