Deposition, diagenesis and reservoir quality of an Oligocene reefal-margin limestone succession: Asmari Formation, United Arab Emirates

Abstract

An outcropping stratigraphic section of the Asmari Formation near the city of Al Ain in the United Arab Emirates is examined using a measured section, thin sections, and both geochemical and petrophysical analyses. This formation is the only known strata of Oligocene age outcropping in southeast Arabia and should not be confused with strata of overlapping age and the same name in southwest Iran. The 230-m measured section is divided into four intervals of distinctive depositional facies, representing progradation of a reefal margin and its surrounding environments. Siliciclastic content and dolomitization tend to increase with distance from the coarse-grained platform margin.A key finding is how the strata have undergone extensive diagenesis, although buried to <1 km depth in Miocene-Pliocene time, but nevertheless have moderate to high porosity and permeability. Original macropores were almost entirely eliminated by calcite cementation, whereas much secondary macroporosity was formed by early bioclast dissolution and late non-fabric-selective dissolution. The mud-poor samples have undergone “porosity inversion”, whereas total porosity in mud-dominated facies consists mainly of micropores (defined in this study as pores below petrographic resolution or around 30 μm diameter). Although pore systems consist mainly of micropores and moldic macropores, permeability shows overall correlation with total porosity, possibly because of the late, exposure-related dissolution. Investigation of this shallowly buried Asmari example provides useful background for understanding the petrophysical evolution of similar shallow-marine limestones that are now petroleum reservoirs after having undergone the additional effects of deep burial diagenesis.