Diagenetic history and source rock potential of the Upper Jurassic Diyab Formation, offshore Abu Dhabi, United Arab Emirates

Abstract

The Upper Jurassic Diyab Formation is a highly argillaceous dolomitic limestone unit underlying one of the most prolific hydrocarbon reservoirs in the world: the Arab Formation. The Diyab is thought to be the primary source for the Arab in the United Arab Emirates. Thin sections from cores in offshore Abu Dhabi show the Diyab to contain dolomitized grainstones at the base with associated glauconitization and chertification grading upward to olive-green, argillaceous, organically-rich, dolomitic limestones in the upper third of the Diyab, creating a marly texture which persists through to the top of the section. Late diagenetic calcite (and subordinate anhydrite) cement, which include coarse spar as well as blocky and poikilotopic fabrics, occlude much of the secondary porosity and are interpreted to have occurred during burial diagenesis. Dolomite is in the form of euhedral rhombohedra which primarily replace the matrix of the limestones. Allochemical grains, which include peloids, intraclasts and bioclasts, have recrystallized to low Mg-calcite and are partially glauconitized in the lower part of the section. Pressure solution has caused fracturing and stylolitization, fractures being filled by sparry calcite and stylolites by a bituminous residue.