Abstract

The so-called “dense zones” (intervals of very low porosity) separating the thicker, high-porosity reservoir intervals of Lower Cretaceous limestone strata in the Abu Dhabi subsurface have been studied in cores from a giant onshore oilfield. Although holding no oil reserves, the dense zones are important elements of reservoir architecture and essential components of any sequence stratigraphic interpretation, yet little substantive work has been reported regarding their description and origin. The present study of the two dense zones enclosing the upper, ca. 50-m-thick reservoir zone of the Kharaib Formation reveals that they have similar ranges of bulk chemical composition, with much higher aluminum, iron, potassium, thorium, and uranium than the intervening reservoir zone, but are very different from one another in depositional texture. The upper dense zone (Hawar Member), here referred to as “DA,” consists mainly of peloid–orbitolinid packstone, whereas the next-lower dense zone, “DB,” consists mainly of mudstone, also containing common discoidal orbitolinids. Both dense zones display cycle-like fluctuations in gamma ray activity that correlate throughout the field area, reflecting alternating darker and lighter rock color due to varying content of clay and pyrite. Organic matter is generally low (average 0.2–0.3 wt. % total organic carbon), consistent with intense bioturbation throughout both dense zones. Clays consist of illite–smectite with 15–25% expandable layers and subordinate dickite. Unlike the reservoir limestones, both dense zones contain abundant pyritized grains. DA was deposited in shallower water depths than DB, but comparison with the range of water depths represented by the reservoir zones is problematic because of uncertainties introduced by the higher turbidity and nutrient levels inferred for the dense zones. The high clay content of the dense zones accounts for their extreme early loss of porosity and is explained as the result of their stratigraphic position, associated with (either preceding or immediately following) third-order sea-level lowstands.