A new depositional model and sequence stratigraphic interpretation for the upper Jurassic Arab "D" reservoir in Qatar

Abstract

Deposition of the Arab Formation on the Arabian Plate followed a eustatic sea-level high during the Oxfordian that deposited the open-marine shelfal carbonates of the Hanifa and Jubaila Formations. Oolite/peloidal shoals and local coral-algal stromatoporoid banks were subsequently deposited on the platform margin. These acted as barriers and led to the differentiation of intrashelf basins from open-marine (Tethyan) waters to the east. During the subsequent Kimmeridgian lowstand, gypsum wedges were laid down in the intrashelf basins. Slight changes in water depth, which exposed or flooded these barriers, are believed to be responsible for the cyclic nature of the Arab Formation sediments. Arab Formation cycles show a 4^th order frequency but have thicknesses more typical of 3^rd order Vail-type sequences. This is probably explained by the 4^th order flooding events merely topping-up pre-existing accommodation space of tens of metres water depth in the intrashelf basin. Diagenesis associated with movement of hypersaline brines may have been responsible for the development of widespread dissolution porosity and dolomitization. The laminated, organic-rich, bituminous lime mudstones of the Hanifa/Jubaila Formations are the probable source of oil in the Arab Formation in Qatar. The main reservoir types are oolitic-peloidal grainstones and dolomitized limestones.