Online Journal for E&P Geoscientists

Van Steenwinkel, Mia ^*1; Droste, Henk ²
(1) Rock-It, Hombeek, Belgium. (2) Shell International E&P, The Hague, Netherlands.

The Middle Jurassic Tuwaiq Mountain Formation in Oman is only present in the WNW, as a result of truncation related to uplift and subaerial exposure of the eastern plate margin. The Tuwaiq represents a carbonate shoal complex, which prograded from eastern Oman towards the west into an intra-shelf basin, located in western Abu Dhabi and Qatar. This formation yields good reservoir carbonates and is sealed by deep-water aphanitic limestones.

Well correlations, together with a biostratigraphic review have revealed that these aphanitic limestones in Oman have been wrongly associated with the Hanifa Formation in the UAE and Saudi Arabia. Instead, they represent a much younger aphanitic limestone facies deposited in front of an eastward prograding Late Jurassic-Early Cretaceous carbonate shelf, equivalent in age to the Jubaila to Hith.

The Hanifa s.s. Saudi/UAE correlates in Oman with a transgressive limestone unit that has previously been identified as Tuwaiq. The unit is ca. 30 m thick, pinching out towards the east. It consists of good-reservoir mid-ramp grainstones, grading upward to outer-ramp wackestones. The base of this unit is a sequence boundary or a transgressive surface; the top is a major regional tectonic unconformity, which corresponds to uplift and subaerial exposure of the eastern plate margin at the end of the Middle Jurassic.

Hence, the Hanifa Formation s.s. Saudi/UAE represents an offlapping wedge to the west of the Tuwaiq shoals in Oman, with its shallow-water pinch-out reaching the Lekhwair area in Oman.

The Tuwaiq reservoir below this transgressive unit is shallow-water carbonate grainstone, with macroporous, interparticle porosity, in which leaching has provided oversized pores and darcy-range permeabilities. Leaching may have been caused by subaerial exposure at the base-Hanifa sequence boundary and/or the late Middle-Jurassic truncation surface. The low-angle truncation surface overlain by the aphanitic limestones may provide traps with a stratigraphic component with a lateral seal provided by deep-water limestones of basal Tuwaiq.