Abstract
ABSTRACT
The Middle Permian to Early Triassic Khuff Formation occurs in the subsurface of the United Arab Emirates at depths that range from 3,688–6,188 m (12,097–20,297 ft) in Abu Dhabi and Dubai, and as outcrops in mountainous areas of the northern United Arab Emirates. The formation consists of a shallow-water carbonates that include limestones, dolomitic limestones with subordinate anhydrite and dolomites. It reaches a thickness of as much as 625–970 m (2,050–3,182 ft) in the subsurface and 125–960 m (410–3,149 ft) in outcrops. The Khuff Formation is interpreted as a second-order composite sequence represented by the KS1 through KS7 third-order sequences. The Khuff transgressive systems set starts with the KS7 event and ends at the maximum flooding surface of KS4. The highstand systems set starts in the upper portion of the Khuff with a second-order maximum flooding surface (MFS-4) and ends with a sequence boundary at the top of KS1 that characterizes the top of the Khuff Formation. The formation is subdivided into ten facies units distinguished on the basis of their depositional textures that represent an overall regressive carbonate-evaporite sequence. Based on the paleoecology, sedimentary structures and lithology, four distinct depositional settings can be recognized: (1) supratidal (sabkha), (2) lagoon, (3) shoal and (4) shallow shelf. The formation can be broadly subdivided into two major carbonate units deposited in two different hydraulic regimes, which are separated by an anhydrite bed (the Middle Anhydrite marker).
A detailed petrographic study of the Khuff carbonates reveals a complicated diagenetic history. Four diagenetic settings have been identified: (1) marine phreatic, (2) mixed phreatic, (3) meteoric phreatic, and (4) burial. The Khuff Formation has both primary and secondary porosity. Most open pores are a result of interparticle, intercrystalline, dissolution vug or enlarged mouldic porosity. The diagenetic features in these sediments are mainly partial cementation, dolomitization and the development of secondary anhydrite. Porosity ranges from 6–20% and permeability from less than 1.0 to more than 500 md. Horizontal permeability is greatly enhanced by subvertical partings of the open pores, common in microcrystalline dolomites. Stylolites are common, but unimportant as vertical barriers. Extensive fracturing of the reservoir has produced a dense network of intersecting vertical and subvertical fractures. These fractures have a significant impact on the enhancement of the effective porosity and permeability.
The Khuff Formation has large volumes of proven gas reserves in Bahrain, Iran, Qatar, Saudi Arabia and the United Arab Emirates and minor oil in Oman. While the Khuff Formation forms prolific gas and condensate reservoirs in the offshore United Arab Emirates, no hydrocarbons have been found in the onshore area. Locally here the reservoir is capped by the shales and dolomites of the overlying Sudair Formation (Early Triassic). The formation is sourced from Silurian Qusaiba shales.