Geology and Hydrocarbon Potential of the Triassic Succession in Abu Dhabi, U.A.E.

Abstract

ABSTRACT The Triassic rocks in Abu Dhabi have been recognised in all the deep tests drilled to date in the offshore and onshore areas. They consist of a sequence of shallow marine mixed carbonates, with interbeds of shales, sandstones and anhydrites. Thickness ranging between 1440 and 3190 ft. was encountered in wells at depths between 10,500 and 16,600 ft. subsea. The sequence is subdivided, in ascending order, into three formations: the Sudair, the Gulailah and the Minjur. The Sudair and Gulailah Formations, although exhibit gradual thickening in an easterly and southeasterly direction, yet they show remarkable lateral consistency, both in lithology and log response. This allow further subdivision of each into a number of units correlatable and mappable all over the area. The Minjur is dominantly a terrigenous clastic unit, developed in the onshore area, but is absent in almost all the offshore areas. Faunal assemblage is rare, however it was possible to delineate three distinct biozones within the Triassic-Upper Permian sequence. In addition, palynologic investigation helped to define six distinct palynozones in the sequence, also the nature and limits of boundaries of the Permian/Triassic and the Triassic/Jurassic. The Sudair and Gulailah Formations were deposited within shallow lagoonal and Sabkha evaporite environments. The sequence suggests a gradual progradation towards the east and northeast through time, accompanied by gradual shallowing, through microsedimentary cycles, resulting from minor marine transgressive and regressive fluctuations. Mud-flat and tidal-flat environments prevailed over most of onshore Abu Dhabi during the later stages of the Triassic, when the clastics of the Minjur were deposited. The Triassic sequence was deposited under tectonically stable conditions, however evidence indicate that the early phase of Qatar Arch development, and the southeast Mender paleohigh occurred during the Triassic time. Porosity in the Triassic is poor in general, represented by primary and secondary types. It is controlled by lithology, depth of burial and diagenesis. The Triassic sequence has moderate source rock development, however source rock horizons are likely discontinuous. Sapropelic kerogen constitutes the dominant type of organic matter, however, humic type is present but of minor quantity. The rocks have been subjected to various degrees of thermal alteration ranging from mature to intensively mature stages. Interpretation of maturation regime indicate that most of offshore Abu Dhabi has been in the wet gas/oil zone at the level of Lower Triassic. Oil phase-out is expected below 13,000 ft., whereas only dry gas is expected below 15,500 ft. in the basin. The remaining hydrocarbon generation potential is low. Pronounced gas shows were found in the Triassic in some offshore structures, and the study indicates that the promising area for potential hydrocarbon accumulations lies in the northern part of Abu Dhabi where structures are better defined and sedimentation, burial and maturation history are favourable. The Gulailah Formation holds the best prospectivity, while the sedimentary pattern and characteristics of the Minjur Formation could develop stratigraphic entrapment for hydrocarbons in the area.