U.S. DEPARTMENT OF THE INTERIOR
U.S. GEOLOGICAL SURVEY
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By
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Open-File Report 99-50-C
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The lower Paleozoic section along the southern rim of the Arabian platform
is comprised of mainly continental clastics, with some marine intercalations,
which form important hydrocarbon reservoirs in the Ghaba and Fahud Salt
Basins. A thick sequence of rift-fill terrigenous and shallow-marine siliciclastics
of the Haima Supergroup (Droste, 1997) overlies the Ara Formation. These
sediments were derived mainly from the south during a period of rift-fill
sagging and downloading (Al-Marjeby and Nash, 1986; Sykes and Abu Risheh,
1989; Droste, 1997). In the Ghaba Salt Basin, sediments of the Haima Supergroup
fill and cover the margins of the basin reaching thicknesses in access
to 6 km along the central axis (Droste, 1997). Pre-existing, highly variable
topography caused major variations in sediment infill and syndepositional
movement of the underlying salt (halokinesis) and differential subsidence
across basement highs influenced thickness and lateral extent of these
clastics (Aley and Nash, 1985: Heward, 1990: Loosvelt and others, 1996).
Numerous unconformities are present throughout the Paleozoic in Oman. Several erosional surfaces related to Ordovician glaciation separate transgressive open-marine to regressive deltaic cycles of the Safiq Group; however, most of the Safiq Group and overlying Silurian to Devonian is not preserved. In particular, two major and very broad uplift and erosional events in eastern Oman removed most of the Silurian and Lower Devonian sediments and the interval between mid-Devonian and Upper Carboniferous; these erosional events are recognized in deep wells from the main producing fields in the Ghaba and Fahud Salt Basins. Late Carboniferous time is marked in Oman by glaciation and subsequent deposition of glacial clastics of the Al Khlata Formation (Levell and others, 1988) and shallow marine and fluvial clastics of the Gharif Formation, both of which comprise the Haushi Group (Hughes-Clark, 1988) and are important hydrocarbon reservoirs throughout Oman. Clastics of the Haushi Group (fig. 5), confined mainly to interior Oman derived from southern sources, represent the transition from a dominantly clastic system to a prolonged phase of carbonate deposition. |
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| Development
of a regional shallow carbonate platform during a Middle Permian marine
transgression deposited the widespread lower Khuff Formation (fig.
5) (Sharief, 1982) which forms a major regional seal above the clastic
reservoirs of the Gharif Formation. Subsequent transgressions resulted
in blanket deposits of the Jurassic to Cretaceous (Cenomanian) sequence
of mainly cyclic shelf carbonates over northern Oman. This includes the
carbonate rocks of the Jurassic Sahtan and the Cretaceous Kahmah (Thamama)
and Wasia Groups (fig. 5). In general, regional
changes in sedimentation during the Jurassic and most of the Cretaceous
in northern Oman were controlled mostly by eustatic fluctuations rather
than tectonics. The youngest prograding carbonate sequence of this succession
is the Natih Formation of latest Albian to Early Turonian. Organic-rich
marls of the Natih Formation accumulated in intrashelf, restricted basins
during periods of global anoxic events.
A major change in tectonic style and depositional setting took place at about the Cenomanian-Turonian boundary mostly because of collision and partial subduction of the eastern Arabian sub-plate; these compressional tectonic events are responsible for thrusting, fore-bulge, and downwarping of a foreland basin in northern Oman, ultimately forming the Omani foredeep (Loosvelt and others, 1996). Flexural extension from downwarping initiated and/or reactivated normal faulting along a northwest-southeast trend, such as those associated with Natih and Fahud fields (fig. 2). Moreover, the Late Cretaceous was a period of pronounced salt movement in the Ghaba and Fahud Salt Basins. Salt movement is associated with a large number of producing oil fields, specifically in the southeastern Arabian Gulf region in the offshore and in the onshore Ghaba and South Oman Salt basins. This structural event is also roughly coincident with a worldwide eustatic sea-level rise during the Late Cretaceous (Vail and others, 1991). Combined, these events resulted in a change from a shallow, stable platform to a deep-water marine environment. In the Ghaba and Fahud Salt Basins, a significant unconformity is present between the Wasia |
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