USGS Open-File Report 99-50D (Province Geology

U.S. DEPARTMENT OF THE INTERIOR
U.S. GEOLOGICAL SURVEY

Ghaba Salt Basin Province and Fahud Salt Basin Province, Oman -- Geological Overview and Total Petroleum Systems

By
RICHARD M. POLLASTRO

World Map with Province

Open-File Report 99-50-C

	Group and the overlying, deeper marine, pelagic shale and carbonate facies of the thick Fiqa Formation (Santonian-Campanian) of the Aruma Group (Hughes-Clarke, 1988). Shallow-water carbonate deposition was re-established during the Maastrichtian portion of the Aruma (fig. 5). Early Tertiary sediments, including carbonates of the Hadhramaut and arid continental clastics and marine rocks of Fars group, disconfomably overlie the Aruma. Petroleum System Overview Several potential petroleum-source rocks have been identified in the stratigraphic section of Oman and range in age from Proterozoic to mid-Tertiary; however, organic-rich source rocks of Tertiary age are thermally immature (Grantham and others, 1988; 1990). Grantham and others (1988; 1990) and Al-Ruwehy and Frewin (1998) have identified five chemically distinct types of crude oils in Oman that are best identified by distinct biomarkers and carbon isotope values. Four of these oil types are linked to source rocks from the following: 1) a broad group of oils from the Infracambrian Huqf Supergroup, 2) the Lower Silurian Sahmah Formation of the Safiq Group (Droste, 1997), 3) the Middle Cretaceous Natih Formation, and 4) the Upper Jurassic Diyab (Grantham and others, 1988; 1990) or Tuwaiq/Hanifa equivalents (Lake, 1986; Al-Ruwehy and Frewin, 1998) source. A fifth type, referred to as `Q-type' or `questionable source' oils, has characteristics and occurrences that suggest a separate source unit of the Huqf Supergroup (Grantham and others, 1988, 1990) presumably prevalent in the Ghaba Salt Basin (Guit and others, 1995; Al-Ruwehy and Frewin, 1998; Richard and others, 1998a, b). The unidentified `Q' source rock of the Huqf Supergroup is tentatively interpreted as Lower Cambrian in age (Lake, 1996) and has recently been referred to by Richard and others (1998b) as the Dhahaban source rock inverval at the top of the Ara Salt. Some geochemical characteristics of these five oil types are compared in table 1. The geographic distribution of these oils is shown in figure 6.
			Lake (1996) summarized the main source rocks of Oman and identified seven specific source- rock units within the Huqf Supergroup. These source rocks or equivalents, other than the unknown `Q' source, have been identified from well penetrations in the South Oman Salt Basin (2011); however, the presence of similar source beds is predicted to occur in portions of the Ghaba and Fahud Salt Basins. Terken (1998; in press) differentiated the major oil families of Oman using a cross plot of C₂₇ sterane percentage versus total oil carbon isotope value (fig. 7) . Moreover, similar cross plots of oils sourced from Huqf rocks and oils produced from fields of the Ghaba and Fahud Salt Basin, referred to as North Oman Huqf oils (fig. 7), could be distinguished from oils sourced from Huqf rocks that were produced from South Oman fields (South Oman Huqf oils) (fig. 7). Therefore, based on the data of Terken (1998; in press), the term `North Oman Huqf' oils will be used in this study to define the Total Petroleum Systems of the Ghaba and Fahud Salt Basin Provinces. Hydrocarbon accumulations have been recognized in carbonate and clastic units throughout the stratigraphic section of the Ghaba and Fahud Salt Basins, but mostly within reservoirs of Cretaceous, Carboniferous-Permian, and Cambrian-Ordovician age because of their proximity to excellent overlying seals. Major accumulations and plays have been identified in the Infracambrian Huqf Supergroup of the South Oman Salt Basin (2011) where these units can be reached by drilling (Al -Marjeby and Nash, 1986; Syke and Abu Risheh, 1989; Mattes and Conway-Morris, 1990; Boserio and others, 1995; Hartstra and Graham, 1996; Lake, 1996; Onderwaater and others, 1996; Amthor and others, 1998). Recently, potential for Huqf deep-gas in Buah Formation carbonates was revealed in the Makarem 1 (ST-2) well along the Makarem-Mabrouk High, Central Oman Platform Province of north-central Oman, suggesting further potential for other pre-salt Huqf deep-gas targets in the Ghaba and Fahud Salt Basin Provinces, particularly around salt diapirs.