USGS Open-File Report 99-50D (Total Petroleum

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

porosity, commonly fractured, grainstones and chalky carbonates of the Lower Cretaceous Shu'aiba Formation and Middle Cretaceous Natih Formation. Although some fields producing from Natih reservoirs are sourced by the organic-rich facies of the Natih Formation (the Natih(!) Total Petroleum System discussed later), a large volume of oil and gas in Natih reservoirs is sourced from the Huqf. Most production, however, is from the Shu'aiba Formation (Thamama-Khamah Group, Early Aptian) (Murris, 1980; Alsharhan and Nairn, 1997). The Shu'aiba is overlain by the Nahr Umr Formation of the Wasia Group, a widespread transgressive shale, which forms a regional seal for these reservoirs (fig. 5). The intergrity of this seal is excellent as hydrocarbons produced from the Shu'aiba are commonly different from those produced in Middle Cretaceous Natih reservoirs overlying the Nahr Umr shales (Brennan, 1985). In both the Shu'aiba and Natih Formations, shallow-water, shelf-margin carbonate buildups (mainly rudistid reefs) and associated grainstones (debris shoals) formed on and around low relief structural highs (mostly formed by salt pillows and tilted, upthrown fault blocks) comprise the best reservoirs (Frost and others, 1983; Harris and Frost, 1984). Uplift from both tectonic and halokinetic movements produced secondary (mostly moldic and vuggy) porosity from subaerial erosion and meteoric diagenesis. Porosities ranging from 30 to 40 percent have been recorded in the Shu'aiba at Yibal, Al Hawaisah, Natih, Fahud, and Daleel fields.

Sandstones of the Haushi Group (Gharif and Al Khlata Formations-Khuff limestone seal) form reservoirs in some fields. The Barik sandstone member (Mabrouk Shale seal) and Miqrat Formations (Al Bashair Shale seal) of the Lower Cambrian to Lower Ordovician age parts of the Haima Supergroup are primarily deep gas reservoirs with reservoir potential recently recognized in Precambrian carbonates of the Buah Formation.

Similar to the Ghaba Salt Basin, most of the fields of the Fahud Salt Basin are structurally complex, salt-induced anticlines and domes that have been broken up into several fault blocks by crestal collapse features (Nederlof and others, 1995). Specific common trap styles are faulted closures, dip closures, and faulted-dip closures (Lake, 1996).

Source rock character
The geochemical characteristics of North Oman Huqf-type oils are discussed in the previous

section on the North Oman Huqf/`Q' — Haushi(!) TPS and are also summarized in table 1. North Oman Huqf oils have d¹³C compositions between about -33‰ and -35‰ and C₂₇ sterane from 10 to 45% and can be distinguished other Huqf-sourced oils (i.e., South Oman Huqf and `Q' types, see fig. 7).

Burial history, generation, and migration
Generation of hydrocarbons from Huqf source rocks in north and central Oman has been discussed by Sykes and Abu Risheh (1989), Visser (1991), Guit and others (1995), Borgomano and others (1996), Droste (1997), and Amthor and others (1998). Burial history for the North Oman Huqf source rocks in the Fahud Salt Basin Province is shown in figure 15 and figure 16.

Vitrinite reflectance equivalents of Huqf source rocks within the the North Oman Huqf — Shu'aiba(!) TPS ranges from about 2.0 to 4.0 percent (Sykes and Abu Risheh, 1989; and Visser (1991). Burial history reconstructions by Visser (1991) suggest that an early minor stage of oil generation occurred in Middle and Lower Huqf source rocks during the early Silurian (fig. 15). Peak oil generation occurred during Late Permian/Early Triassic (~250 ma); gas generation began during the Cretaceous (~110 Ma). Modelling by Amthor and others (1998) suggest that gas expelled from Huqf source rocks in the Fahud Salt Basin charged structures across the Fahud Salt Basin and reached the Makarem high during a period ranging from 80 Ma to present day (fig. 16).

Fahud-Huqf Combined Structural Assessment Unit (20160101)
One all-inclusive assessment unit, designated as the Fahud-Huqf Combined Structural Assessment Unit, has been assigned to the North Oman Huqf — Shu'aiba(!) TPS of the Fahud Salt Basin Province. The assessment unit boundary is defined and described by the Total Petroleum System boundary and includes all reservoirs (Haima, Haushi, Shu'aiba, Natih, among others) and styles of structural traps (domes, anticlines, fault blocks, etc.) of the known inclusive fields that produce hydrocarbons from Huqf sources in the Fahud Salt Basin proper (fig. 14, table 2). Thus, the assessment unit boundary outlines the maximum geographic extent at which similar undiscovered fields may exist in the North Oman Huqf — Shu'aiba(!) TPS. The Fahud-Huqf Combined Structural Assessment Unit shares a common border to the