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

oil is from a source rock in an undrilled Huqf level, had a specific flourishing biomass, contained type I and type II organic matter, and is present in a limited extent within the salt in the Ghaba Salt Basin. Other specific source rock characteristics, such as thickness and TOC, of the `Q' source rock(s) are still unknown. The `Q'-sourced oils are recognized by having a high (>50%) total C₂₇ sterane abundance relative to other steranes and d¹³C measured at -30.4‰. Additionally, the `Q' oils have a distinctive compound which is not found in other oils and yet to be identified in a known source rock (Al-Ruwehy and Frewin, 1998). Pure `Q' oil end members have been identified in Haima and Natih reservoirs in Saih Rawl and Bahja fields.

Most oil accumulations in central Oman are probably mixtures from both Huqf and `Q' source rocks. Commonly, `Q' oils are found in continuous strings of accumulations below the regional Khuff seal on northerly plunging structural highs. The Huqf oils, however, are found at all stratigraphic levels in structures related to salt domes and fault zones (Guit and others, 1995) suggesting a vertical mechanism for Huqf hydrocarbons as opposed to a predominantly horizontal migration for `Q' oils.

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 curves for the North Oman Huqf source rocks in the Ghaba Salt Basin Province are shown in figure 12 and figure 13.

Vitrinite reflectance equivalent (VRE) of Huqf source rocks within the North Oman Huqf/`Q' — Haushi(!) TPS ranges from about 0.6% to 4.0% (Sykes and Abu Risheh, 1989; and Visser (1991). Basin modeling by Amthor and others (1998) suggests three periods of hydrocarbon generation in the Ghaba Salt Basin: (1) during Haima (Andam-Safiq) deposition (about 520 - 373 Ma); (2) during Akhdar-Kahmah deposition (270 - 100 Ma); and (3) during Aruma/Tertiary deposition (80 - 0 Ma). Deep reservoirs of the Haima Supergroup

were charged with oil mainly during the second stage. Gas was generated by Huqf sources as early as 510 - 375 Ma in the central Ghaba Salt Basin. Gas generated during central Ghaba Salt Basin inhibited migration of the western flank gas charge from reaching structures along the eastern margin of the basin.

Stratigraphic distribution of the Huqf source rocks relative to the Ara Salt (i.e., pre-salt, intra-salt, post-salt) and salt movement also plays a critical role in the migration and charging of reservoirs. For example, modeling by Borgomano and others (1996) in the Ghaba Salt Basin shows at least two stages of Huqf oil generation and reservoir charge with subsequent Huqf gas generation and gas flushing of the second stage of oil charge.

`Q' oils are speculated to be sourced within the Ghaba Salt Basin (Grantham and others, 1988; Sykes and Abu Risheh, 1989; Visser, 1991; Guit and others, 1995; Richard and others, 1998a). However, Richard and others (1998b) more recently suggest from seismic data that the `Q' oils may in fact be derived from two different "kitchens": 1) a source along the western margin of the Ghaba Salt Basin, and 2) a possible source in the south Fahud Salt Basin. In these presumed `Q'-source kitchens, `Q' charge appears to postdate Huqf charge with Huqf oils generated as early as Ordovician (Visser, 1991). Older structures in the Ghaba Salt Basin and closest to the `Q' source rock seem to have been flushed later with light `Q' oil with lateral migration along north-south trending structural highs. `Q' oils are speculated to be sourced within the Ghaba Salt Basin (Grantham and others, 1988; Sykes and Abu Risheh, 1989; Visser, 1991; Guit and others, 1995; Richard and others, 1998a). However, Richard and others (1998b) more recently suggest from seismic data that the `Q' oils may in fact be derived from two different "kitchens": 1) a source along the western margin of the Ghaba Salt Basin, and 2) a possible source in the south Fahud Salt Basin. In these presumed `Q'-source kitchens, `Q' charge appears to postdate Huqf charge with Huqf oils generated as early as Ordovician (Visser, 1991). Older structures in the Ghaba Salt Basin and closest to the `Q' source rock seem to have been flushed later with light `Q' oil with lateral migration along north-south trending structural highs.