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

Gharif. The glacio-lacustrine Al Khlata Formation consists of laterally continuous sandstones, intercalated with thin shale layers which have no sealing capacity (Levell and others, 1988). The regionally deposited Rahab Shale at the top of the Al Khlata stratigraphic boundary and between the Al Khlata and Gharif Formations commonly forms a seal for Al Khlata reservoirs (fig. 5). In most fields, however, the Al Khlata Formation and laterally-continuous, porous deltaic sands of the Lower Gharif form one continuous reservoir and are sealed by 20 to 30 m of the Haushi Limestone member (Guit and others, 1995; Nederlof and others, 1995). Middle Gharif reservoirs are composed of fine-grained sandstones, siltstones, and shales with up to 20 percent porosity but with low permeability and are sealed with a thick (about 90 m) claystone. Alluvial, mostly laterally-discontinuous channel sands of the Upper Gharif are sealed by basal carbonates of the Lower Khuff Formation (Guit and others, 1995; Nederlof and others, 1995).

Other major reservoirs are porous, commonly fractured, grainstones and chalky carbonates of the Middle Cretaceous Natih Formation (Fiqa Formation shale seal) and Lower Cretaceous Shu'aiba Formation (Nahr Umr Formation shale seal) (fig. 5). Sandstones of the Barik sandstone member (Mabrouk Shale seal) and Miqrat Formations (Al Bashair Shale seal), Haima Supergroup (L. Cambrian to L. Ordovician) are primarily deep gas reservoirs; potential for deep gas is also being explored in Precambrian carbonates of the Buah Formation, Huqf Supergroup (fig. 5).

Most of the fields of the Ghaba Salt Basin, particularly those producing from Haushi reservoirs, 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 and geochemistry
Infracambrian Huqf source rocks contain structureless, type I and type II oil-prone organic matter (Grantham and others 1988, 1990; Mattes and Conway-Morris, 1990; Lake, 1996). Huqf

source rocks produce a rather broad group of North Oman Huqf-type oils which are recognized by having high abundances of of C₂₉ steranes(greater than 50 percent) relative to other steranes and d¹³C measured at -37.1‰. South Oman Huqf-sourced oils can be distinguished from North Oman Huqf-sourced oils in cross-plots of isotopic and C₂₇ sterane percent compositions (fig. 7). North Oman Huqf oils have d¹³C compositions between -31‰ and -35‰ and C₂₇ sterane as high as 45 or 50%, whereas South Oman Huqf oils are between - 35‰ and -37‰ and C₂₇ sterane <25%. Richard and others (1998a, b) have identified the Shuram Formation of the Huqf Supergroup as the primary source of Huqf oils in Haushi reservoirs of north Oman. The Shuram contains carbonate source rocks that are thick (about 450 m), laterally extensive, and average about 2 % TOC (Lake, 1996).

The main geochemical characteristics of the Huqf-type oils, which correlate well with extracts from Huqf source rocks, are high sulfur (1.5-2.0%) content, the presence of a homologous series of long chain, methyl substituted alkanes, the so-called `X'-branched compounds (Klomp, 1986), high (45-90%) C₂₇ sterane percent, and high-negative (-28 to -33‰) carbon isotope. The geochemical character- istics of North Oman Huqf-type and `Q'-type oils are summarized in table 1.

The `Q'-type oils were first identified by Grantham and others (1988) from fields in central Oman and later by Sykes and Abu Risheh (1989), Guit and others (1995), Al-Ruwehy and Frewin (1998), and a, b and others (1998) in more northern fields. These commonly light (average API gravities of about 40), relatively low-sulfur (about 0.2%), mature oils are found in Paleozoic clastic reservoirs and occur where Huqf Group rocks are deeply buried, at depths greater than those routinely penetrated by drilling, and unsampled. The `Q' oils also contain the so called `X-compounds', evidence which argues for the presumed Infracambrian age of the `Q' source rock. Grantham and others (1990) and Guit and others (1995) speculate that the distinctive `Q'