CHAPTER 3 FARS GROUP
Published:January 01, 2010
Authors:James and Wynd (1965).
‘Fars’ is an Iranian rock-unit term widely and loosely applied around the Gulf area outside Iran to embrace younger Tertiary sediments that were, in many areas, not further subdivided. The base is effectively defined by a region-wide event involving a hiatus or marked change of sedimentary regime during the Late Eocene – Oligocene. In Oman, the sediments following this event are diverse, but have been little studied. In the oil-producing areas, they are considered under two headings: carbonates, hosting important aquifers (Al Lamki and Terken, 1996), and varied clastic-evaporite sequences, some clastics also being laterally variable aquifers. The more significant carbonate developments in Interior Oman occur at the base of the Fars and these have been included under the Taqa Formation.
Type Section: Fars Province, southern Iran.
Lithology: In northern Oman (Figure 3.1), the Fars Group is represented by the Dhulaima Formation, consisting largely of dolomites, limestones, evaporites and shales. The dolomites are often rich in gastropods and bivalves. In South and Central Oman, the Group is represented by the Taqa Formation (generally much thinner), consisting largely of limestones and dolomites.
Boundaries: The Fars Group unconformably overlies the Hadhramaut Group and, in Dhofar, Mesozoic units.
Distribution: The Group occurs throughout Interior Oman up to outcrop areas to the north, east and south. It is limited to the west, at approximately E55°15′ by a north-south outcrop of the Hadhramaut Group. It is found at surface, or covered by Quaternary only.
Subdivision: The Fars Group is divided into the Dhulaima Formation and the Taqa Formation.
Early Miocene MFS Ng10, and their Oligocene MFS Pg50 (previously Ng10), MFS Pg40 and MFS Pg30 surfaces within the Taqa Formation of southern Oman.
Age: Oligocene – Pliocene, ca. 33–1.8 Ma (?2.6 Ma depending on ultimate status of the Gelasian Stage). Although Hughes Clarke (1988) stated an Oligocene to Pliocene age for the Fars Group in general, the oldest and youngest limits of the Group are poorly defined in Oman. Often only Miocene fauna are recorded with any degree of confidence (Figure 3.2). The presence of Nummulites intermedius is used to confirm an Early Oligocene age (in northern Oman), but care must be taken as reworked Oligocene and Eocene taxa can be recorded in Miocene Fars sediments.
Biostratigraphy: Biozones F78 (Peneroplis farsensis, Miocene) and F77 (Miogypsina sp., Nummulites intermedius, Oligocene) (formerly referred to as FZV and FZU, respectively).
Authors: Proposed herein. Previously referred to as the Clastics and Evaporites Formation (PDO usage, see Mohammed et al., 1997), discussed/illustrated as a clastic-evaporite sequence by Hughes Clarke (1988).
The Dhulaima Formation is synonymous with the Fars Group in North Oman.
Type section: Dhulaima-1 (Figure 3.3).
Subsurface recognition: The Dhulaima Formation is generally drilled with good returns. The Rate of Penetration varies with lithology, which is dominated by calcareous dolomites and evaporites with bioclastic and often argillaceous limestone and calcareous shale towards the base. The dolomites are rich in gastropods and lamellibranchs. Lithologically, the dolomites and evaporites do not differ from those of the Rus Formation. There is generally a negative drillbreak (Rate of Penetration) at the lower boundary (e.g. Lekhwair-251, Figure 3.5).
Boundaries: The top boundary is picked mainly on faunal evidence for Biozone F78 (FZV) and, in the basal part for F77 (FZU) (Oligocene - marked by Nummulites intermedius, Figure 3.2). The unit is characterised by an erratic and variable Gamma-ray trace and a low trend in the Density curve. The top boundary can sometimes only be picked using offset well Gamma-ray log correlation using expanded Gamma scale (e.g. 5–40° API is suggested) to enhance the log character (see Barik-8, Figure 4.6). There generally is a negative drillbreak (Rate of Penetration) at the lower boundary (Figure 3.5).
Distribution: The Dhulaima Formation occurs in northern Oman only. It is very variable laterally and intra-formational correlations are extremely difficult to carry out.
Deposition: Thought to have been deposited under mainly shallow-marine conditions with significant clastic influence.
Age: Oligocene – Pliocene, ca. 33–1.8 Ma. The Oligocene calibration in northern Oman relates to the non-dolomitic sections in northwest Oman (e.g. well Dhulaima-1, Figure 3.3).
Biostratigraphy: Distinct fauna, predominantly of Miocene age (F78) with the basal part being Oligocene (Nummulites intermedius, F77, Figure 3.2). See Group and Taqa Formation discussions.
The Taqa Formation is equivalent to the Fars Group in South and Central Oman (Figure 3.1).
Type and reference sections: Taqah area (E54°24′, N17°02′), and west of Ras Raisut (E54°00′, N16°56′), Dhofar. The thickness in the type area is ca. 1,100–1,400 m. Additional reference sections are Barik-8 (see Figure 4.6) and Nimr-143 (Figure 3.6).
Lithology: A dominantly carbonate sequence (Figure 3.4d), including chalky and porous marine, sometimes reefal, limestones. It features strongly recrystallised fossiliferous limestone and calcareous dolomite.
Subsurface recognition: Whilst drilling the lithology of the Taqa Formation is distinctive, comprising strongly recrystallised fossiliferous limestone and calcareous dolomite. It is often difficult to recognise due to limitations of sample collection and erratic returns. Post-drilling, the Taqa is recognised by the microfossil content (F78 Biozone) and through Gamma log correlation.
Boundaries: It unconformably overlies the Hadhramaut units and, in Dhofar, Mesozoic units. It passes up with transition into undifferentiated Fars clastics and evaporites.
Distribution:Hughes Clarke (1988) stated that the Taqa Formation is developed mainly to the south, in Dhofar, along the southeastern coasts, but also in the Lekhwair area of North Oman. This reflects the areas with most marine influx during the Oligocene – Miocene. The flat-lying thin limestone forming the surface of the Jiddat al Harasis plains (Central Oman) is a deposit of the global high-stand of sea level during the Middle Miocene. However, according to Mohammed et al. (1997) it is mostly confined to South and Central Oman.
Deposition: Shallow-marine (mostly very shallow to restricted), including reefal and lagoonal environments.
Age: Oligocene – Miocene, ca. 33–5.3 Ma. The Taqa has a distinct fauna of Miocene age. Although Mohammed et al. (1997) stated that there is no evidence of Oligocene faunas in southern Oman, a Late Oligocene age has been noted via palynological analysis and there are rare records of Biozone F77 in the south. Note also there are well-documented Early Oligocene vertebrate discoveries in Dhofar (Thomas et al., 1999, and related papers). Sharland et al. (2004, see also 2001) place their renumbered Early Miocene MFS Ng10, and their Oligocene MFS Pg50 (previously Ng10), MFS Pg40 and MFS Pg30 surfaces within the Taqa Formation of southern Oman.
Biostratigraphy: Biozone F78 (Peneroplis farsensis, previously FZV, Miocene) with Late Oligocene palynofloras (not zoned) in southern Oman, and Biozone F77 (Miogypsina sp., Nummulites intermedius, previously FZU, Oligocene) (Figure 3.2).
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