Fahahil Fm
Type Locality and Naming
Q.P.C. Well Dukhan No. 66 [DK-0066], lat 25°20’53” N, long. 50°48’47” E, elevation 23 m (76 ft), completed 18.5.1960, between drilled depths 1964 and 2021 m (6444 and 6631 ft). The formation takes its name from a locality on the Dukhan anticline near the type section. [Note: In the original Sugden’s reports of 1953/56, he assigns DK-0026 as type locality (spudded in 1952). Also, the above coordinates are not for DK-0066 but rather for DK-0026. It is possible that Standring, who took over the Lexicon from Sugden after 1959, wanted to change the type locality from DK-0026 to DK-0066 but forgot to change the coordinates.]
W. Sugden (unpublished company report)
Synonym: “Lower, carbonate part of the Arab D Member”, Powers, 1968. “No. 4 Limestone” of Qatar.
Reference Section:
Lithology and Thickness
(After F. Gosling, unpublished report). Top. 1. Dolomitic lime mudstone with streaks of dolomite and anhydrite, dolomitic packstone; two foot anhydrite bed near top. 6.4 m (21 ft). 2. Interbedded grainstone, wackestone and packstone of skeletal grains in lime mudstone matrix. Upper 9m predominantly grainstone. 42 m (139 ft). 3. Alternating dolomitic lime mudstone and dolomite. 8.2 m (27 ft). Base. Thickness is 57 m (187 ft).
Relationships and Distribution
Lower contact
Darb Fm; contact conformable. At the junction of porous dolomites of the Fahahil Formation, above, with dense lime mudstones of the Darb Formation, below.
Upper contact
Qatar Fm; contact conformable. At the junction of dolomite, anhydritic lime mudstone of the Upper Fahahil Formation, below, with massive anhydrites of the basal unit of the Qatar formation, above.
Regional extent
Extends throughout the oilfield area of Qatar, Bahrain and Saudi Arabia, but is unknown from Kuwait northwards. Its equivalent can be recognised in Abu Dhabi.
GeoJSON
Fossils
In 1. Prethocoprolithus sp., Aeolisaccus dunningtoni Elliott. In 2. Kurnubia jurassica (Henson), Nautiloculina oolithica Mohler, Salpingoporella annulata Carozzi, Burgundia trinarchii var. Dehorne, Cladocoropsis mirabilis Felix, Stromatoporina choffati (Dehorne), S. romanica (Dehorne), S. somaliense (Zuffardi-Comerci), S. annulata, Clypeina jurassica Favre and Richard, C. cf. hanabatensis Yabe and Toyama, Lithoporella melobesioides Foslie. In 3. K jurassica.
Age
Depositional setting
Additional Information
This unit is easily defined in western Qatar and much of Saudi Arabia. In eastern Qatar, increasing dolomitization of the Upper Darb Fm makes it virtually impossible to distinguish a stratigraphic equivalent of the base of the formation as defined in Dukhan. Much increased dolomitization of this nature, farther to the east, probably is reflected in the conditions met in the Umm Shaif oilfield, where the name Arab Darb has been introduced.
The lower unit (3) of the reference section could be related to the underlying Darb as easily as to the Fahahil Fm. It has been demonstrated (F. Gosling, unpublished report) that the Fahahil marks a transition from deeper to shallower water conditions. It is this transitional nature, together with the unit’s great commercial value as an oil reservoir, which prompted W. Sugden to segregate it as a distinct formation, instead of incorporating it either in an Arab Fm (as in Saudi Arabia) or in the Darb Fm.