Gulailah Fm
Type Locality and Naming
Q.P.C. Well Kharaib No. 1, lat. 25°27’09” N, long. 51°11’56” E, elevation 41.5 m (136 ft), completed 29.6.1953, between drilled depths 2454 and 2615 m (8052 and 8578 ft). The formation takes its name from a locality near the type section.
Synonym: “Dolomite-Anhydrite series” of early unpublished reports on Arabia. “Jilh Formation”, Steineke and Bramkamp, 1952
Reference Section:
Lithology and Thickness
Top. 1. Dolomite, grey and dark grey, fine grained, dense, calcareous, anhydrite, partly argillaceous, with interbedded anhydrite, grey and white. Authigenic quartz common. 27 m (88 ft). 2. Dolomite, grey, fine grained, dense, partly calcareous. 25 m (84 ft). 3. Anhydrite, grey, alternating with calcareous dolomite, grey, fine grained, dense, argillaceous. Occasional thin beds of shale, dark grey, calcareous. Authigenic quartz. 11 m (38 ft). 4. Limestone, grey, fine grained, dense, partly dolomitic, with streaks and nodules of anhydrite. Very argillaceous in upper and lower few feet. Lower half pellety in parts. 37 m (122 ft). 5. Limestone, grey and grey-brown, fine grained, dense, pellety in part, dolomitic, grading to dolomite, grey, fine grained, dense, anhydritic with interbeds of white to grey-brown anhydrite. Authigenic quartz throughout. 35 m (116 ft). 6. Limestone, grey, fine grained, dense, grading to dolomite, calcareous, containing a little grey anhydrite. 23.8 m (78 ft). Base. Thickness is 160 m (526 ft).
Relationships and Distribution
Lower contact
Suwei Fm, contact probably conformable; placed where brown to grey shales and marls of the upper Suwei underlie limestones and dolomites of the basal Gulailah Fm.
Upper contact
Izhara Fm; contact unconformable, placed at the top of the highest anhydrite of the Gulailah Fm. The chronological list of current rock units indicates the next younger unit as Hamlah Fm
A similar relationship is seen in Q.P.C. Well Musaiymir No. 1, but in Well Dukhan No. 65, an additional unit, the Hamlah Fm, intervenes between the Gulailah and the Izhara – See Hamlah Formation.
Regional extent
Deep wells on the Dukhan structure and at Musaiymir in eastern Qatar. Probably penetrated by deep wells in offshore Qatar (“Khail Formation” Dominguez, 1965). Is the lateral equivalent of the Jilh Formation as developed in subsurface areas of Saudi Arabia.
GeoJSON
Fossils
In unit 4. Rare Ammodiscus sp. and Glomospira sp.
Age
Depositional setting
Additional Information
When first penetrated, in Well Kharaib No. 1, the Gulailah was believed to correlate with part of the Dhruma Fm of Saudi Arabia, while the underlying red beds of the Suwei Fm were equated with the basal Jurassic, Marrat Fm. With the completion of the Dukhan Deep Test Well it became apparent that this correlation was incorrect, the Gulailah being lithologically similar to the Jilh Fm of nearby areas of Saudi Arabia and in the same relative stratigraphic position.
The top of the Gulailah Fm is marked by a widespread unconformity, the equivalent of the pre-Marrat unconformity of Saudi Arabia. This results from a period of late Triassic to early Jurassic uplift and erosion which were effective both regionally and locally over individual structures. The time interval represented by this hiatus saw the deposition of the continental sands of the Minjur Fm in areas closer to the Arabian Shield.
In Qatar, this unconformity causes cut-out of the Hamlah Fm between Dukhan and Kharaib, accompanied by marked thinning of the Gulailah in the same, easterly direction.