Hith Platform - No modern same-scale counterpart

Hith_basins

Late Jurassic Tithonian palaeogeography of the Arabian Gulf. Deposition of the four carbonate-anhydrite cycles of the Arab Formation and the overlying Hith Anhydrite constitute the dominant regional reservoirs and caprocks. The western Rub’ Al-Khali and Gotnia basins were inland salterns characterised by halite rather than platform-wide anhydrite, which is the dominant evaporite salt in the Arab and Hith formations.

The Late Jurassic (late Kimmeridgian) Arab Formation within and around the Arabian Gulf hosts some of the most prolific oil fields in the world, including Ghawar and Safaniya fields. This prolific petroleum system includes several regional organic-rich source rocks, stacked reservoirs and multiple seals. Jurassic strata were deposited across a broad platform that is sometimes characterized as “layer cake.” Chronostratigraphic correlation from outcrops to wells, well to well, and from country to country, defines a late Jurassic regional anhydritic sealed carbonate platform encompassing large intrashelf basins with grainier rims. At times of hydrographic isolation, the platform evolved into a widespread saltern that with further isolation was characterised by halite-filled depressions known as the Gotnia and Rub Al-Khali salt basins ).

The proposed regional stratigraphic framework for the Jurassic is as follows; 1) Upper Triassic-?Lower Jurassic continental clastics (Minjur Formation and equivalents) and the subsequent pre-Toarcian unconformity, indicating regional erosion and non-deposition over the Arabian platform. 

2) A Toarcian sequence (Marrat Formation and equivalents) providing a basal Jurassic regional datum, except in Oman. 3) Late Toarcian and Aalenian corresponding to a substantial sea-level lowstand and a regional depositional hiatus. 4) The Middle Jurassic Dhruma Formation corresponding to four different sequences with a significant intervening hiatus. The Upper Dhruma Member, together with the Tuwaiq Mountain Member, forms the topmost sequence (Figure). The correlation between the Dhruma, Tuwaiq Mountain, Hanifa and Jubaila formations, to their equivalents in other Arabian Gulf countries, still requires more precise definition. 5) The Arab and Hith Anhydrite formations are Tithonian, based on their sequence assignment, while the Sulaiy Formation is mostly Berriasian and straddles the Jurassic- Cretaceous boundary.

Hith_stratigraphy

Stratigraphy of the Late Jurassic sediments of the southern Arabian Gulf.

The four Tithonian Arab Formation carbonates (A-D members) were deposited as transgressive to early highstand deposits, while the Tithonian Arab, Gotnia and Hith anhydrites are perhaps late highstand deposits which overstep inland salterns (Gotnia and western Rub’ Al-Khali basins). Each carbonate and overlying anhydrite cap in the four Arab cycles (Arab A-D) are interpreted as complete third-order cycles. The equivalents to the Kimmeridgian Jubaila Formation and Tithonian Arab carbonates are absent because of non-deposition in Kuwait. The Tithonian Hith Anhydrite provides a regional stratigraphic datum in the Arabian Peninsula, except in Oman and the eastern United Arab Emirates (UAE). There the Arab and Hith Anhydrite formations are absent as they have been eroded or were never deposited, so the former eastern and southeastern extent of the Hith is not well understood. The main reservoirs within this prolific petroleum system are oolitic-peloidal grainstones and dolomitised limestones, within a regionally extensive, mostly muddy, carbonate platform that is now called the Arab Formation.

The Arab Formation is typically divided into four cycles (A through D from base to top); all cycles can produce, but the Arab D is the most productive over much of the Gulf. Each cycle shoals up from a more marine base to restricted-marine, variably dolomitised, limestone to mudflat dolomites and evaporites. The lower portion of the Arab D is typically a bioclastic lime mudstone or wackestone with normal-marine fossils. Away from topographic highs, local organic-rich mudstones were deposited in intrashelf basins and are lithologically identical to similar intrabasinal successions in the underlying Diyab Formation. The lower Arab D is overlain by a middle unit of clean washed calcarenite, composed mainly of sorted and rounded skeletal particles and ooids (grainstone-packstone), and an upper variably developed thin dolomitic mudflat sequence, which grades across a metre or two of anhydrite-plugged carbonate into massive to nodular anhydrites of the Arab D capstone. This anhydrite was deposited within a regional sulphate saltern/pan with local, but still widespread, evaporite mudflats atop local highs. Evaporitic mudflats also encircled the depositional edges of the basin. 

Hith_Arab_cross_section

Schematic east-west cross-section of the Arab and Hith formations, offshore Abu Dhabi. Note: Datum is possibly affected by erosion and evaporite dissolution with the amount of truncation increasing to the east. Facies relationships between wells are schematic .

The preserved upward shallowing trend of the Arab-D reservoir is manifested laterally by a regional eastward thickening interpreted to be the result of an eastward progradation across the shallow Late Jurassic epeiric shelf and into the relatively deep Arabian intrashelf basin. Regionally, atop the anhydrite-capped marine-dominated limestone and dolomite of the Arab D are the more restricted cyclic carbonates-anhydrites of the Arab A-C. These sediments in the United Arab Emirates show depositional relationships that correspond to the outline of the current and possibly the late Jurassic edge of the evaporite basin, although exact detail has been obscured by edge dissolution and erosion. Wireline signatures in the anhydrite caps to the various A-C cycles in oil fields in both Saudi Arabia and eastern Abu Dhabi indicate evaporite mudflats made up of mosaics of sabkhas, ephemeral brine pans and salinas (personal observation) and are very similar to signatures in shoal water portions of the Arab D capstone. As in the Arab D cap, the proportion of subaqueous anhydrite increases in the capstone away from the crests of many fields, showing that many current highs were also highs in the Jurassic. Unbreached anhydrite caps in all Arab D cycles are highly effective seals, for example, the anhydritic cap to the Arab D holds back a 400m-oil column in Ghawar Field, Saudi Arabia. Wireline signatures in the crest of the field indicate a mosaic of salinas and sabkhas.

The Asab Formation in the eastern United Arab Emirates is laterally equivalent (although not necessarily time equivalent) to the Arab Formation. Definitions of Arab-A, -B, and -C vary from field to field across the Gulf and do not necessarily represent the same chronostratigraphic unit. The combined Arab-ABC is defined as a lithostratigraphic unit composed of high-frequency dolomite-anhydrite shallowing-upward cycles capped by the massive Lower and Upper Hith Anhydrite. The top of the underlying Arab-D reservoir in the eastern UAE is typically picked at the top of a clean gamma-ray trend, which is usually an oolitic and bioclastic grainstone section. Its top typically coincides with the first occurrence of a gamma kick, indicative of a black marker limestone that commonly contains pyrite. However, in my opinion, this marker is not always a depositional unit and may be a dissolution residue created by fluid crossflow along the underside of the anhydrite sealing the Arab D. Although a useful correlative marker of the reservoir top, it probably does not represent a time-line. If we assume the Arab D is the base of the section throughout the Arabian Peninsula, then the difficulty in breaking out three separate cycles (Arab A-C cycles) atop the Arab D reflects the high degree of textural similarity of all restricted and dolomitised evaporite-capped shoaling cycles. The inherent lack of an entrained time-significant fossil assemblage and the vagaries of syndepositional evaporite dissolution in all evaporitic mudflats makes detailed well-to-well correlation at the level of the Arab A-C very difficult.

After deposition of the Arab D, ongoing higher-order eustatic changes formed the numerous parasequences that make up the widespread A-C cycles of the Arab Formation in the epeiric shelf interior. Eustatic fluctuations drove periodic marine flooding episodes on the platform, followed by filling of accommodation space, hydrographic isolation and capping by evaporites and exposure surfaces. With each transgression, another more open marine platform unit defined the base of a cycle and was deposited as a marine to mesohaline, skeletal-poor peloid wackestones/mudstones. Local shoaling on the epeiric platform led to progressively higher bottom energies, culminating in thin local buildups of ooid and peloid grainstones that are now producing intervals in the Arab ABC. Progressive platform restriction followed, via a combination of active carbonate aggradation and high order falls in relative sealevel (3rd to 5th order), which periodically exposed the shelf margin within an environment of net evaporation across the arid platform. This drove numerous brining-upward depositional cycles in the Arab ABC, with most, but not all culminating in the deposition of mudflat gypsum. Gypsum beds were porous at the time the various Arab ABC sulphates were deposited, and brine reflux drove the formation of mimetic dolomites in the immediately underlying limestones. Dolomitisation is less pervasive, but still significant, in terms of reservoir quality in the underlying Arab D (see Chapter 10, Warren 2016 for detail).

Both the Arab and the Khuff formations accumulated at different times along the southern shore of the Neo-Tethyean Ocean, when sediment accretion atop an original Hercynian horst and block terrain periodically isolated large parts of the platform interior. This led to the punctuation of the carbonate platform record with evaporite beds throughout much of the Mesozoic. Regionally, the Mesozoic in the Middle East was a period of tectonic activity centred on the opening and closing of the Tethys. The Late Permian ocean backed onto the vast landmasses of Pangaea and in both opening and closing stages of the supercontinent, the ocean created large subsealevel marine margin areas where epeiric carbonate seas formed that were periodically isolated as intracratonic sags and then filled to hydrologic base level with bedded salts. Similar evaporite-entraining epeiric carbonate ramps also characterised the Infracambrian of the Middle East, the lower Palaeozoic of North America and Australia, the Late Palaeozoic of America and Russia, and the Triassic and Jurassic of Europe.

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