YEMEN - The Geology.

Yemen is located on the south-western part of the Arabian Plate. The Arabian Shield became welded onto the African Shield by a plate collision in late Pre-Cambrian time. An extensive marine platform developed on the north-eastern flank of the shield during Paleozoic-Mesozoic time. This platform built into the Tethyan seaway, and during late Jurassic a unique combination of conditions resulted in the Arab Oil System. These included high organic productivity, repeated regressive shoal conditions, anoxic lagoonal oil source rocks, and evaporitic seals.

The Afro-Arabian Plate drifted to the north-east, closing the Tethyan seaway. It eventually collided with Eurasia in mid-Late Tertiary time, forming the Zagros mountains and Iranian fold belt. At the same time, the Arabian Plate was detached from Africa by means of the Aden-Red Sea rift which split the shield.

In its position on the trailing edge of the Arabian Plate, and being high on the Arabian Shield, Yemen had a stable geologic history, with erratic distribution of continental sediments throughout the Paleozoic period. The first marine transgression of the country occurred in the mid-Jurassic, from the south and south-east. This transgression was associated with a broad sag which preceded the opening of the Indian Ocean and resulted in deposition of the transgressive Kohlan sandstone and widespread shallow shelfal deposits of the lower Aniran limestone ("Massive Amran facies"), grading upwards into the "Marker Shale facies".

The actual opening of the Indian Ocean in the Upper Jurassic was coincident with a rift event which cut across the shield, resulting in the Ma'rib-Jawf graben, where oil production at Alif now is located. Rapid subsidence of the graben floor provided accommodation for thick syn-rift deposits of the "Turbidite facies" which are restricted to the graben. Away from the graben, shallow marine conditions continued.

The rift event lasted less than 10 million years and in Tithonian time was followed by a post-rift sag. This sag, combined with a lowering of eustatic sea level, resulted in the exposure and erosion of broad shelfal areas, and deposition of the detritus in the sag basin. The Alif sandstone was the result. It was followed by the regional Arabian evaporitic cycle of the late Tithonian, and massive salt deposits filled the sag basin. Away from the sag basin, erosion probably continued.

In the Cretaceous through Paleocene time the Tawilah-Medj Zir sandstone was deposited in low-lying areas in a marginal marine to continental environment, during a time of stable tectonic conditions. Stable conditions persisted until the development of the Red Sea-Gulf of Aden rifts in the Oligo-Miocene period, which resulted in a massive uplift along the rift boundaries, crustal thinning, extension, and massive outpourings of Yemen Volcanics, intrusion of dikes along fractures, and local intrusion of basic and acidic plutons, stocks and volcanic necks. Obviously, high heat flow was associated with this crustal thinning and igneous intrusion. This volcanic activity continued in the Quaternary.

Uplift and eastward tilting continues to the present, with historical volcanic flows north-west of San'a', active fumaroles and volcanic vents east of Dhamar, and sporadic earthquake activity.

The stratigraphic column of Yemen comprises of sediments ranging from Paleozoic to Recent and can be subdivided to the following units:

Pre-Cambrian-Infra-Cambrian Basement Complex. This is composed of eroded igneous and metamorphic rocks including gneiss, schist, meta-volcanics, and meta-sediments as follows:

Wajid (Cambrian to Carboniferous): The Wajid consists of sandstone and minor carbonates. The age ranges. It is unconformably overlying the Permian Akbra formation or the Jurassic Kohan formation.

Akbra (Upper Carboniferous to Lower Permian): The Akbra formation is considered roughly contemporaneous with and consists mostly of mudstone of fluvio-glacial sediments. It is equivalent to the Unayzah formation in southern Saudi Arabia.

Kohlan (Bathonian to Kimmeridgean): The Kohlan formation is the basal Jurassic sandstone. It is a transgressive clastic sequence found throughout the basin as localised erosional remnants. This unit is composed of sandstone with minor amounts of conglomerate, shale and limestone. The age was poorly known (Hunt, 1994), however, in the Bathonian. Yemen was subjected to a marine transgression (AI-Thour, 1997). The equivalent unit is distributed in Somalia and Ethiopia as the Adigrat sandstone.

Shuqra (Oxfordian to Kimmeridgian): The Shuqra formation was deposited in a broad shallow shelf environment. The unit is predominantly a platform carbonate with local reefal build-ups. The basal part is often sandy due to a reworking of the Kohlan formation. The carbonate is composed of wackestone and packstone and is locally silty and argillaceous. Interbedded mudstone is common. The lowermost part could be the Callovian.

Arwa (Kimmeridgian to Tithonian): The Arwa formation is composed of argillaceous limestone, deep-water lime mudstone, and locally minor packstone, wackestone, grainstone, and black shale. It represents the initiation of rifting in the Ma'rib-Jawf Shabwa rift system. It conformably overlies the Shuqra formation.

Meem (Kimmeridgian to Tithonian): The Meem formation consists of various facies including shallow to relatively deep marine carbonate, shallow marine sandstone, turbidite and organically rich mudstone. It is conformably overlaying Arwa formation.

Lam (Tithonian): The Lam formation is similar in the lithology to the Meem formation. However, Lam is dominant in sandstone. The sandstone is generally interpreted to be turbidite and occurs in thin beds within the mudstone.

Tawillah (Hauterivian to Maastrichian): The Tawillah group consists mainly of fluvial to continental clastics predominantly sandstones interbeded with siltstone, marl and shale. The Tawillah group unconformably overlies the Amran group.

The Arabian Peninsula is part of the Pre-Cambrian Arabian-Nubian Shield and can be subdivided into two adjacent structural regions:

The internal stable region (Arabian Platform), which includes the Pre-Cambrian basement rocks and the overlying sedimentary cover.

The mobile belt region, which includes Zagros, and the Oman mountain Series.

During the Late Paleozoic to the Early Mesozoic the trends of the basement rocks were oriented N-S, NW-SE, NE-SW, and subordinately E-W. These influenced inter-basins rifting direction or elevated uplift/high as differing stress fields built up during various phases of Gondwana fragmentation.

During the Late Jurassic to the Early Cretaceous the ancient NW-SE Najid Faults trend were rejuvenated, with final separation of the Arabian Plate from the African Plate in the Neogene along the ENE-WSW (Gulf of Aden) and NNW-SSE (Red Sea) trend. This had profound impacts on the geology of the area.

In the northern and eastern parts of the Arabian Plate, the basement is depressed and thickly covered with sediments (Arabian Platform Zone), whereas in the southern and western parts, the Pre-Cambrian crystalline basement and its sedimentary cover are uplifted and partly exposed (Arabian Shield Zone). The major parts of Yemen are in the Arabian platform and its tectonics and geological architectures are directly related to the tectonic evolution of the Arabian Plate.

The tectonic evolution of Yemen resulted in the formation of the following uplifts and sedimentary Basins:

Paleozoic: including the southern flank of the Rub' al-Khali (empty quarter) and the eastern Socotra Basin. They are probably related to the Karoo rifting episode of Africa (Permian-Triassic phases).

Mesozoic: including ad-Dhali, Sab'atayn, Say'un-Masila, Jiza-Qamar, Balhaf and the south Socatra Basin, which trends NW-SE (Najid Fault System).

Tertiary: including Mukalla-Sayhut, Aden-Abyan which extends parallel to the ENE-WSW Gulf of Aden trend and Tihama Basin along the NNW-SSE Red Sea trends.

The major basement uplift (Mahafid Uplift) separates a-Dhali and Sab'atayn Basins, while Jabal al-Aswad separates the Hajar sector of the Sab'atayn Basin from the Balhaf Basin. The Jahi-Mukalla high separates the Sab'atayn-Balhaf Basins from the Say'un-Masila Basin and Fartaq high separates the Say'un-Masila Basin from the Jiza-Qamar Basin. The Hadramaut Arch separates the Mesozoic Basins from the Rub' al-Khali Basin.

The current petroleum concession map of Yemen (July,2005) contains 87 blocks: 11 oil producing blocks (see Oil & Gas Market Trends No. 26); 21 exploration blocks; 13 blocks under MoU's and PSAs; and 42 open blocks.