YEMEN - The North-Western Sector.

The north-western sector of Yemen is smaller than that of the south-east, with an area of 195,000 sq km and a population of about 16.3m. The south-eastern sector has an area of about 333,000 sq km and a population of 5.2m.

It was mainly a series of big oil discoveries in the south-eastern sector which caused a civil war from May to July in 1994.

There are six main sedimentary basin in Yemen: the Red Sea Basin in the west, the Yarin Basin in the north-west, the oil/gas rich Sab'atayn (Ma'rib-Shabwa) Basin of the shared area between the north-western and south-eastern sectors of the country, the equally rich Sir-Say'un Basin in the south-east bordering with Saudi Arabia's part of Rub' al-Khali, the Qamar-Jiza Basin in the east of the country bordering with Saudi Arabia and Oman, and the Gulf of Aden Basin in the south.

The Tihama region of the north-western sector, a hot semi-desert strip about 65 km wide, separates the Red Sea from the rugged volcanic mountain ranges of the interior. Rising steeply to 5,000-12,000 feet, these highlands have deposits of rock salt, silver, gold, coal, copper and other minerals, plus a sedimentary basin of about 30,000 sq km between the ancient city of Ma'rib and Wadi al-Jawf areas.

Offshore along the Red Sea coast there are fairly wide areas of shallow waters, from the border with Saudi Arabia down to Bab el-Mandeb. In tectonic terms, some of the areas within the Red Sea Basin are interesting parts of the Arabian Shield. A stable shelf with Mesozoic-Cenozoic cover stretches far into the interior, from a point near Hodeidah, and along the north-western part of the Gulf of Aden.

The tectonic relationship between the Owen Fracture Zone and the Gulf of Aden is important for the study of their sedimentations. The magnetic zone, or new oceanic crust, of the Sheba Ridge is relevant to their study, together with the axial portions in the Owen Fracture Zone and the Gulf of Aden which form the southern limits of the main hydrocarbon reservoirs of the Middle East.

Onshore prospects first became the subject of interest in the early 1950s, when a European geologist on a private visit to the ancient city of Ma'rib encountered an intriguing fault. He found the surface rocks to be of particular interest. These eventually were identified as Amran limestones.

Subsequent reporting to his company failed to arouse enthusiasm, however, partly because of hostile tribes in that area. The ruling Zaidi Shi'ite Imams of North Yemen had sealed their territory's interior from the outside world. The numerous tribes of the north-west were armed and suspicious of foreigners. An unsuccessful search for oil along the Red Sea at the time was another discouraging factor.

In 1963, Aramco geologists indicated that a large area of sandstone and siltstone outcrops overlying the crystalline basement in the south-west of Saudi Arabia extended to north Yemen. Three years later it was said they were part of the Wajid formation, of Paleozoic sandstone about 950 metres thick. In 1968 Aramco data based on surface and subsurface studies suggested the Wajid in Yemen was of the Permian age. In the 1970s the outcrops were defined as being Cambro-Ordovician in age. A UNESCO map for Yemen in 1985 showed them as Ordovician.

The source area for the Wajid is not clearly defined, with some geologists saying it to be from elevated eastern areas adjacent to the Hadhramout Arch of south-eastern Yemen. There was the theory that the source was the crystalline basement block in north-western Yemen's interior.

An older Paleozoic sequence was found to be truncating along these features from the north and north-east to the south and south-west, with adjacent exposures in Ethiopia. But more recently it was said that a Cambro-Ordovician age for the Wajid outcrops was unlikely.

Ma'rib/Jawf Background: Despite the long neglect of Yemen's interior by foreign companies, the geology of that area remained the subject of interest, particularly in the north-east of Ma'rib. In 1979, the San'a' government sponsored an aero-magnetic survey. The results in 1980 caught the attention of Moujib al-Malazi, a Syrian exploration consultant to Ray Hunt's Hunt Oil Co. of Dallas.

Malazi studied data showing a narrow but deep graben with a thicker sedimentary basin than originally thought. He shared his assessment with Dr. Ian Maycock, exploration manager of Hunt Oil's London office. The two concurred that Pre-Cambrian rock formed the basis for the entire area, later known as the Sab'atayn Basin.

There was a sharp boundary in the north-west with a Jurassic formation, which in Saudi Arabia has yielded prolific oil reservoirs.

The theory was that the NW/SE trending graben had started to rift in Jurassic times and accumulated a good thickness of sediment. In December 1980 Maycock took the data and Malazi's assessment to Ray Hunt in Dallas. Within days Hunt sent a team to Yemen for a further scouting of that area and started negotiations with San'a' government.

Addressing a London Geological Society conference in 1990, Maycock explained that the government's 1980 survey had indicated the possibility of a 30,000 sq km sedimentary basin and a fill of 3,500 metres. Later successes achieved by Hunt Exploration Co., he said, were a fine example of rich rewards gained by perseverance and imaginative exploration in frontier areas.

The Ma'rib/Jawf area was mostly covered in sand but included a diapric salt anticline with some associated bituminous shales. Close to the basin centre, there was a basement outcrop. Field work along the margins revealed a Mesozoic sequence 750 metres thick, including important Upper Jurassic sands rich in oil.

On evidence of likely source rocks, the prime targets for Hunt were: possible reservoirs, evaporites to provide structuration and seals, amenable government regulations and, most important, a potentially large oil basin. Subsequent studies concluded that the then shared North-South Yemen border zone nearby, called Wadi al-Ain (later to be known as Jannah block), also contained related strata.

The Amal and Iyad oilfields in West Shabwa of the southern sector, only about 55 km east of Ma'rib's Alif field, were found to be of a similar Upper Jurassic reservoir. Later it was proven that they all had the same basin, which aroused the interest of many companies. But West Shabwa has been a disappointment.