COMPLEX GEOLOGY DISCUSSED BY NOTED ARABIAN SCIENTIST

Saleh M. Billo
King Saud University
Riyadh, Saudi Arabia

The Arabian Gulf expands as an arm of the Arabian Sea between the Arabian Peninsula and southwest Iran (Fig. 1).

Oil in the region is found mainly in thick Cenozoic and Mesozoic sediments juxtaposing a major geosyncline located along the southern margin of the Asian continent. Since nearly 66% of all the world's proven oil reserves lie in the Arabian Gulf, oil companies are stepping up exploration there. Drilling and seismic activities have increased, new oil fields have been discovered, and new amplified oil and gas reserves have been annexed from several producing fields. Although demand for oil in the non-communist world fell by 11% between 1979 and 1985, this decline in energy consumption is a short term marvel, and that demand will grow by 2.5 to 3.0% per year through the year 2000.

GENERAL GEOLOGY

The Arabian Gulf or its ancestor the Tethys seaway is a relic of an oceanic waterway between Gondwana and Laurasia. The Tethys Sea was a greater Mediterranean that extended along the whole southern part of Eurasia and incorporated sites of the present Alps and Himalayas. During most of its geologic history since the Precambrian, the Arabian Craton, (Fig. 2) was tilted toward the Tethyan trough, a great sedimentary basin presently pervaded by the Arabian Gulf.

STRUCTURE

The Precambrian basement complex, exposed in the central part of western Arabia, emerges again in the eastern section of the Iranian mountains. The area between these two outcrops of Precambrian rocks is a great basin. The trough of this basin is now retained by the valleys of the Tigris and Euphrates rivers and by the Arabian Gulf, (Fig. 2, 3).

Two major tectonic environments are delineated by the cratonic area to the southwest and the orthogeosyncline to the northeast, (Fig. 2). The cratonic area is the relatively stable interior region whose rigidity is controlled by the Precambrian basement. The other is the great mobile belt of Taurus, Zagros, and Oman Mountains, adjoining the stable region on the north and east. Saudi Arabia supervenes entirely within the stable region. The interior stable region includes the Arabian Shield as well as the Arabian Shelf. The main structural elements within the Arabian Shelf are the Interior Homocline, the Interior Platform, and several basinal areas (Fig. 2). These demarcations are probably cognate to epeirogenic movements within the basement complex.

The Arabian Gulf area sustained two periods of orogeny, during Late Cretaceous and Late Tertiary periods. The first produced broad north-south trending symmetrical folds of low amplitude southwest of the Arabian Gulf. The second and relatively major orogeny turned up during Pliocene time which produced the Zagros and Kurdish mountains. Powerful, deep-seated stresses in the east caused mountain building concurrent with the submerging of the eastern Arabian Gulf region. The Arabian mainland was little influenced except for being tipped slightly toward the Arabian Gulf basin. The oil fields of southwest Iran and eastern Iraq are in anticlines along the western flank or foreland of these mountain ranges (Fig. 2). On the western side of the Arabian Gulf the older diastrophism was least acute, and the folds in eastern Saudi Arabia are of a broad, gentle nature, generally trending north and south.

APPLIED STRATIGRAPHY

The basic development of rock units is the Arabian shield which occupies approximately one-third of the Arabian Peninsula in the west and crops out occasionally along the southern coast and in eastern Iran, (Fig. 3).

The Arabian Shield is flanked by sedimentary rocks of Paleozoic, Mesozoic and Cenozoic age, which thicken outward into the Tethyan trough. The shield has been remarkably stable since the dawn of the Cambrian time, conforming only to gentle, epeirogenic movements. Different invasions of the sea upon the old basement complex resulted in the deposition of an aggregate total of about 5,500 m (18,000 ft) of sedimentary rocks ranging in age from anticipated Cambrian to Pliocene (?). The Tethyan trough was relatively stable during most of the Paleozoic, and began to subside during Permian time.

The marine deposits, which in the outset were deposited nearly horizontally, have been tilted gently in an easterly direction by a general and gradual sinking of the Arabian Gulf basin, (Fig. 4). The sediments in many localities have been warped and swelled out under high stress into anticlines or domes containing sandstone and limestone reservoirs of some of the largest oil fields in the world.

HYDROCARBON OCCURRENCES

The Arabian Gulf region is a hydrocarbon province where oil and gas seeps have been recognized since Biblical times.

The first well was drilled in 1908 penetrating the Tertiary Asmari limestone, the chief reservoir rock in Iran, at 500 ft in the Masjid-i-Sulaiman field, (Fig. 5). By 1965 petroleum exploration and production has steadily increased, and more than fifty major oil fields have been discovered (Fig. 5,6). Today the Arabian Gulf is by far the most important area in the world in terms of potential oil production for the immediate future. The Arabian Gulf basin has the greatest conflux of large oil fields in the world chiefly in Iran, Iraq, and the GCC states (Fig. 1,6). Much of the production is secured mainly from Middle Cretaceous sandstone and Upper Jurassic limestone reservoir rocks, which are folded into big anticlines (Fig. 3,4). Various pools accommodate 5 billion bbl or more each. The most important producing carbonate formation in Saudi Arabia is the Arab formation (Upper Jurassic) composed of manifold calcarenitic limestone and dolomite members, each 20-200 ft thick, alternating with anhydrite layers, and the sequence winds up with the thick Hith anhydrite. These sediments are presumed to mimic mudbanks seemingly wave maneuvered grading into anhydrite to the west.

FORMATION EVALUATION

Productive holes drilled for oil and gas have successfully avouched the Miocene, Oligocene, Eocene, Cretaceous, Jurassic, Triassic, and the Permian as the oil-producing horizons in the region, (Fig. 4,5). The Carboniferous reservoirs have evinced minor "shows", but their petroleum potential has been appraised as non-commercial. Immense gas and bitumen seepages that flaunt as natural surface landmarks for oil and gas (petroleum) within the Arabian Gulf geosyncline have spurred the search for economic production for approximately half a century.

EXPLORATION METHODS

Petroleum exploration in the Arabian Gulf area started out by employing empirical means to study and observe sites and events under which oil has actually heaped up as surface seeps. The scientific approach evolved as drilling and production schemes were enhanced. Surface mapping is very active southwest of the Gulf but the existing thrust faults to the north (Fig. 3) circumscribe the application of this method. Seismic refraction surveys, sonic (acoustic), electric, velocity, gamma ray-neutron logs, and air photos are the methods now used. Discovery of new reserves requires keeping abreast with manned satellite photographic coverage for skylab of Gemini and Apollo, and mustering all the know-how and knowledge of modern petroleum technology.

GEOMETRY OF RESERVOIR ROCKS

The reservoir rocks stipulate a probabilistic model of an ancient carbonate platform that is attached to a landmass, namely the Arabian subcontinent, along the south coast of the Arabian Gulf (Fig. 2), with lagoonal sediments being deposited in the west and north where they are associated with anhydrite, calcareous sand in the form of offshore bars near the present coast, and deeper water mud in the east. The present Arabian Gulf is today comparatively shallow, being nowhere deeper than 300 ft. The deepest water culminates near the northern or Iranian shore. The Arabian Craton was inclined toward the Tethyan geosyncline now occupied by the Arabian Gulf. The Arabian Gulf geosyncline demarcates the anticlinal traps of the Trucial coast, Saudi Arabia and S. Iraq from the structural belts of E. Iraq and SW Iran (Fig. 4,6). The producing oil fields of Iran, Iraq, and the Arabian Gulf (Fig. 6) are located within the various cinctures of folding (Fig. 2,4) but generally limited to the proximity of the Arabian Gulf geosyncline in a long narrow belt trending NW-SE and extending from Turkey on the NW to the Arabian Gulf on the south (Fig. 2,6). The Zagros Mountains of Iran parallel the belt along the NE border and the Arabian forelands adjacent to the continental plate bound the area on the SW margin. The rocks in this narrow belt were originally deposited in the Tethyan trough lying east of the Arabian Shield and fringed by several stable highland masses which controlled the deposition of the known petroliferous sediments. (End Part 1 of 2.)