Geology of Afghanistan,

The geology of Afghanistan is structurally complicated, consisting essentially of a succession of narrow northeast-trending terranes of continental fragments of Paleozoic to Tertiary age and Mountains. These have moved northward, colliding obliquely with the Asian continental land mass. The last arriving fragment was the large Indian continental block. It docked obliquely, imparting much additional folding and faulting and causing changes in structural trends. The accreted blocks are separated by sutures along which ophiolites are present. The latterapparently are the only remnants of subducted oceanic crust, representing oceanic spaces of unknown widths.

See Provinces for Minerals per Area

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Three principal areas of thick sedimentary rock are present in Afghanistan:

1) North Afghanistan basin in the north,
2) southwestAfghanistan drained by the Helmend River, and
3) Katawaz area on the southeast. The second and third of these areas are highly folded and faulted, and petroleum potential is negligable. They are not considered further here. Only the North Afghan basin hasappreciable prospects for oil and gas. The sedimentary rocks of this basin extend northwestward into Turkmenistan and Uzbekistan, where they form the Amu-Dar'ya gas-oil province. See Internet Geology News Letters 22, 25, 42, 89, and 116.

Drilling began in Afghanistan in 1956, and the first discovery was the Angut oil field in 1959. The period from 1959 through1966 saw 50 wildcats drilled in the basin, and three primarily gas fields were discovered: Etym-Tag in 1960, Khvajeh Gugerdak in 1961, and Khvajeh Bulan in 1964. Details of further exploration are lacking, but it appears that from 1966 to 1981 two additional small oil fields and one large gas field (Dzhar-Kuduk in 1971) plus at least two other small gas fields were discovered. Most of the discoveries have been restricted to the northwest part of the North Afghan basement high. By the beginning of 1974 discoveries of recoverable gas totaled 3.5 tcf and additional indicated reserves of 1.235 tcf. Recoverable oil reserves, probably including condensate, stood at about 80 million barrels.

Exploration in the adjacent Amu-Dar'ya gas-oil province of Turkmenistan and Uzbekistan has been quite successful. Dauletabad-Donmez gas field in Turkmenistan with perhaps 60 tcf of recoverable gas is but 40 km (25 miles) from Afghanistan. Although exploration of the Amu-Dar'ya province appears to be at least in an early stage of maturity, that in the Afghan counterpart is not. The level of activity for an area of this size appears to be low.

Until the beginning of 1979 reportedly only 7700 km (4800 mi) of seismic profile had been shot, and 292,109 m (958.364 ft) of wildcat hole had been drilled. This is equivalent to a single rig drilling over the course of 20 years. The success rate is only 8 percent. The status of exploration after the Soviet invasion of 1980 is unclear.

The average geothermal gradient of producing wells of the North Afghan high is about 1.9 degrees /100 ft (34.6 degrees C/km). This gradient is considered to be average for the North Afghanistan basin. Assuming that the geothermal gradient has been stable since early in the Mesozoic, it appears that: 1) the Jurassic beds are in the thermal gas window in all the basin areas; 2) the Cretaceous beds are in the thermal gas window in the deeper parts of the Afghan-Tajik sub-basin but are partly in the oil window on the basin perimeter and in part of the Turanian platform area; 3) Paleogene and some Neogene beds are in the oil window on the basin perimeter.

The North Afghanistan basin as well as the adjoining Amu-Dar'ya gas-oil province is gas-prone. This reflects the fact that 85 percent of the hydrocarbons were generated by Jurassic source beds, which carry largely humic organic matter and are in the thermal gas window in most of the basin. One small oil field (Angut) in the Cretaceous and several minor Paleogene oil fields on the Uzbek side of the Afghan-Tajik sub-basin are the only oil discoveries to date.

Two generations of traps seem to be present. The earliest traps are low-amplitude drapes within the Turanian platform over basement fault blocks. Closure on these structures attenuates from maximum on the Jurassic beds to zero on the lower Tertiary. These traps are expected to contain oil and gas pools in the Jurassic and Lower Cretaceous.

The younger traps are those attributed to Himalayan (Alpine) orogeny associated with collision of India with the Asian landmass. Trap development probably began in the eartly Neogene and has continued to the present, peaking probably in the Pliocene. Basin subsidence also attained a maximum at that time.

It appears that the first phase of hydrocarbon generation began early in the Cretaceous and continued until late in the Cretaceous, during which time interval the principal source rocks (Lower-Middle Jurassic and less rich Upper Jurassic) were passing through the oil window. Subsidence accelerated during the Neogene so that most of the Jurassic section and the Cretaceous in the deeper parts of the basin entered the thermal gas window. Any oil accumulation under the Upper Jurassic evaporite seal was probably cracked to gas or expelled from the trap by newly generated gas.

Five plays are recognized in the North Afghanistan basin that together are assumed to contain more than 95 percent of the hydrocarbons. They are: 1) Upper Jurassic drapes, area of 11.6 million acres; 2) Neocomian drapes, area of 3.44 million acres; 3) folded Neocomian reservoirs, area of 9.10 million acres; 4) folded Paleogene reservoirs, area of 8.13 million acres; 5) western fold belt, area of 2.45 million acres. Total assessed undiscovered recoverable petroleum in these five plays is 300 million barrels of oil, 9.6 tcf of gas, and 145 million barrels of condensate.

Taken from Kingston and Clarke, 1995, Petroleum Geology and Resources of Afghanistan: International Geology Review, vol 37, p. 111-127, four maps, six play analysis summaries. Copyright 2001 James Clarke. You are encouraged to print out this News Letter and to forward it to others.