The geology of Afghanistan is described in relation to hydrocarbon occurrence and future potential. Current gas production, exported to Russia, is abstracted from structural closures in Cretaceous - Neogene sediments on the margin of a Neogene - Recent depression northwest of the Hindu Kush mountain range. Source rocks are thought to be Jurassic paralic sediments and/or Cretaceous platform limestones, the latter outcropping extensively across much of central Afghanistan north of the Heat fault zone, a major fracture separating two tectonically different regions. Future exploration should be directed towards the search for similar structural closures with other Neogene - Recent depressions bordering the Hindu Kush, particularly in the north and west.

Substantial thicknesses of Mesozoic and Paleogene clastic and calcareous successions are reported to occur in the southern part of the Katawaz fault block, a geotectonic unit between two major fracture zones in SE Afghanistan. Aeromagnetic and field studies indicate a basin up to 9000m thick containing suitable source and reservoir rocks but tight folding, lack of seal lithologies and excessive depth of erosion lessen the chances of finding hydrocarbon pay zones.

Future discoveries are more likely to be gas than oil as the geothermal gradient, both present and historis is thought to be high in view of the tectonic position within a collision zone. Nevertheless, oil seeps, bitumen and fetid limestone in central and western Afghanistan suggest dissociation may not be complete.

Introduction

Afghanistan occupies a critical position west of the Himalayan sector of the Alpine orogenic belt. A tectonically complex mountainous core is surrounded by extensive areas of permian - Cretaceous - Paleogene calcareous sediments overlain, in peripheral basins, by Neogene - Recent deposits. These basins are peripheral basins, by Neogene - Recent deposits. These basins are of Particular interest as prospect areas for hydrocarbons.

Natural gas was discovered in 1960 in northern Afghanistan and production commenced in 1967. The gas field is located 180 km W production commenced in 1967. The gas field is located 180 km W of Mazar-i-Sharif and a few kilometres south of Sheberghan, the Provincial capital of Jowzjan Province. Provincial capital of Jowzjan Province. Gas is transmitted in two pipelines, one eastwards to supply a thermal power station and urea plant in Mazar-i-Sharif, the other 200 km northwards to consumers in the southern asiatic states of the USSR.

Since the establishment of the Afghan Geological Survey in 1955, the country has been mapped at reconnaissance level by various Missions from France, Germany and Russia and the published results of these surveys form the basis of this resume together with a previous assessment of petroleum potential in southern Afghanistan by Schreiber, et al.

Regional geology and tectonic framework

Afghanistan lies within the Tethyan paleogeographic regime extending latitudinally as a broad mobile fold belt between two major cratonic masses, the Bukhara portion of the Eurasian craton to the north arid the Indian-Arabian portion of Gondwana to the south. The stratigraphic record of Afghanistan and neighbouring regions has been interpreted in the plate tectonics concept by a number of authors who show that the area has been affected by at least three orogenies, the last of which marks a collisonal event between subcontinental India and continental Asia in the Oligocene and contemporaneous with the Alpine orogeny of western Europe

The effect of these orogenies has, of course, reduced considerably the hydrocarbon potential of the older portions of the stratigraphic column which, even in the Devonian, contain possible source and reservoir rocks. Post-Paleozoic rocks occupy almost two thirds of Afghanistan and of these, at least half are calcareous, so that there is considerable scope for the occurrence of gas and oil.

The tectonic framework of Afghanistan is determined principally by several major, deep-seated, fracture zones separating eight geologically different areas (Fig. 1). These are :

  1. North Afghan block an extensive platform to the north of the Herat - hindu Kush fault zone.

  2. Pamir block a structurally complex region in NE Afghanistan which is part of the giant Pamir syntaxis and composed largely of Pre-Jurassic igneous and metamorphic rocks. Pre-Jurassic igneous and metamorphic rocks.

  3. Herat Fault Zone comprising a narrow, highly faulted horst feature composed of Precombrian Traissic igneous and metamorphic rocks, striking latitudinally across central Afghanistan and joining eastwards with the Hindu Kush fault. Note : the Afghan sector of the Hari Rud river system flows along this fracture zone but the term "Hari Rud Fault" is used to refer to the segment of the meridional Ural - Oman fracture zone positioned close to the Iran/ Afghanistan border and is the tectonic lineament marking the west edge of the Farah and Seistan blocks (see below).

  4. Farah block comprising folded Jurassic - Cretaceous mostly calcareous sediments with fold axes trending WSW and overlain by a Paleogene volcanic—sedimentary succession, situated in W. Afghanistan.

  5. Helmand block in central Afghanistan consisting of pre-Cretaceous igneous and metamorphic terrain with faulted pre-Cretaceous igneous and metamorphic terrain with faulted Cretaceous and younger sediment in the south.

  6. Seistan block a large Quaternary depression occuying most of SW Afghanistan.

  7. Katawaz block in SE Afghanistan. The northern part of this block forms the "Kabul wedge" between the Chaman and Quetta fault zones where they meet to impinge on the Herat-Hindu Kush lineament. The southern part of the block is occupied by a thick succession of Paleogene flysch.

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