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 and six play analyses listing untested trap area,
percent of untested trap area productive, average effective pay, ratio
of oil to gas, oil recovery (BBLS/AF), gas recovery (MCF/AF), NGL
recovery; most likely probability for oil, gas, NGL, and BBOE.
Copyright 2001 James Clarke. You are encouraged to print out this
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