Characteristics of petroleum systems and resources potential in the Afghan-Tajik Basin

The Afghan-Tajik Basin is one of the most prospective superimposed foreland basins in Central Asia. Based on the updated data and using the method of comprehensive evaluation of geology and petroleum system analysis, the authors focused on the hydrocarbon distribution and the main controlling factors, evaluated the undiscovered recoverable resources, and discussed the potential of oil and gas resources and the future exploration areas. The results indicate that two petroleum systems are developed in Afghan-Tajik Basin, i.e., the Lower-Middle Jurassic-Callovian-Oxfordian petroleum system and Eocene/Lower CretaceousBukhara petroleum system. In this region, oil and gas are mainly distributed in the southwest Gissar uplift and the Surhandarya depression. The basin is characterized by the richness of gas and lack of oil, about 74.0% of the natural gas reserves are distributed in the southwest Gissar uplift, 71.5% of the oil reserves are distributed in the Surhandarya depression, and almost all condensate reserves are distributed in southwest Gissar uplift. Stratigraphically, the hydrocarbon is mainly stored in Jurassic strata, followed by Cretaceous and Paleogene units. In the basin, the gas is rich in sub-salt stratigraphic trap, and the oil is concentrated in post-salt structural trap, and the distribution is controlled by the favorable reservoir-cap assemblage, effective source rocks and the Cenozoic tectonic movement. Petroleum resources assessment shows that the amounts of undiscovered oil, gas and condensate resources are respectively 202.74 MMbbl, 8680.15 Bcf and 126.65 MMbbl, with an oil equivalent account of 1776.09 MMboe. The LowerMiddle Jurassic-Callovian-Oxfordian play is the most favorable exploration target.