The search for non-anticlinal traps has become very important in the southeast of part of Fergana depression to the south of the Andizhan group of anticlines as far as where the Paleozoics crop out on the Karachatyr high. In this sector the Neogene Baktriy sediments rest on an erosion surface that cuts across sediments that range in age from lower Miocene to Paleozoic. (The Baktriy Series derives its name from Bactria, an ancient name for a region of Central Asia. It was conquered by Alexander in 328 B. C.) The age of the underlying sub-cropping sediments increases toward the south.
At the same time that the Baktriy sediments were being deposited in the subsiding Fergana graben, compressive orogenic forces in the bordering areas on the south produced new folds in the basin, particularly in the Baktriy sediments.
At the base of the Baktriy is the so-called productive Horizon I, or Formation C. These porous sandstones pass in places into impermeable varieties, which act as seals for hydrocarbons migrating up dip in the underlying sediments. Where Horizon I is porous the pools can extend into it as in Yuzhnyy Alamyshik field. These Baktriy sandstones are continental and not very persistent. Correlation is difficult because of absence of fossils.
Two plays are recognized in the Neogene section: Miocene and Pliocene. The boundary between them is drawn at the base of productive Horizon I. This horizon is marked clearly on the electrical logs by high resistivity values. Thickness of Horizon I decreases from 120 m on the northwest to zero on the southeast, indicating a persistence in the overall tendency for the floor of the basin of deposition to slope from south to north both in pre-Baktriy time and afterwards.
Laboratory modeling and experimental studies suggest that in the Fergana depression migration of hydrocarbons from the source rocks could have taken place not earlier than the end of the Miocene. Generation of hydrocarbons began when the source beds were at depths of 1200-1500 m and proceeded intensively when they reached depths of 2.5-4 km. Analysis of isopach maps indicates that this intensive migration was in late Pliocene-Anthropogene time. A late time of migration is also substantiated by the fact that none of the oil in the Paleogene strat traps is oxidized.
Nets of profile wells to depths of 1200 m are recommended to explore for the non-anticlinal traps discussed here.
Taken from Ubaykhodzhayev, Abidov, and Ibragimov, 1986,
one map and one cross section; digested in Petroleum Geology,
Vol. 23, No. 3/4. Copyright 2003 James Clarke. You are
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