Discovery of Makhram and Mingbulak fields by the first wells on these large anticlines has stimulated interest in the Paleogene complex in structurally deeper zones, spurring regional studies. Seismic surveys indicate that in much of the Central graben area of Fergana depression and in a small sector along the north border of the depression the Paleogene rocks are at depths of 6 km and more. Karadzhidin, Kokand, and Kirov highs can be reached by the drill at depths of about 6 km. Kokand and Kirov highs are of prime interest because they are large and in the same tectonic position as Makharam field, which is just to the southwest.
Some investigators hold that where the Paleogene is at depths greater than 6 km, attention should turn to the Neogene. Favorability of this part of the section is not disputed; however, success of exploration has been extremely low. Pools are generally low in the section and sporadic because of lithologic inhomogeneity and non-persistence of permeable horizons. No hydrocarbon shows were recorded in the Neogene of Mingbulak,Karadzhidin, Kokand, and Kirov asreas.
The nature of the junction of the Central graben with the borders of the depression has been the subject of detailed study. Horizontal displacement has now been recorded on both the south and north borders. Impetus for study of this overthrusting onto the depression has come from the discovery of Obi-Shifo field on the south shoulder in 1984. The pool is controlled by a fault-related fold sealed in part by the overthrust.
The largest amount of horizontal displacement has been found along a fault between the Central graben and the south shoulder of the depression. Overthrusting there is 2-3 km for the Paleogene sediments. To the west this fault splays out into a series of feather faults, and total amount of overthrusting is only a few hundred meters. On the north border of the depression overthrusting has taken place along the North Fergana fault on almost the entire length of the depression. In contrast to the south border where the allochthon overthrusts anticlinal structures, such traps are not present on the north border.
The problem of Paleozoic oil is complex. Small accumulations of oxidized oil have been found in rocks of Silurian and Middle and Late Carboniferous age in the mountainous frame of the depression and in Carboniferous and Upper Permian rocks of the Kara-Chatyr, a Plaeozoic inlier in the eastern part of Fergana depression. Weak flows of oil and gas have been recovered from Paleozoics on the south shouler in Yuzhnyy Alamyshik and Khodzhiabad fields. A commercial flow of oil at about 100 cubic meters per day was recovered form Triassic-Upper Permian rocks in Boston field.
The Silurian and Carboniferous deposits are largely deltaic and contain much organic matter. Carbonates along with reef bodies are present in the Carboniferous.
Taken from Glumakov, Tabachnikova, and Abidov, 1992; digested
in Petroleum Geology, vol. 32, no. 1, one map and one cross section.
Copyright 2002 James Clarke. You are encouraged to print out
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