Fergana depression is in the Central Asian republics of Uzbekistan, Tadzhikistan, and Kyrgyzstan. Please see also the News Letter no. 13, October 4, 1999. In spite of a long history of oil-gas exploration in this depression several questions remain unanswered: 1) Nature of the deformation and depth of occurrence of the Paleogene rocks in the central part of the depression; 2) scale of overthrusting and assessment of the Mesozoic and Cenozoic in the overthrust zones along the north and south borders of the depression; and 3) oil-gas potential of the Paleozoics.
The Paleogene rocks are the main exploration target, and they occur at depths of 6-7 km and more over a considerable part of the depression. Play of this target is restrained on the one hand by limited capacity for deep drilling and on the other by absence of a reliable understanding of the structure. As a consequence there are differences in opinion as to degree of deformation and depth of occurrence of the Paleogene sediments.
According to Azimov and others the Neogene in the central part of the depression is weakly deformed, whereas large anticlinal zones are possibly present in the Paleogene. The crests of these structures in the Paleogene are at depths of up to 6 km and consequently within reach of the drill.
Study of the central part of the depression has been largely by geophysical methods. Concepts as to its structure have been based largely on reflection surveys. These ideas contrast with those based on electrical surveys, and the cause of this is unclear. For example, the electrical surveys indicate a chain of linear anticlinal structures in the Central graben including from east to west Mingbulak, Zapadnyy Mingbulak, Kurgancha, Buvaydy, and Kokand. Within these areas the depth to the marker electrical horizon changes from -5.5 km on the Kokand high to -4,5 km at Mingbulak. Seismic reflection studies do not show such linear folds and variation in depth to a marker horizon in the Turkestan Beds. They indicate a deep depression in the sector between Kokand high on the west and Mingbulak high on the east.
Common depth point surveys made later indicate the same picture of deformation in the Central graben as did the reflection surveys. The Gumkhana and Mingbulak highs and the Damkul and Karadar'in downwarps are clearly expressed. Overthrusting was determined in the zone of junction of the Central graben and the south shoulder of the depression.
Multi-pay fields were discovered by parametric wells on the Makhram (well 1) and Mingbulak (well 3) highs. Mingbulak well 3 confirms the geologic prediction as to presence of oil at depths of about 6 km. Also, the top of the Paleogene Turkmen Beds, which is generally a marker reflecting horizon, is 300 m higher than had been expected from the seismic surveys. The reflector here is actually the boundary between the Paleogene and the Cretaceous. This reflector, which had thus been incorrectly identified, had been the basis for structural models. Continued in Part II.
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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