The Yablunov-Yarov buried anticlinal zone of the central part of the Zhdanov downwarp was detected in 1972-74 by seismic surveys, and deep drilling began in 1976. The first well yielded commercial flows of gas and condensate from Tournaisian and Visean sediments of the Carboniferous. The field was named the Yablunov.
The structure measures 12 by 9.5 km and has closure of about 900 m. It is cut by faults into blocks. Yablunov high is a salt-assist structure that flattens out upward in the section. The salt tectonics were the most active in Early Carboniferous time. The structure had already formed by the time that the Lower Carboniferous sediments entered the gas window. At no subsequent time did it lose its closure.
The sediments of the Tournaisian Stage are largely sandstone and siltstone; they form a single hydrodynamically interconnected reservoir in which a gas pool of the massive-blanket type formed. This pool also includes sandstones in the upper part of the Devonian. There is no hydrodynamic connection between blocks.
Sixteen productive horizons have been discovered and studied in Yablunov field. These are Moscovian, Bashkirian, Visean, and Tournaisian as well as Devonian. They are combined into three stages: Tournaisian, Visean, and Middle Carboniferous. The pools of the Tournaisian Stage have the largest reserves of the field. The height of the pool on the central block is 520 m. On other blocks it ranges from 86 to 377 m. These pools extend over large areas. The reserves in the Visean sediments are concentrated on the central block, where they account for 28 percent of the total reserves of the field. An oil pool is present in the upper Visean above the gas pools. It appears to be sealed lithologically. The pools in the Bashkirian and Moscovian Stages are in the high parts of the down-dropped north and east blocks. At the crest of the central block the Bashkirian sediments are almost completely eroded, and the Moscovian sandstones are water-bearing.
The pool in the Tournaisian-Devonian rocks is gas condensate, and those in the Visean rocks are gas condensate with an oil ring and oil. The oil is viscous and high in tar and paraffin. The pools in the Bashkirian are gas condensate and heavy oil. The latter is used for roads. The oil of the Moscovian pools is viscous, oxidized, and practically immobile. It lost its volatiles during subsidence in pre-Permian time when the seal was lost.
Yablunov field is the largest in the region in Lower Carboniferous rocks. Total thickness of the oil-gas-bearing Paleozoic sediments in the field is 1900 m, extending from a depth of 3160 m to 5100 m.
Several large depressions of the Central Graben of the Dnieper- Donets depression were yet to be explored or had been only moderately explored as of 1986. Several low amplitude structures have been recognized within them.
Taken from Baranovskaya and others, 1986; digested in
Petroleum Geology, vol. 23, no/ 9/10, one structure map.
Copyright 2003 James Clarke. You are encouraged to print out this
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