Carbonate rocks are oil-gas-bearing over a wide stratigraphic range from Tournaisian in the Lower Carboniferous to Lower Permian in the Dnieper-Donets depression of the Ukraine. During the years 1980-1990 about 20 percent of drilling targeted on carbonates; yet, they provided omly 4-8 percent of the growth of reserves. This low success is attributed to inadequate understanding of the litho-facies characteristics of the carbonates.
Schematic maps have now been prepared of litho-facies distributions in carbonate rocks of the lower Visean and Tournaisian of the Lower Carboniferous for the central and southeastern parts of the Dnieper-Donets depression. Continental facies predominate along the borders of the depression, passing into near-shore marine carbonate-clastics and marine carbonates, and finally into deep-water clays in the axial areas of the depression. Maximum accumulation of carbonates took place in the central and southeast parts of the south border zone. The boundaries of these border zones are known in but a general way but will be better defined with further study.
In the central part of the south border zone of the Dnieper-Donets depression the basal part of the Tournaisian section consists of clayey carbonate rock with subordinate beds of siltstone and sandstone. The limestones are dark gray, organo-clastic, and carry thin beds of black shale. The rock is penetrated by thin veins and films of bituminous material. These alternating clay-carbonate rocks are a deep-water marine facies. They are overlain by 200-300 m of limy sandstone. Higher in the section these sandstones pass into massive brownish gray limestone, which is extensively fractured and cavernous in places. It is organo- clastic and largely algal. Recrystallization is intense.
Carbonates are less abundant in the upper Visean and upper Serpukhovian sections. In these complexes the carbonates are not an independent target of study but are treated along with the clastics. Reef buildups have been described in this part of the section over in the northwest part of the Donets Basin.
Favorable sites for deposition of biohermal buildups in the study area are highs in the relief of the basin floor, individual high blocks, crests of anticlines, flexures on monoclines, and areas of uneven sedimentation on the basin floor.
Abnormally high formation pressures in several fields at depths
greater than 5000 m is interpreted as the result of high geodynamic
activity, redistribution of stresses in the rock, and introduction of gas
and water along zones of deep faults. Indicative of this are geochemical
anomalies in which the formation waters have a low mineralization and
contain the micro-elements chromium, titanium, vanadium, zinc, lead,
and mercury, these being characteristic of deep petrogenesis. A heavy
carbon isotopic composition is characteristic of pools associated with
migration of hydrocarbons from deep kitchens. (Taken from Zyuz'kevich,
Krivosheva, and Machuzhak, 1994; digested in Petroleum Geology,
vol 30, no. 1, 1996, two maps and two cross sections)
Copyright 2001 James Clarke. You are encouraged to print out this
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