From: "James Clarke" Subject: MIME-Version: 1.0 Content-Type: text/plain; charset="iso-8859-1" Content-Transfer-Encoding: 7bit X-Priority: 3 X-MSMail-Priority: Normal X-Unsent: 1 X-MimeOLE: Produced By Microsoft MimeOLE V4.72.3110.3 Lower Carboniferous reef traps, Part I

Lower Carboniferous Reef Traps, Dnieper-Donets Depression, Part I.
Internet Geology News Letter No. 169, November 1, 2002.

Carbonate reservoirs are host to 35-48 percent of the oil resources of the world and 23.5-28 percent of the gas. The importance of carbonate reservoirs is expected to increase with depth. At depths greater than 4 km only 18 percent of the petroleum is in clastic reservoirs, and 82 percent in carbonates. Reef traps are very important exploration targets. For example, in the Permian Basin of the North American craton 90 percent of the reserves are associated with large reef reservoirs. Analysis of 41 oil-gas provinces of the world that are favorable for reef reservoirs has shown that recoverable petro- leum resources in oil equivalent are 12.3-13.5 billion tons (86-95 BBOE). This is 10-11 percent of the hydrocarbons of these provinces.

In the Dnieper-Donets depression of the Ukraine the Lower Carbon- iferous carbonates are considered to be very favorable. Discoveries have already been made at depths greater than 4.5-5 km in Machekh, Kampan, and other fields. Most of the fields in carbonates of the study area are in reef deposits. Average porosity of this reef rock is 5-7 percent; however, values of 10-15 percent and more are not rare. Such reservoirs are expected to contribute significantly to increasing the hydrocarbon resources of the Ukraine in the coming years.

The theoretical basis for optimizing reef exploration in the study area is a morphological-genetic classification. The first complete classification of these carbonate traps was compiled by A. Ye. Lukin (1986). It was based on driling and seismic information on reef- bioherm bodies that formed during Tournaisian and early and late Visean cycles of sedimentation. It has now been found that chains of reef-bioherm masses of each tectonic-sedimentational cycle form belts that extend along the south and north border zones of the depression with closure on the west flank of the Srebnen depression.

The reef belt of the Dnieper-Donets depression may be classified as a structure of the first order. The chains of Tournaisian and lower and upper Visean reef-bioherm bodies are then second-order features. The reef-bioherm bodies themselves are third-order features. The development and location of the reef belt were controlled by the long- lasting north and south border faults of the depression. During the rift stage of the depression in Devonian and early Visean time these faults were channels for movement of fluids; volcanic activity intensified; and heat transfer was accomplished. In the sineklize stage of the late Visean to Early Permian they were a factor in structure formation and in development of lithofacies zonality, and this was favorable for reef formation.

Taken from Machilina, 2000. Digested in Petroleum Geology, vol. 36, no. 1, p. 1-8; two figures.
Copyright 2002 James Clarke. You are encouraged to print out this News Letter and to forward it to others. Earlier News Letters are available at: http://geocities.com/internetgeology/
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