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 II

Lower Carboniferous Reef Traps, Dnieper-Donets Depression, Part II.
Internet Geology News Letter No. 170, November 19, 2002.

The discontinuous chains of Tournaisian and Visean reef-bioherm bodies of the Dnieper-Donets depression of the Ukraine, in addition to occurring in close proximity to zones of deep rift faults, are associated with the border zones and flanks of a carbonate platform. Both simple and complex morphologies are found among these reef-bioherm bodies. The first have simple exterior and internal structure and are small. The second are characterized by facies and ecological differentiation, cyclicity, and/or multiple crests. Examples are the complex Kampan reefoid 3.5 km long, and the six-kilometer Selyukov biohermal mass, as outlined by seismic survey.

Most of the large organic buildups are located directly on the flanks of basement highs and depressions. For example, the Machekh bioherm is directly on the flank of the Landariy basement depression, which is at depth of 7.5-8.25 km. The basement beneath this structure is cut by faults, and each successive block is downdropped with respect to the previous block in the direction toward the axial zone of the basin. Similarly, the Rodnikov biostrom and Kampan reefoids are located above flanks of basement structures.

Many large organic buildups are associated wtih swell-like highs and their flanks that developed from Devonian volcanism (Rudov, Klin, and other bioherms) or salt diapirism (Molchanov, Pereshchepin, Bogatoy bioherms, and others).

The six main morphological-genetic types of organic buildups of the Dnieper-Donets depression and their classification as hydrocarbon traps are now described. The first three are relatively simple, controlled by two factors: paleogeomorphology and lithology.

1. The paleogeomorphological type of reef trap is an elevation above a carbonate platform of a solitary or multi-crest bioherm or reef buildup. Height above the platform of all those presently known in the study area is 50-70 m. The rock is largely algal limestone of Tournaisian or early Visean age and biochemical limestone and dolomite of late Visean (early Tula) age. These bodies may be almost isometric an plan or linear. Reefs of the barrier type are bounded on the back side by compact carbonate rocks of shallow-water shelf or lagoonal facies and on the frontal side by basin facies. Reservoir properties are best in the cores of such features.

2. Reef traps of the lithogenetic type include: a) small (15-30 m high) organic banks that have formed in the back-reef zone of the barrier system on the north flank of the Srebnen downwarp; b) intra-shelf bioherms; and c) small bioherms (amplitude up to 60 m).

As a rule all types of traps are poorly expressed in the overlying sediments, and the reservoirs within them are sealed by clayey limestones of shallow-water shelf facies. Small traps here have proved to be oil-gas-bearing.

Continued as next News Letter 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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