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 III

Lower Carboniferous Reef Traps, Dnieper-Donets Depression, Part III.
Internet Geology News Letter No. 171, November 25, 2002

The next four morphological-genetic types of organic buildups of the Dnieper-Donets depression are as follows:

3. Among reef traps of the lithologic-paleogeomorphological type the most favorable are the following three sub-types:

The first sub-type are organic buildups that are expressed morpholo- gically on the carbonate platforms and also in the overlying sediments. Biohermal and biomorphic resevoirs are sealed by clay-carbonate sediments of shallow-water shelf facies. The seal for the upper part of the reef traps is generally clayey rock.

The second sub-type differs from the first by occurence of reef-bioherms close to salt domes. For example, the Chernuzhin biohermal massif, which has a core 220 m high, is sealed by the large Pozyakov salt dome. Reservoirs are likely on the periphery of this massif.

The third sub-type are asymmetrical buildups located at the junction of the carbonate platform and the depression. As a rule the back-reef part is sealed by dense carbonate rock, and the frontal part by bituminous clays of basin facies. This type of trap is found on the gentle or terrace flanks of the carbonate platform.

On the whole the reef traps of the lithological-paleogeomorphological type have a wider zone of oil-gas accumulation in comparison with the paleogeomorphological type.

In the following three types the main factors in their formation are structure, tectonics, and cyclicity.

4. Reef-biohermal traps of structural-stratigraphic type have formed on structures or swell-like highs in the basement relief. Structure-forming tectonic oscillations of the basin floor and changes in sea level led to stratigraphic breaks and cyclic structure of their reservoirs.

5. Reef traps of lithologic-structural-paleogeomorphological type are associated with flexures on monoclines, where large (80-130 m) bioherms are found, and also with weakly expressed highs where intra-basin organic buildups have formed (Bilichev bioherm). A distinguishing feature of the intra-basin reefs and bioherms is that they are surrounded on all sides by contemporaneous depression facies. They have grown always on relatively uplifted areas of the basin floor. Tectonic or eustatic factors contributed to cyclicity in these features. Optimum reservoir properties are found in the cores of these buildups.

6. Reef traps of the combined type are characteristically bounded by faults, which may serve as conduits for transmission of hydrocarbons or act as seals for pools.

The oil and gas in the Tournaisian reef massifs may have been derived from source beds in the Upper Devonian and/or Tournaisian. The oil and gas of the Visean reef bodies was from vertical and lateral migrstion from organic-rich Domanik-like (Rudov) beds at the base of the Visean.

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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