During the Carboniferous huge carbonate platform deposits collected
along the margins of the North Caspian depression. Many of these were
atolls; others were massifs of such tremendous size that they form arches
(Astrakhan); while others developed high relief in great part due to
Late Carboniferous erosion (Tengiz). Astrakhan, Tengiz, and Karachaganak
are giant deposits, all sealed by Lower Permian salt. Another such
carbonate body is Yuzhnyy, to the south of Tengiz. It is not oil-bearing,
however, because the Permian salt does not extend that far south.

An additional possible carbonate platform massif has now been indicated
by seismic surveys in association with the Karpinskiy ridge, which is a
complex of Paleozoic rocks that have been thrust north-northeastward onto
the southwest border of the North Caspian depression. This feature is an
eastward continuation of the Dnieper-Donets depression on the west, which
passes eastward into the folded Donets Basin and thence into the Karpinskiy
ridge. This region has been studied by geophysical surveys for more than 40
years. In 1955-65 a net of east-west and north-south seismic profiles were
run on a spacing of 40-60 km. These established the main structural
features: From west to east are highs with intervening lows. The eastern
part of Karpinskiy ridge was regarded as favorable for oil and gas. The
western sector was neglected.

Study of the western sector was renewed in the early Seventies with
several seismic surveys; however, subsequent drilling failed to make any
significant discoveries. In 1994-96 attention turned to reinterpretation of
the seismic data along with incorporation of new data. A significant
feature registered on one of the profiles is a high-velocity interface in
the sedimentary section, indicating presence of a thick carbonate body. It
is probably an analog of the Astrakhan carbonate body, and its thickness
should be not less than 3 km. Thus there appears to be a carbonate massif
in a transition zone between the Karpinskiy ridge proper and the Sarpin
downwarp of the North Caspian depression.

This transition zone is some 50 km wide. Within it the structure of
the Paleozoic rocks differs from that to the north (North Caspian
depression) and to the south (Karpinskiy ridge). Thus the northern part of
what had been regarded as part of the Karpinskiy ridge is now recognized as
a separate terrane, within which appears to be a carbonate body that is an
analog of such carbonate massifs as the Yuzhnyy, Tengiz, and Astrakhan.
These massifs form a single chain of intra-basin carbonate buildups that
extends along the margin of the North Caspian depression. The presently
available information is insufficient for determining the spatial position
of this massif. All proposed models for this area, including this one,
require further examination. The important thing is the possible presence
of a new carbonate target for oil-gas exploration (Petroleum Geology, vol.
34, no. 1, p, 53-60, 2000)

Copyright 1999 James Clarke. You are encouraged to download this News
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