More than 50 relatively small oil fields have been discovered in an area of about 5000 sq km to the west of giant Romashkino and Novo-Yelkhov oil fields, which are in the central part of the Volga-Ural oil-gas province. This area has been covered by a net of common depth point seismic profiles at 1.5-2.5 km/sq km. Most of the reserves are associated with drape structures on Upper Devonian Frasnian-Famennian reef buildups.
Zone "P" is the area immediately west of Novo-Yelkhov field. It is about 160 km long and 30 km wide, extending in a northeast direction. It is characterized by a high density of reef structures at some 10-20 in an area of 100 sq km. Next to it on the northwest is zone "B", which has the same trend as is zone "P" but is somewhat larger. Here Carboniferous erosional truncations have been intensive, and oil potential decreases on approach to the Kama-Kinel system of downwarps to the west.
The relationship of the erosional truncations (Vereya and Visean) to the reef structures along with comparison of structural plans of the various horizons indicates that diagenesis led to increase (with change of aragonite to calcite) or decrease (calcite plus magnesium ions to dolomite) in volume of a reef body over an extended time. As a result systems of radial and concentric faults developed above reef bodies, and these served as paths for vertical migration of magnesium ions and hydrocarbons.
These reef structures are anomalously dense dolomitized bodies, the vertical dimensions of which are commensurate with the horizontal. These bodies should be surrounded by a draping of fractured rock, along which oil has migrated vertically. The pools in the reef buildups are controlled not only by clay seals (Vereya and Tula-Serpukhovian) but also by dolomitized sub-vertical seals.
Wells drilled through reef bodies on into the clastic Devonian were not successful because no highs were present beneath the reefs. Close to the southeast margin of zone "P" the drilling disclosed three highs on the crystalline basement, two of which had associated reefs. By analogy with modern reefs it is clear that the reefs of zone "P" developed on a shallow-water shelf. Then to the southeast was a deep-water region, where reefs developed on elevated parts of the sea floor.
There is a group of reef buildups in zone "B", which on the Lower Carboniferous are 2-3 times taller than similar reefs of zone "P". This is explained by intensive subsidence of the sea floor while these reefs were forming. A clear indication of short stages of uplift during Vereya time is the presence of four erosional truncations to the west in the Kama-Kinel system of downwarps. (Could these phenomena be related to sea level changes due to glaciation in the Southern Hemisphere at this time? - JC)
Taken from Cheredov, 1997; digested in Petroleum Geology, vol, 32, no. 3, 1998. one map and one illustration of reefs. Copyright 2001 James Clarke. You are encouraged to print out this News Letter and to forward it to others. Earlier News Letters are available at our web page: http://geocities.com/internetgeology/ Internet Geology News Letter is distributed without charge in the interest of our science of petroleum geology. To be added to the distribution list please send your e-mail address to: jamesclarke@erols.com For information on the journal Petroleum Geolgy, please telephone or FAX to 703 759-3754.