With discovery of the super-giant Romashkino and Novo-Yelkov fields in Upper Devonian Frasnian clastics more than 50 years ago, interest in the Middle Devonian was neglected. Pools were found in the Middle Devonian only by occasional wells. Market conditions dictate that increases in reserves be gained without great capital expenditure. This is possible with Romashkino field, where local pays of high-quality oil can be foumd in the Devonian and in particular in the Eifelian and Givetian of the Middle Devonian. A difficulty is the limited size of these pools.
The Middle Devonian section at Romashkino consists of Eifelian rocks overlain by Givetian rocks. In ascending order within the Givetian are the Aftonin, Vorob'yev, Ardatov (Starooskol), and Mullin Horizons. Within the Ardatov is the "Middle Limestone" . The "Upper Limestone" is in the Pashiy Horizon, which is at the base of the Upper Devonian Frasnian section.
A characteristic feature of the pays of Vorob'yev and Ardatov Horizons is low electrical resistivity where they are commercially oil-bearing. The problem here is distinguishing between oil-bearing and water-bearing strata.
Up until the Eighties most of the production in Romashkino field was from Horizons D-I and D-0 of the Pashiy Horizon. Only a few wells penetrated to the "Middle Limestone". During the Eighties drilling became focused on the Eifelian and Givetian. Effectiveness was low, however. Studies of the Vorob'yev and Ardatov Horizons were made in1993-94.
The number of wells that penetrated the "Middle Limestone" at Romashkino is more than 3200, and this is 17 percent of the total number of wells drilled on the Devonian. The number of wells is much fewer that reached productive Strata D-III and D-IV, which are in the Ardatov beneath the "Middle Limestone". About 5 percent of the wells reached basement.
As a guide for exploration of the Middle Devonian, an isopach map was compiled for thickness of the section between the base of the "Middle Limestone" and the base of the "Upper Limestone". This compilation used data from about 20,000 wells.
Thickness distribution indicates that comparatively small- amplitude downwarps of north-south and north-northeast trend are present in the relief of the "Middle Limestone". These ancient downwarps are traced over the entire cetral part of the Tatar arch and continue in the form of individual linear zones on the north, northeast, south, and southeast flanks. These syn-sedimentary downwarps are compensated almost completely on the top of the "Upper Limestone".
All parts of Romashkino field are favorable for finding commercial oil pools in Strata III and IV of Ardatov age. A total of 265 prospective areas are recognized. This play is expected to add 90 million tons (630 MM barrels) to recoverable reserves. Studies similar to this are recommended for all areas of Tatarstan where Middle Devonian rocks are present. (Taken largely from Larochkina and others, 1997; digested in Petroleum Geology, vol. 32, no. 1, 1997, two structure maps)
Copyright 2000 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/ This News Letter is distributed without charge in the interest of the science of petroleum geology. To be added to the distribution list please send your e-mail address to: jamesclarke@erols.com