Radioactivity methods are not decisive in oil-gas exploration; never the less they are used widely in early stages of study of a prospective area. Ground surveys in 1957-66 in Bashkortostan and Perm Region demonstrated that rings of elevated values of natural radioactivity were present along the margins of oil fields, and low values were characteristic of the central part of these fields within the rings. No radioactivity anomalies were found in association with dry structures.
Aero-geophysical surveys were then made in poorly studied parts of the north and west of the Volga-Ural oil-gas province in the area of the Vyat swell as well as on Shkapov and Sergeyev oil fields of Bashkortostan with aircraft flying at heights of 40, 60, and 100 m.
The area at Sergeyev field has subdued relief, no river net, and is swampy. Low values of natural radioactivity at up to 4 mkR/h are found above the oil pool, and values of 5.5-6.5 mkR/h along the periphery. The high values lie along a ring that repeats the configuration of the pool.
Conditions are similar but less pronounced above Shkapov field, which is located in the valleys of the Ik and Meleuz Rivers. Natural radioactivity is at 3.5-4.5 mkR/h above the pool and at 5-6 mkR/h along the periphery. The higher values occur along a ring, although there are breaks in its continuity.
Surveys on the Vyat swell even with comparatively low-sensitivity instruments showed that the Syt'yan structure is marked by a broad ring anomaly, in the center of which the natural radioactivity is 2-3 mkR/h lower than along the periphery.
Some conclusions from the above study: 1) Oil pools are marked by areas with lower values of radioactivity, the configuration and spatial position of which coincide approximately with the outline of the pool. The decrease in radioactivity does not exceed 25 percent of background, and variations are not abrupt. 2) Beyond the margin of the pool is a zone of elevated radioactivity that forms a ring. Values there are approximately 10 percent above background. 3) Where a pool has multi-pays, the anomaly corresponds with the average margin of the pool. 4) The shape of the radioactivity anomaly is affected by lithology of the outcropping rock, topographic relief, ground water level, and vegetation.
Results of radiometric surveys in conjunction with magnetic data are the basis for predicting 27 favorable sectors and possible structures in the Orenburg part of the Cis-Ural foredeep. Similar surveys were made at scale of 1:25,000 in 1990 in the northern part of Perm Region. These surveys disclosed several anomalies in the radioactivity field in overthrust zones of the Cis-Ural foredeep. To the west of the foredeep the information is less reliable due to swamps and vegetation. Never the less, 14 favorable anomalies have been recognized there in an area of 7500 sq km.
In Tatarstan aero-geophysical surveys in 1994 on a scale of 1:25,000 covered 1,500 sq km on the southeast flank of Tokmov arch and 6,000 sq km on the north margin of South Tatar arch. No petroleum discoveries have been made in the first of these, although seven favorable sectors have been recognized. Oil fields are present in the second sector. (Taken from Mavrichev and Molodtsov, 1999; digested in Petroleum Geology, vol. 34, no. 1, p. 1-9, 2000, 4 maps and 2 cross sections)
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