The Upper Devonian carbonate deposits of the Volga-Ural oil and gas province are characterized by a wide distribution of bioherms, that control a large number of oil deposits. Deposits in the Upper Devonian carbonate strata are unevenly distributed over the area and section and are confined mainly to the territories of vast paleoshelfs. The main risk factor in prospecting for hydrocarbon deposits is the quality of seals.
The article discusses the structural features of the Bashkir and South Tatar paleoshelfs at the Middle and Late Famennian stages of development, which determined the patterns of distribution of reservoirs and seals in the Upper Devonian carbonate strata. The South Tatar and Bashkir paleoshelfs differ from each other in their internal structure. The main oil deposits on the South Tatar arch are belong to the deposits of the middle and upper Famennian, and on the Bashkirian - to the deposits of the Lower Famennian. The Famennian seals are composed of clayey-carbonate rocks. Their accumulation is associated with the stages of maximum transgression, when the shelf area totally sank below the wave base, as well as with the stages of maximum regression, when the barrier zone limited water exchange in the shelf. At that time, organogenic carbonates without clay admixtures, with bad seal properties, accumulated in the hydrodynamically active side zone. The Visean claystones are regional seals in the study area. The seals identified in the carbonate strata of the Famennian Stage are classified as zonal and local. The facies heterogeneities of the Upper Devonian carbonate shelves revealed decades ago and supplemented by us make it possible to predict the distribution of seals and reservoirs over the area. The proposed technique can be applied in neighboring regions.
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