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Type sections from the terrigenous Lower Visean oil and gas-bearing complex in the Middle Volga Region

UDK: 553.98(470.4)
Key words: Bobrikovskian horizon, type sections, sedimentation settings, reservoir forecast
Authors: S.V. Astarkin (KogalymNIPIneft Branch of LUKOIL-Engineering in Tyumen, RF, Tyumen), О.P. Goncharenko, (National Research Saratov State University, RF, Saratov), Yu.A. Pisarenko (Lower Volga Scientific Research Institute of Geology and Geophysics, RF, Saratov), V.P. Morozov (Kazan (Volga Region) Federal University, RF, Kazan)

The Bobrikovskian horizon within the junction zone of the Ryazan-Saratov trough and the Zhiguli-Pugachev swell is peculiar for extreme variations of its thicknesses and stratigraphic completeness of the section. This is accounted for by the Early Visean regressions and transgressions. Diverse facies conditions used to contribute to formation of non-anticlinal (stratigraphic and lithologic) oil and gas traps currently associated with the major prospects for hydrocarbon exploration. Therefore, investigations aimed at specifying the structure of the Bobrikovskian horizon and reconstructing the settings of its formation is highly topical.

The major aim of the study consisted in typifying the sections from the Bobrikovskian producing horizon within the junction zone of the Ryazan-Saratov trough and the Zhiguli-Pugachev swell.

Analyses of the well log data and thorough lithological examination of the core material fromboth, the earlier described and the newly drilled wells have resulted in recognizing three principal section types in the Bobrikovskian horizon and in revealing their areal distribution regularities. Variations in distribution of the effective thicknesses, reservoir-rock porosity and permeability are accounted for by the rocks lithologic changeability both, along the well section and over the area, as well as by sedimentation settings in the paleobasin. Section typifying complies with the paleostructural plan of the area and is largely related to the paleogeomorphological features of the basin structure and of the sediment accumulation. Analysis of the peculiarities in the examined section types – facies sedimentation settings - has made it possible to recognize the section intervals most promising in terms of searching for HC deposits. Sections of the second type are regarded as prospective ones; those may be associated with both, deposits of anticlinal type and traps of non-anticlinal type. The latter ones may occur in the regional zones of layer pinch-outs in the trough walls.
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