Lithofacial characteristics of reservoir rocks of the Upper Paleozoic subsalt complex of the eastern and south-eastern parts of the Pre-Caspian depression

UDK: 553.98
DOI: 10.24887/0028-2448-2023-5-36-39
Key words: lithofacies characteristic, Pre-Caspian basin, pre-salt deposits, Pre-Caspian depression, oil and gas potential, reservoir, hydrocarbon system
Authors: D.O. Almazov (VNIIneft JSC, RF, Moscow), A.S. Rakhmatullina (Gubkin University, RF, Moscow), K.I. Dantsova (Gubkin University, RF, Moscow), A.S. Monakova (Gubkin University, RF, Moscow), S.F. Khafisov (Gubkin University, RF, Moscow)

The article is about formation conditions and the main patterns of distribution of potential reservoir rocks of the pre-salt complex of the Upper Paleozoic of the eastern and southeastern parts of the Caspian depression. The studies were based on a complex of geological and geophysical data, including seismic data, borehole geophysics, core data, as well as on literature data, using examples of already studied deposits close in genesis to those under study. The lithofacies environment of the formation of both carbonate and terrigenous complexes are considered. Structural, textural, material features and a set of lithotypes of the carbonate natural reservoirs make it possible to use J. Wilson sedimentation model with the identification of most favorable zones for the formation of the reservoir potential. According to this, the formation of the carbonate complex took place in relatively shallow waters with different hydrodynamics. The boundstones of the facies zone of organogenic structures and the grainstones of the upper slope are characterized with the highest porosity coefficient. Moreover, an obligatory condition for the presence of reservoir rocks with improved characteristics is the confinement of deposits to hypergenesis zones.

The formation of the terrigenous complex took place during and under the conditions of the growing mountain structure of the Urals, when the marginal trench was filled with clastic material carried from the orogen. Such deposits are characterized by a rhythmic structure and a gradation distribution of fragments within rhythms. In such rhythms, potential reservoirs will be confined to the bases of the rhythms, for example sandstones have the highest porosity in the studied wells. However, deposits of this genesis will be characterized by high vertical anisotropy due to the relatively thin alternation of rocks. Nowadays, the carbonate complex where large deposits of oil and gas have already been discovered in is the most studied. The terrigenous pre-salt complex of the Caspian depression has been poorly studied, however, the oil and gas potential of deposits of a similar genesis has already been proven in the Permian deposits of the United States.


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