Geochemical criteria for characterization of the Devonian and Carboniferous organic-rich formations of Bashkortostan and their petroleum generation potential

UDK: 553.98
DOI: 10.24887/0028-2448-2018-6-46-49
Key words: the Republic of Bashkortostan, organic-rich formations, organic matter, vitrinite reflectance, pyrolysis, geochemical criteria, bitumen
Authors: R.Kh. Masagutov (Bashneft PJSOC, RF, Ufa), O.D. Ilemenova (BashNIPIneft LLC, RF, Ufa), V.N. Minkaev (Bashneft PJSOC, RF, Ufa)

The paper reviews the composition, specific features, lateral and vertical distribution of the Devonian and Carboniferous organic-rich formations. The study is aimed at the detailed understanding of the organic-rich sediments and their role as a source for petroleum generation in Bashkortostan. The organic-rich sediments typical of the Domanic formation are widespread throughout the territory of the Cis-Ural Bashkiria whereas those of the Upper Frasnian, Famenian and Tournaisian are confined only to structural lows – the axial parts of the Shalym, Aktanysh-Chishmy and Inzer-Usolsk paleo depressions of the Kama-Kinel System of Troughs. The results of pyrolysis (Rock-Eval) on samples from the Domanikian Formation, as well as the domanicoid sediments from the Tournaisian and Famenian sequences are compared and analyzed. The samples from the Domanic formation are characterized by significantly higher TOC values as compared to the Tournaisian and Famenian samples. The obtained results of pyrolysis and measured vitrinite reflectance indicate highest level of thermal maturity of the organic matter in the Domanic formation which corresponds to МК1-МК2 stages of catagenesis (mesocatagenesis) or the main oil window. The organic matter of the Tournaisian-Famenian sediments is characterized by relatively low thermal maturity. This observation is based on the value of vitrinite reflectance which indicates the ПК3–МК1 early stages of catagenesis (proto- and mesocatagenesis), also supported by the Pyrolytic Oil-Productivity Index values and Тmax. The difference in the level of maturity of the organic matter in the Domanic formation as compared to the Tournaisian and Famenian is explained by the difference in the initial composition of the deposited organic matter, tectonic evolution and burial history. The results of gas-liquid chromatography and chromate-mass-spectrometry are used for comparative characterization of bitumen from the Domanic formation and organic-rich formations of the Famenian to liquid hydrocarbons discovered in the Paleozoic reservoirs of Bashkortostan. The parameters of various groups of normal alkanes and isoprenoids in the analyzed samples of the Paleozoic oils and bitumen from the Domanic Formation may be easily correlated whereas the bitumen from the Famenian is quite different based on the geochemical criteria. The findings support the genetic origin of the bitumen of the Domanic Formation and petroleum found in the Devonian and Carboniferous plays of the Cis-Ural Bashkortostan. The organic-rich formations of the Upper Frasnian, Famenian and Tournaisian may also be regarded as an additional source of hydrocarbons.


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