Geochemical evidence for petroleum potential of Domanic deposits in the Republic of Tatarstan

UDK: 553.98(470.41)

Key words: Domanic deposits, oil generation potential, maturity, kerogen, organic matter

Authors: R.S. Khisamov (Tatneft PJSC, RF, Almetyevsk), V.G. Bazarevskaya, PhD, T.I. Tarasova, O.V. Mikhailova, S.N. Mikhailov (TatNIPIneft, RF, Bugulma)

Detailed survey of Domanic formations in Tatarstan has been under way since 2012. To date, sufficient amount of data has been obtained and analyzed. The analysis shows that Domanic formations in Tatarstan are presented by domanikites occupying a large starved basin of the Sargaev-Domanik-Mendymskian age and having TOC of 5 – 20 %, and domanikoids being analogous to Upper Frasnian-Tournaisian shallow-water bioherm-carbonate deposits and having TOC of 0.5 – 5%. These deposits occupy the central areas of the Kama-Kinel system of starved downwarps and the total territory of Tatarstan. 
According to data analysis, domanikites are presented by nonuniform interbedding of carbonate-silica rocks with limestones and dolomites. Organic matter varies from very low to high concentrations. But, regardless of high variation range, the majority of the core samples (55%) have organic matter content over 4%. Whatever organic matter content is, domanikites exhibit HI from 300 to 600 mg·HC/g, which is typical of oil generating kerogen, type 2 (sapropelic). Generally, rock maturity is rather low; according to pyrolysis data, Tmax value averages to 425oC, corresponding to protocatagenesis termination zone. Hence, domanikites are at the beginning of the main oil generation phase. However, deposits maturity within the South-Tatar uplift is higher compared to the Melekessk depression, which can obviously be attributed to specific geothermal conditions in this area, as well as to formation of deposits with high sulphur content, which affected rock maturity distribution and the type of kerogen capable of generating early pre-main-phase oils. These oils contain heavy components (resins and asphaltenes) and sulphur. Analysis of Tatarstan oil composition and density supports this idea. 
Domanikoids are presented by carbonates with varying content of organic matter which, in general, is rather low. As compared to domanikites, domanikoids have higher content of humus substance. Rock maturity is low; according to pyrolysis data, Tmax averages to 424oC, which corresponds to protocatagenesis zone. Productivity index (PI) conforms to Tmax values and corresponds to protocatagenesis zone, as well. 
So, both domanikites and domanikoids exhibit good hydrocarbon potential, being at the end of protocatagenesis and at the start of mezocatagenesis processes throughout the entire territory under consideration. However, considering the potential for early pre-main-phase oil generation and presence of confirmed oil accumulations in Domanic facies associated with natural fracture zones, we can conclude that these deposits show good promise for discovering new unconventional oil reservoirs. To produce oil from such reservoirs, optimization of thermal recovery processes, as well as acid and multi-stage fracturing methods is required.
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