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Geology & Geochemistry of Combustible Minerals No. 1 (182) 2020, 52-61.
Yurii KHOKHA, Oleksandr LYUBCHAK, Myroslava YAKOVENKO, Dmytro BRYKInstitute of Geology and Geochemistry of Combustible Minerals of National Academy of Sciences of Ukraine, Lviv
This paper considers the issue of determining the maximum hydrocarbons amount that can be generated by kerogen using thermodynamic methods. It is shown that the chemical composition of natural gas or gas condensate contains information about the generative capacity of kerogen from which it was formed. Based on experiments of type II and I kerogen pyrolysis and thermodynamic calculations by entropy maximization method, we propose a new method for determining the amount of kerogen from which gas was formed, which contains 1 dm3 of methane at a given ratio of butane isomers. The obtained data are interpreted as an indicator of kerogen maturity in the context of the depth of its destruction.
This method is applied to theWestern oil and gas region of Ukraine hydrocarbon deposits. The analysis of kerogen transformations in the region sedimentary strata, using criteria of the GASTAR diagram, is carried out. We assessed the trends of kerogen conversion in the region in the areas of “maturity” and “biodegradation” in the ratio of ethane/propane (C2/C3) to ethane/isobutane (C2/i-C4). It is shown that the majority of deposits in the Western oil and gas region developed in the direction of maturation and only a small group of gas deposits – biodegradation.
To establish the gases genesis in the region, we built a graph of the two geochemical indicators dependence – the methane/ethane ratio (C1/C2) and ethane/propane ratio (C2/C3). It is shown that some of the gas fields is formed due to the conversion of organic material of oil deposits. At the same time, gas condensate fields in the region, with few exceptions, are formed due to the primary destruction of kerogen.
Based on the results of the calculations, maps of the methane (generated by type II kerogen) amount distribution were constructed. It is established that kerogen, which was the source material for hydrocarbon deposits of Boryslav-Pokuts oil and gas region, has practically exhausted its gas generation potential. Instead, kerogen from gas and gas condensate fields in the Bilche-Volytska oil and gas district still retains the potential to generate hydrocarbons.
kerogen, butane isomers, thermodynamic modelling, gas-generating potential.
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