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Oil-Source Rock and Gas-Source Rock Correlations in the Dniepr Donets Basin (Ukraine): Preliminary Results

Abstract

The Dniepr Donets Basin (DDB) is a Devonian rift-structure located within the East European Craton. It is filled with Devonian syn-rift sediments and a thick Carboniferous to Mesozoic post-rift succession. The basin hosts more than 200 oil and gas fields, mainly in Carboniferous clastic rocks. Oil deposits are found in the shallow NW part, whereas gas deposits prevail in the deeper central and SE parts. Potential source rocks occur in various stratigraphic levels. Rocks with TOC contents exceeding 1 % and containing kerogen type III-II are widely distributed in Devonian to upper Visean horizons. The most prolific interval is an upper Visean black shale (Rudov Beds) with more than 5 %TOC. Highly oil-prone black shales occur in Serpukhovian intervals in the NW part of the DDB. Oil-prone Lower Serpukhovian and gas condensate-prone Middle Carboniferous coal is widespread in the S and SE part of the basin. In the 1990s USGS investigated about 40 oil and 18 gas samples. Some biomarker and stable isotope parameters are available from the USGS database. In the present contribution we use these data together with data from 13 oil and condensate samples, which were determined within the frame of the present study and compare them to published data measured on rock extracts. Pristane/phytane (Pr/Ph) ratios of accumulated oils and condensates vary significantly between 0.7 and 3.9. Typically samples with low Pr/Ph ratios are characterized by light C-isotopy. The dibenzothiophene/phenanthrene (DBT/Ph) ratio is low (<1) in all studied samples. Low Pr/Ph ratios (<1.3) are typically found in extracts from Devonian rocks and from Rudov Beds. Higher Pr/Ph ratios are derived from Serpukhovian rocks. Pr/Ph ratios from other Upper Visean rocks range from 1.1 to 2.3. Extracts from Devonian rocks are characterized by lighter carbon isotopic composition than extracts from Visean and Serpukhovian rocks. Unfortunately no data for Rudov Beds are available yet. With the exception of Lower Visean carbonate rocks DBT/Ph ratios of potential source rocks are generally low. This suggests that different source rocks contributed to the accumulated oil and condensate. Isotopic compositions of methane and ethane from gas samples in the DDB suggest generation from source rocks containing a type II kerogen with varying maturity (0.6 – 1.5 %Rr). Based on isotopic composition, gas from the southeastern part of the basin may be derived from coal-bearing successions.