Generation potential of the Devonian rocks of the Volyn-Podillya Plate

Since the 1960s, two gas fields (Lokachi and Velyki Mosty) have been discovered in the Devonian sequence of the Volyn-Podillya Plate, and numerous gas shows have been observed in the Lokachi, Olesko, Horokhiv, and Oglyadiv areas. Organic-rich rocks within the Volyn-Podillya Plate are widespread in the Lower, Middle and Upper Devonian strata. They are represented by terrigenous, clayey and carbonate layers. The objective of this study is to investigate the generation potential of the Devonian rocks of the Volyn-Podillya Plate and the possibility of their participation in the petroleum system of the region. Geochemical studies covered the entire territory of the Volyn-Podillya Plate as well as the entire chronological interval of the Devonian strata. Rock-Eval pyrolysis studies showed that the content of total organic carbon in the Lower Devonian organic-rich rocks ranges from 0.01 to 0.45 % (average values 0.12 %). The organic matter in these rocks contains mainly kerogen of marine origin type II, and has undergone primary and/or secondary oxidation processes. The content of total organic carbon in Middle Devonian sediments varies: in rocks of the Eifelian Stage from 0.02 to 0.64 % (average values 0.08 %), in Givetian from 0.01 to 2.35 % (average values 0.19 %), in Frasnian from 0.04 to 1.43 % (average values 0.08 %), in Famennian from 0.07 to 0.10 % (average values 0.09 %). The thermal maturity level of the Lower (Lochkovian Stage), Middle (Eifelian, Givetian Stages) and Upper (Frasnian, Famennian Stages) Devonian ranges from immature to overmature rocks. The Middle and Upper Devonian rocks are dominated by type II marine kerogen, which underwent primary oxidation during sedimentation and/or secondary hydrothermal oxidation of dispersed organic matter during dolomitization. The pyrolysis temperature Tmax varies from 422 to 527 °C, demonstrating that the degree of thermal transformation of kerogen ranges from immature to overmature, with a significant part of the sediments being within the zones of generation of liquid and gaseous hydrocarbons, which indicates the significant role of Devonian sediments in the formation of the petroleum system of the region. Geochemical studies of the generation properties of the Middle and Upper Devonian sediments within the Volyn-Podillya Plate showed that they can be considered as oil and gas source rocks in the Upper Paleozoic sequence.