Lithological and geochemical characteristics of the Middle Devonian strata of the Lviv depression in the aspect of their oil and gas bearing prospects

Abstract

The Middle Devonian deposits within the Lviv Depression of the Volyn-Podillya plate are largely underexplored and are of great interest for further exploration for hydrocarbons. The presence of two discovered gas fields and the occurrence of granular and fractured reservoir rocks within the entire Lviv Depression point that the deposits of this age range are prospective for further exploration works for hydrocarbons. The authors conducted mineralogical and petrographic studies of these strata in order to study different types of reservoir rocks. Petrographic studies of terrigenous rocks showed that the reservoir rocks are composed of fine-grained and medium-grained sandstones, as well as fine-grained and coarse-grained siltstones. The matrix in these rocks is contact-porous and contact, composed of dolomitized calcite (4–19 %) and hydromica (3–13 %). Regardless of the type of matrix, the pore space in rocks is formed by intergranular spaces of 0.05 to 0.5 mm size. Siltstone-sandstone deposits represent the granular-type reservoir rocks, the filtration properties of which are formed by the intergranular space, while fractures are of subordinate importance. Terrigenous rocks form gas-bearing horizons in Middle Devonian (Eiffelian and Zhivetian) in the Lokachi field of the Lviv Depression. Carbonate rocks are represented by a wide range of lithological types from slightly dolomitized biodetrital limestones to secondary dolomites. Dolomitization and recrystallization form fracture-like microcaverns with a size of up to 0.5 mm and result in a high porosity of up to 9 %. In carbonate reservoir rocks fracturing is prevailing, while porosity has a subordinate value. Studies of the molecular composition of natural gases from reservoir rocks of the Middle Devonian of the Lokachi field showed that their predominant component is methane. Its content is 92.7–95.4 vol %. The rest of the methane homologues account for 1.45–2.16 vol %. The total share of non-hydrocarbon gases – nitrogen, carbon dioxide, helium and hydrogen are 3.102–5.082 vol %. In order to clarify the origin of the Middle Devonian gases of the Lviv Depression, further studies of the carbon, nitrogen, and hydrogen isotopic composition of these gases and the study of the generation properties of the Lower and Middle Devonian rocks of the studied region are necessary.