Organic geochemical study of source rocks and natural gas and their genetic correlation in the central part of the Polish Outer Carpathians

Abstract

Geochemical characteristics of organic matter (quantity, genetic type, maturity and petroleum potential) were determined in profiles of Lower Cretaceous–Miocene strata of the Dukla and Silesian units of the Polish Outer Carpathians in the Tarnów-Dębica-Nowy Sącz-Gorlice-Jasło area. This determination was based on Rock-Eval, isotopic, and biomarker analyses of 159 rock samples. Oligocene Menilite beds reveal the best hydrocarbon potential within the investigated formations (depositional units of the so-called flysch strata) and can be considered as primary source rock in the study area. Marine and mixed marine–terrestrial kerogen present in these rocks is immature in Silesian Unit and mature (oil window) in Dukla Unit, respectively. The Lower Cretaceous–Eocene formations (Veřovice, Lgota, Godula and Istebna beds, Variegated Shales, and Ciężkowice beds) of the Silesian Unit reveal variable amounts of mostly gas-prone kerogen and can be considered as additional source of hydrocarbons. The Upper Oligocene–Lower Miocene Krosno beds and Miocene Chaotic beds have a variable organic matter content, usually over 1 wt%, but very low maturity and low hydrocarbon potential predispose them as a source of microbial methane. Ten natural gas samples from sandstone reservoirs of the Upper Cretaceous–Lower Miocene strata in the Silesian Unit of the Outer Carpathians were analysed for molecular and isotopic compositions in order to establish their origin and role of thermogenic and microbial processes. Thermogenic gases without the admixture of microbial methane were encountered only in one well (Bystra-4) in the block A of Szalowa-Heddy-Bystra gas deposit. Thermogenic gas component was generated at a maturity level of 0.7 to 1.0% of vitrinite reflectance (Ro) scale from mixed Type II/III kerogen dispersed in the Oligocene Menilite beds from a few fields within the Silesian Unit. Microbial methane had been generated earlier from the same organic matter, during microbial carbon dioxide reduction process. A significantly high microbial component occurs mainly in the gases from Kobylanka (gas horizon), Gorlice-Glinik, and blocks C and B of the Szalowa-Heddy-Bystra fields. The microbial gases were generated within the Oligocene Menilite beds and/or Miocene Chaotic beds within local zones in the study area. Carbon dioxide originated both from thermogenic and microbial processes.