Magnetic Properties of Sandstones Within the Zone of Hydrocarbon Migration Near the Boundary of Dukla and Silesian Units (Western Carpathians, Poland)

This study represents the application of magnetic techniques in hydrocarbon prospection within the Outer Western Carpathian petroleum play, specifically focusing on the Eocene–Oligocene rocks of southeastern Poland. Magnetic minerals serve as a sensitive proxy for diagenetic processes, particularly concerning hydrocarbon generation, migration, and biodegradation. We investigated the magnetic characteristics of surface rock samples from Dukla and Silesian Units, which have been altered by hydrocarbons migrating through pore spaces and tectonic fissures. The magnetic mineralogy of 29 specimens was analyzed through thermal demagnetization (Lowrie's test), complemented by hysteresis loop and anhysteretic remanence susceptibility measurements to assess domain states and ultrafine grain size ranges. Scanning electron microscope analysis of mineral composition revealed that traces of bitumen and abundant pyrite were common in almost all samples. Ultrafine maghemite emerged as the predominant magnetic mineral within the host rock, with some samples exhibiting varying amounts of ultrafine magnetite, often associated with iron sulfides. Areas adjacent to calcite veins stained with hydrocarbons displayed enhanced magnetic properties, primarily due to increased magnetite content. Additionally, aggregates of barite were identified as accessory minerals within these veins. Our study indicates that detrital iron oxides underwent partial dissolution, leading to the formation of new authigenic minerals such as paramagnetic pyrite and, likely, ultrafine magnetite. These constituents were subsequently transformed partially or completely into maghemite during the oxidation stage of diagenesis. Importantly, the presence of hydrocarbon appears to facilitate the formation of ultrafine (<100 nm) magnetic particles, with magnetite being particularly characteristic of tectonic migration pathways.