Origin, secondary processes and migration of oil and natural gas in the central part of the Polish Outer Carpathians

Abstract

The origin, migration pathways, and the influence of secondary processes of oil and natural gas accumulated in Lower Cretaceous to Lower Miocene strata of the central part of the Polish Outer Carpathians are investigated based on organic geochemical analyses. Oil and thermogenic hydrocarbon gases were mainly generated from both types II and III kerogens in the Oligocene Menilite beds of the Silesian, Skole and Dukla nappes. Moreover, some gases originated from Type III kerogen of Lower Cretaceous Veřovice shales in the Silesian Nappe and Lower Cretaceous Spas shales in the Skole Nappe. These gases migrated and subsequently mixed with oil and gas produced from Type II kerogen in the Menilite beds. The most extensive biodegradation and water washing were observed in oils from Harklowa and Iwonicz fields, and in oil from the BF (Franek) dug-well in Bóbrka-Rogi field (one of the world's oldest with exploitation started in 1854) of the Silesian Nappe. Oils most affected by evaporative fractionation come from Krościenko and Iwonicz Zdrój fields. Natural gas originated mainly from low-mature thermogenic processes (“oil window”) with some primary microbial gas contribution (primary carbon dioxide reduction), and many gas samples show secondary alteration caused by biodegradation. Carbon dioxide originated from both thermogenic kerogen decarboxylation and microbial processes.