Natural gases from Middle Cambrian and Carboniferous-Lower Permian reservoirs of the onshore Polish Baltic region were studied for their molecular and stable isotope compositions. The following gas species were analysed: ^12,13C in CH₄, C₂H₆, C₃H₈, n-C₄H₁₀, i-C₄H₁₀ and CO₂, ^1,2H in CH₄, ^14,15N in N₂, and stable isotopes of all noble gases. Due to significantly different geological settings and genetic types of source rocks of Eastern and Western Pomerania, the molecular and isotopic compositions of natural gases of these two regions exhibit distinct differences. Hydrocarbon gases associated with oil accumulated in the Middle Cambrian reservoir of Eastern Pomerania were generated during low-temperature thermogenic processes from a single source rock containing Type-II kerogen at one phase of petroleum generation. Non-associated hydrocarbon gases accumulated in the Carboniferous (Mississippian and Pennsylvanian)-Lower Permian (Rotliegend) reservoirs of Western Pomerania originated during at least two phases of gas generation by thermogenic processes from mainly Type-III kerogen and a small component of mixed Type-III/II kerogen. Noble gases are in general heavily enriched in radiogenic and nucleogenic isotopes such as ⁴He, ⁴⁰Ar and ²¹Ne accumulated in the reservoirs. Weak contributions of mantle-derived He and Ne are observed. Radiogenic ⁴He/⁴⁰Ar ratios are higher than the average production rate ratio of about 5 for radiogenic ⁴He/⁴⁰Ar in crustal materials, which might have been caused by a selective supply of ⁴He that is lighter than ⁴⁰Ar from crustal rocks, or (U + Th)/K ratio might be higher than the average in crustal block. Carbon dioxide from gases of both the Western and Eastern Pomerania were mainly generated during thermogenic processes of transformation of organic matter, although gases of Western Pomerania can contain an endogenic component. Molecular nitrogen from the Eastern Pomeranian natural gases was mainly generated during low-temperature thermal transformation of organic matter and derived from NH₃ and NH₄ of crustal fluid, whereas molecular nitrogen from Western Pomeranian natural gases contains a significant component from the destruction of organic matter at a higher maturity level which may have been caused by a high heat flux from the volcanic activity during late Pennsylvanian–early Rotliegend ages, and has a bigger component release from NH₄-rich illites.

Carbon isotope stratigraphy of the basal Zechstein (Lopingian) strata in Northern Poland and its global correlation

The origin, migration pathways, as well as the influence of secondary processes of oil and natural gas accumulated in lower Cretaceous to lower Miocene strata of the western part of the Polish Outer Carpathians (between Kraków and Pilzno towns) based on results of organic geochemical analyses are investigated in this paper. Oil and thermogenic hydrocarbon gases were generated mainly from type II kerogen, and type II and III kerogen mixed in various proportions. These kerogens mainly occur in the Oligocene Menilite beds of the Silesian and Dukla nappes. Oils were generated from early to late “oil window”. Secondary cracking was recorded in oils from Dukla nappe, other secondary processes including biodegradation, water washing and evaporative fractionation were also developed to a various extent in many oils. The most biodegraded oils occur in seep S-Li, and the most extensive water washing is observed in the oil from seep S-Sa/1. The evaporative fractionation processes most significantly occur in the selected deepest parts of the multi-horizontal Biecz field. Hydrocarbon gases originated during both microbial and thermogenic processes of organic matter transformation. Natural gas has not been subjected to biodegradation processes. Carbon dioxide is derived from both microbial and thermogenic decomposition processes of organic matter and was generated together with hydrocarbon gases.