Sources of natural gases in Middle Cambrian reservoirs in Polish and Lithuanian Baltic Basin as determined by stable isotopes and hydrous pyrolysis of Lower Palaeozoic source rocks

• Molecular and isotopic compositions of natural gases were analysed.
• Lower Palaeozoic source rocks by hydrous pyrolysis were examined.
• Gaseous hydrocarbons are only of thermogenic origin.
• Upper Cambrian and Tremadocian source rocks are the main contributors.
• N2 was generated from NH4-rich illites and from thermal transformation of kerogen.

Origin of natural gases associated with oil and condensate accumulations within the Middle Cambrian sandstone reservoir of the Polish and Lithuanian Baltic Basin was characterised by means of molecular composition, stable carbon isotopes of methane, ethane, propane, butanes, pentanes and carbon dioxide, stable hydrogen isotopes of methane and stable nitrogen isotopes of gaseous nitrogen. Generated gas from potential Upper Cambrian, Tremadocian, and Llandovery source rocks by hydrous pyrolysis at 330 °C and 355 °C for 72 h was used to characterise thermogenic gas to evaluate correlation parameters based on molecular composition and stable isotopes. The pyrolysis conditions represent gas generation during oil generation, which appears to be the conditions represented by the natural gas accumulations and their low GORs (gas:oil ratios). The dryness of the pyrolysis and natural hydrocarbon compositions compare well, but do not provide a means of distinguishing the contributions of each source rock to the natural gas accumulations. The average δ13C value of the natural methane is 6.9‰ depleted in 13C compared to methane generated in the hydrous pyrolysis experiments. This difference is less for ethane and essentially nonexistent for propane, butanes, and pentanes. Tentatively, this diminishing difference with increasing carbon number is attributed to kinetic effects resulting from higher experimental temperatures. Although the δ13C values of methane and ethane from the hydrous pyrolysis experiments are not useful in direct correlations with natural gas accumulations, δ13C of propane, butanes, and pentanes is useful, and indicates that the Upper Cambrian and Tremadocian source rocks are the main contributors and that the Llandovery source rocks are not significant contributors to the Polish and Lithuanian Baltic natural gases. Polish natural gases with relatively higher methane and ethane are attributed to the mixing of drier, more mature gases from deeper parts of the basin to the west. Carbon dioxide of natural gases was generated during thermogenic processes and gaseous nitrogen was generated from NH4-rich illites of the clayey facies and from thermal transformation of organic matter of the Lower Palaeozoic strata. Hydrous pyrolysis gases have higher concentrations of CO2, H2S and H2 than the natural gases. This difference is attributed to reduction or loss of these highly reactive and soluble gases during migration and entrapment of the natural gases. Although CO2 concentrations between pyrolysis and natural gases are different, the δ13C values of the former fall within the range of the latter.