Origin of methane in quartz crystals from the Tertiary accretionary wedge and fore-arc basin of the Western Carpathians

Quartz crystals from mineralised joints of the Carpathian flysch contain fluid inclusions covering the compositional range from gaseous CH4±CO2±C2+ mixture, through CH4-saturated oil, to CH4-saturated aqueous solution. The C isotope composition of CH4 extracted from the fluid inclusions (−31 and −36‰ V-PDB) is indicative of a thermogenic dry gas released from overmature rocks, although the maturation level of the surrounding flysch corresponds to a wet-gas generating zone. Trapping PT conditions for the CH4-bearing fluids range between 130 and 205 °C and 0.5 and 3.7 kbar. The fluid inclusion-derived crystallisation temperatures are inconsistent with the vitrinite reflectance data. Strongly fluctuating fluid pressures during growth of the quartz crystals indicate the existence of a crack-seal mechanism with recurrent hydrostatic and lithostatic regimes. Fluid pressures exceeding the lithostatic load have been recorded in the Dukla nappe. The overpressures have been generated by fluid volume increase due to thermal decomposition of oil and kerogen to CH4 and pyrobitumen. Occurrence of oil and condensate in the CH4-bearing inclusion fluids trapped at temperatures considerably above oil window suggest a short-termed, transient post-orogenic heating, rather than a prolonged heating related to burial. The thermal event giving rise to the quartz crystallisation has been coincidental with regional collapse and uplift of the Western Carpathians.