MATURITY AND PETROGRAPHY OF BITUMENS IN THE CARBONIFEROUS OF IRELAND

A set of bitumen samples from the Carboniferous of Ireland shows a gen eral trend of increasing maturity southwards across the island. This t rend is consistent with a similar southwards increase in maturity exhi bited by the host rocks (vitrinite reflectance), reflecting proximity to the Hercynian Orogen to the south of Ireland. Increasing maturity i s reflected by decreasing H/C ratio, and increasing reflectance, biref lectance and anisotropy, and is accompanied by heavier carbon isotope composition for the most mature samples. Bitumen reflectance is the mo st sensitive parameter and ranges up to 3.6%. Most of the bitumens occ ur in hydrothermal mineral veins and were generated locally by the pas sage of hot fluids through Carboniferous shales and limestones. Some b itumens contain inclusions of metalliferous and other inorganic minera ls. High sedimentation rates during the Carboniferous caused hydrocarb on generation before the Late Carboniferous Hercynian Orogeny, so that the bitumens experienced thermal overprinting by the Orogeny and show the same maturity trend as the country rocks. Anisotropy is exhibited by all bitumen samples with reflectances of 2.0% or over; mesophase t extures occur in bitumens with reflectances over 3.0%, including one c ase where fluid inclusion homogenization temperatures in coeval minera ls (calcite and fluorite) are less than 200 degrees C.