Oil generation by igneous intrusions in the northern Gunnedah Basin, Australia

The origin of extensive oil staining, recorded over a 120 m interval of the Jurassic Pilliga Sandstone in the fully-cored Bellata-1 well, northern Gun nedah Basin, has been investigated using conventional organic geochemical t echniques. The borehole intersects 451 in of Permian and Triassic strata, o verlain by 640 m of Jurassic and Cretaceous sediments. Igneous intrusions o ccur mainly within the lower part of the Triassic Napperby Formation where the vitrinite reflectance profile shows the effect of significant local hea ting (R-v up to 2.43% within the intruded interval). Lower reflectance valu es (R-v = 0.57-0.74%) occur in the unaffected parts of the Permian sequence , with even lower values in places due to either marine influence or liptin ite-rich lacustrine organic facies. Hydrocarbon biomarkers in the oil stain in the Pilliga Sandstone indicate its derivation from a higher plant-rich, shaley source. A methylphenanthrene index of 1.48 (calculated vitrinite re flectance R-e = 1.29%) for the oil suggests expulsion from a late-mature so urce rock. None of the potential source rock samples analysed appear to hav e attained such a maturation level, except those from the lower part of the Napperby Formation affected by intrusions. Source-specific biomarkers from this formation show several parallels with those of the oil stain, but dif fer from those in the Permian Goonbri and Maules Creek Formations and the u pper parts of the Napperby Formation. A Jurassic source can also be exclude d based on the immaturity of that part of the section and the absence of Ju rassic-specific biomarkers in the oil stain. (C) 2001 Elsevier Science Ltd. All rights reserved.