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 Gunnedah Basin, has been investigated using conventional organic geochemical techniques. The borehole intersects 451 m of Permian and Triassic strata, overlain by 640 m of Jurassic and Cretaceous sediments. Igneous intrusions occur mainly within the lower part of the Triassic Napperby Formation where the vitrinite reflectance profile shows the effect of significant local heating (Rv up to 2.43% within the intruded interval). Lower reflectance values (Rv=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 liptinite-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 reflectance Rc=1.29%) for the oil suggests expulsion from a late-mature source rock. None of the potential source rock samples analysed appear to have 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 differ from those in the Permian Goonbri and Maules Creek Formations and the upper parts of the Napperby Formation. A Jurassic source can also be excluded based on the immaturity of that part of the section and the absence of Jurassic-specific biomarkers in the oil stain.