Extractable hydrocarbon and carbon isotope geochemistry of Lower Cretaceous sediments from Sites 762 and 763 on the Exmouth Plateau, northwest Australian margin

Abstract

The organic geochemical character of rocks selected from Aptian, Valanginian, and Berriasian claystone and siltstone sequences was determined by means of a variety of analytical procedures. These sequences represent distal portions of the Mesozoic Barrow delta, in which petroleum source rocks and reservoirs exist on the Australian continent. The organic matter at the ODP sites is thermally immature type III material. Biomarker hydrocarbon compositions are dominated by long-chain, waxy n-alkanes and by C29 steranes, which reflect the land-plant origin of organic matter. Organic carbon δ13C values ranged from -26‰ to -28‰, consistent with a C3 land-point source. Kerogen pyrolysate compositions and hopane isomerization ratios revealed progressively larger contributions of recycled organic matter as the depth of the deltaic sedimentary layers became greater. -Authors