oa Geochemistry of an Oil Stain in the Paleocene Deepest Core in IODP Expedition 378 Hole U1553D, Campbell Plateau, New Zealand

International Ocean Discovery Program (IODP) Expedition 378 recovered a continuous, > 580 m thick Paleogene sedimentary sequence at Site U1553 on the Campbell Plateau, south of the South Island of New Zealand. An oil stain sample was recovered from the deepest Paleocene core, associated with high amounts of methane and wet gases detected by on-board instrumentation. The n-alkane distribution of the oil stain sample is bimodal, suggesting mixing of a thermogenic migrated oil with a signal from the indigenous Paleocene sediments. Interpreted molecular weight zones are migrated thermogenic hydrocarbons (C12–C17), indigenous Paleocene sediment (C25–C29), and a mixed signal (C19–C24). This is confirmed by the ≥C27 hopanoid and steroid biomarkers in the oil stain sample which are very similar to the immature Unit Vb early Paleocene sediments, including the presence of neohop-13(18)-enes, ββ hopanes, and βαα steranes. Some aromatic compounds (e.g., alkylnaphthalenes, methylbiphenyls, methyldibenzothiophenes, methylpyrenes) indicate a thermal maturity in the early to mid-oil window, consistent with being derived from a migrated hydrocarbon phase. The migrated thermogenic oil and gas were derived from a marine source rock containing a significant contribution from terrigenous and probably coniferous organic matter, deposited in a dysoxic depositional environment.