Evidence for mixing of migrated hydrocarbons and indigenous organic matter 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. Oil stain samples are rarely recovered during scientific ocean drilling, but one was recovered from the deepest Paleocene core in Hole U1553D (579.94 m CSF-A), associated with high amounts of methane and wet gases detected by on-board instrumentation, that led to drilling any deeper being prevented. This oil stain was analysed by gas chromatography–mass spectrometry and gas chromatography–triple quadrupole mass spectrometry, and compared with the indigenous Paleocene organic matter. 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. This is confirmed by the ≥C 27 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. The C 10 –C 20 aromatic compound distributions are in the molecular weight range where there is a mixed signal. 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. Vertical migration of thermogenic oil and gas penetrated the early Paleocene sediments, but did not reach into the Eocene section due a significant permeability barrier associated with a middle–late Paleocene unconformity. Deeper drilling and geophysical investigation are required to understand the putative Cretaceous or older strata in this region. • Organic geochemical study of oil stain sample from IODP Expedition 378 • Mixture of indigenous Paleocene organic matter and thermally mature migrated HCs • High molecular weight components in oil stain are immature, from Paleocene • Medium molecular weight aromatics indicate mixed signal, early to mid-oil window • Migrated hydrocarbons originate from deeper buried Cretaceous or older sediments