Geochemistry of oil seepages and Paleocene organic-rich shale deposits from the onshore East Coast Basin in New Zealand: implications for characteristics, origin and oil-source rock correlation

In this study, four oil seepages and ten organic-rich shale samples of the Paleocene Waipawa Formation from outcrop sections in East Coast Basin, New Zealand were studied. Multi-scale organic geochemical analyses, including total organic carbon (TOC) content, Rock-Eval pyrolysis, biomarker and carbon isotope measurements were conducted and used to determine the characteristics of the organic matter in seep oil and shale samples and then to learn more about their genetic relationship. The studied Waipawa shale samples are characterized by high TOC content up to 10.84wt. %, and mixed kerogen types, with mainly hydrogen-rich kerogen Types II and II/III, suggesting a good oil- and gas- source rock, with substantial potential for oil generation. The studied Waipawa shale sediments are characterized by Pr/Ph ratios between 0.90 and 3.40 and generally low values of C32R/C30 hopane ratio of less than 1, indicating that these shale sediments are clay-rich facies and were deposited under anoxic to relatively oxic environmental conditions. The shale sediments of the Waipawa Formation are also characterzied by high ratios of C23tri/C24tri, C25tri/C26tri, and C24tre/C22tri tricyclic terpanes, alongside high abundance of C27 and C29 regular steranes compared to C28 regular sterane, further indicating a mixed organic matter, with high contributions of marine organic matter input. The high contributions of marine organic matter during the formation of the studied Waipawa shale succession are further corroborated by the outcome of carbon isotopes. The source and depositional-related biomarker ratios and parameters also show a good genetic relationship between the Waipawa shale source rock and those of the studied seep oil samples. However, the studied oil seepages are more mature than the Waipawa shale from the outcrop sections, exhibiting a range of early-mature to peak-mature of oil window, and thus the shale source rock of the Waipawa Formation contributed to these seep oils in the deeper stratigraphic sequence. Thereby, the intensive oil exploration of the Waipawa shale source rock is recommended in the deeper structural units of the East Coast Basin.