Oil Seeps and Potential Source Rocks of the Northern East Coast Basin, New Zealand; A Geochemical Study

Abstract

Biomarker and non-biomarker geochemistry have been used to study a range of oils and possible hydrocarbon source rocks from the northern East Coast Basin of New Zealand. The basin contains Early Cretaceous/Jurassic to recent sediments which were deposited in a dominantly marine environment The main oil seeps at Waitangi, Rotokautuku and Totangi occur in the East Cape-Gisborne region; however, impregnations and other minor seepages are present in Cretaceous and Tertiary formations throughout the basin. In general, the seep oils are moderately mature with a marine signature. They are isotopically light (-28‰ to 29‰) with low sulfur contents (O.lwt% and.0.4wt%). The biomarker geochemistry suggests that, in outcrop, the post-Paleocene sediments are immature and other geochemical data suggest that the organic content of these sediments is insufficient for oil generation. The sulfur-rich Paleocene Waipawa Black Shale, while having excellent source potential (TOC up to l2%), is very thin (up to 70m) and has a low maturity level in outcrop. Higher abundances of bisnorhopane, C29 steranes and oleanane suggest that it was deposited in a marine anoxic environment with a moderate terrestrial input Unusually high concentrations of C36 steranes present in all samples indicate a significant algal contribution to the source. The kerogen is mainly Type II with some Type III, suggesting an oil-prone formation with some gas potential. The Upper Cretaceous Whangai Formation is marginally mature in outcrop, but has lower organic content (TOC up to l.5%) than the Waipawa Black Shale. It has two main members in the northern East Coast Basin. These are the Rakauroa Member which imitates the Waipawa Black Shale in colouring and has higher organic (mean=0.9%T0C) and sulfur (>lwt%) content than other members of the Whangai Shale, and the Upper Calcareous Member, which has a moderate organic content (up to 1.3%TOC present study), a lower sulfur content than the Rakauroa Member, and lies conformably under the Waipawa Black Shale. These members have a lower abundance of C30 steranes than the Waipawa Black Shale, but their biomarkers are comparable to those of the oil seeps. Extensive oil staining occurs in the Whangai Shale in the western margin of the basin, especially near Matawai, and particularly in the Upper Calcareous Member. Underlying Mid to Early Cretaceous formations are mainly mid to outer shelf marine sediments, with slightly more terrestrial organic content than the Whangai Shale. The biomarkers are mostly mature, with fair correlation to the oil seeps. Oil impregnations are rare in these formations, and the organic and sulfur content are lower (TOC's average 0.4%, less than 0.lwt% sulfur) than the Whangai Shale members. The Mid Cretaceous Karekare Formation is up to 2.6km thick with an organic content of up to 1.6%TOC, but has not been sampled thoroughly enough to identify possible organic-rich horizons. The similariry of the biomarker fingerprints of these underlying Cretaceous sediments suggests regional similarities in the depositional environment. Biomarker and stable carbon isotope data for the main oil seeps and impregnations suggest that the oils of the northern East Coast Basin are related, with a common regional source. Their likely source is the Whangai Shale, and more specifically its Upper Calcareous Member, although the Rakauroa Member and Karekare Formation may have some source potential. The Waipawa Black Shale has the greatest generating potential in the basin. While it is not considered a contributor to the northern East Coast Basin seep oils due to its high C30 sterane level and sulfur content, it may be generating hydrocarbons elsewhere in the basin given suitable burial and thermal conditions.