Oil–Coaly‐Source Rock of Oil Seepages From the West Coast Basin, New Zealand: Insights From Biomarker and Isotopic Investigations Coupled With 1D Basin Modeling Analysis

ABSTRACT

Oil seepages were collected from the surface sites through the Western Coastal Basin of New Zealand. This study analyzed three oil seepage samples in order to determine their geochemical characteristics and to better understand the characteristics of their source rock, such as organic matter (OM) content, lithological features, depositional settings, and thermal evolution. The studied oil seepage samples exhibit saturated hydrocarbon (HC) content between 44.4% and 86.5%, confirming a predominantly paraffinic oil character sourced from thermally mature rocks, consistent with the oil‐generative window, ranging from early to peak maturity window. These thermal maturity levels of their source rocks are future supported from maturity‐biomarker parameters. Combined biomarker signatures and stable carbon isotopic ( δ ¹³ C) data from both saturated and aromatic HC fractions indicate that the studied oil seepages were generated from coaly‐bearing source rocks enriched in terrestrial OM that accumulated in fluvial to deltaic depositional environments under oxic conditions. These geochemical signatures of the studied oil seepage samples are attributable to coaly sediments of the Late Cretaceous–Paleocene Paparoa and Eocene Brunner coal measure succession. In this case, the oil seepage sample reflecting lower thermal maturity is interpreted as having been generated from the Eocene Brunner coal measure, whereas those exhibiting higher maturity levels are attributed to the older Paparoa coal measure. This interpretation is independently validated by 1D basin modeling outcomes, which demonstrate that both Paparoa and Brunner coaly‐source rock systems have entered the principal oil generation phase, with EASY %Ro from 0.55 to 0.82. The basin model results also show that the Paparoa coal measure can generate a higher amount of oil than Brunner coal measure, as indicated by extensive kerogen conversion (transformation ratio [TR]) of up to 39%.