Multi‐Scale Organic Geochemical Dataset and 1D ‐ Basin Modelling Analysis for Gas Generation Potential of the Mid Cretaceous Hoiho Formation in the Offshore Great South Basin, New Zealand

ABSTRACT The present work characterises coal‐bearing deposits of the Mid‐Cretaceous Hoiho Formation, located in the offshore Great South Basin, New Zealand, through a multidisciplinary approach involving geochemical analyses, biomarker evaluation, stable carbon isotope measurements, and one‐dimensional basin modelling. This study aims to characterise the organic matter (OM) present within the Hoiho coal facies and elucidate the mechanisms responsible for gas generation. Biomarker analysis combined with carbon isotope data was employed to assess the sources of OM and reconstruct the depositional environment conditions. The Hoiho coals exhibit high total organic carbon (TOC) and sulfur (S) concentrations, indicating marine‐influenced coaly facies. The high TOC and S result in high TOC/S ratios and further suggest that the Hoiho coal facies was deposited in high‐oxygenated environments. This interpretation is further reinforced by the specific biomarker ratios, such as a high pristane/phytane ratio of more than 3 and a low dibenzothiophene/phenanthrene ratio. Hence, the elevated organic carbon, with TOC content reaching up to 79.8 wt. % in the Hoiho coal facies cannot be solely attributed to low‐oxygen depositional settings. Molecular biomarker signatures combined with carbon isotopic data suggest that the OM in the Hoiho coal facies is primarily derived from terrigenous sources and accumulated in fluvial to fluvio‐deltaic environments, as indicated by high T m / T s and C 24 Te/C 26 Ti ratios, along with the predominance of C 29 over its C 27 and C 28 counterparts. This high abundance of terrigenous plants in the studied coal facies indicates that hydrogen‐poor Type III kerogen was the original OM input during deposition. This finding is consistent with the low hydrogen index (HI) values, between 111 and 292 mg HC/g TOC, indicative of predominantly Type III kerogen for mainly gas generation potential. Kerogen transformation modelling reveals that approximately 10% to 54% of the primary Type III kerogen underwent cracking to generate gas from the Late Eocene to present, corresponding to the wet gas generation window, with Easy %Ro values up to 1.72%. Consequently, the Mid Cretaceous Hoiho coal facies can be considered as a promising target for gas development within the deeper stratigraphic intervals of the offshore Great South Basin.