Depositional and organofacies influences on the petroleum potential of an unusual marine source rock: Waipawa Formation (Paleocene) in southern East Coast Basin, New Zealand

Abstract The Paleocene Waipawa Formation is a widespread, ∼2–80 m-thick marine mudstone that occurs in several New Zealand basins. It is relatively enriched in organic matter (OM) and is the inferred source of a sub-commercial oil discovery and several correlated oil seeps. To determine the organofacies and depositional influences on the petroleum potential of the formation, we have undertaken a high-resolution, multidisciplinary study of the Taylor White section in the central East Coast Basin. This section is the thickest stratigraphically coherent surface exposure of the formation and includes contacts with the bounding Whangai and Wanstead formations. Bathymetric indicators within the foraminiferal assemblages indicate a slope setting for the section, deepening from middle to lower bathyal through the Waipawa–Wanstead succession. Significantly, the Whangai Formation in this section is barren of foraminifera. Dinoflagellate and calcareous nannofossil biostratigraphy provides somewhat conflicting age control but a resulting age model indicates a tenfold increase in compacted sediment accumulation rate during Waipawa deposition (from ∼1 to ∼10 cm/ky). Our multivariate statistical analysis of geochemical and paleontological data reveals systematic variation in paleoenvironmental and source rock parameters within the section, with samples forming four distinct clusters or organofacies: OM-rich and OM-poor Waipawa organofacies, Whangai, and Wanstead organofacies. The two Waipawa organofacies are distinguished by enrichments in total organic carbon (TOC) and organic 13 C, dominance of phytoclasts (mainly degraded woody plant matter; 66–98%), and low amounts of amorphous organic matter ( 24 tetracyclic/C 23 tricyclic terpane ratio). A small but significant marine OM contribution within the Waipawa deposition is indicated by unusually high abundance of C 30 steranes of marine pelagophyte origin, whereas the higher sulfur content and redox indicators signal hypoxic to anoxic conditions within the depositional environment. Stratigraphic fluctuations in key geochemical parameters within the Waipawa Formation indicate episodic influxes of terrestrial OM into the slope environment, with consequent reduction in water-column oxygen levels. The inferred oxygen depletion in the water column and sediments together with sulfurisation appears to have resulted in enhanced preservation of OM. Whangai and Wanstead organofacies are characterised by a greater proportion of marine OM but are distinguished from each other by differences in OM preservation: OM is well-preserved with indications of hypoxia in the Whangai organofacies but is poorly preserved in the Wanstead organofacies. Geochemical fingerprinting identifies an interval of Whangai organofacies that separates two phases of Waipawa organofacies deposition. The lower phase comprises four alternations between OM-rich and OM-poor Waipawa organofacies and the upper phase comprises a single pulse of OM-rich Waipawa facies bounded by OM-poor intervals. Overall, Waipawa Formation in this section has TOC and pyrolysable hydrocarbon (S2) values of 0.2–4.6 (mean 2.2) wt% and 0.1–12.4 (mean 3.9) mg HC/g rock, respectively. One-third of S2 values exceed 5 mg HC/g rock, indicating good‒very good bulk petroleum potential, with the remainder having poor‒fair potential. Petroleum potential (S2) increases with increasing TOC (R 2  = 0.85). Hydrogen index (HI) values range from 21 to 295 mg HC/g TOC, averaging just 147 mg HC/g TOC. In line with these modest HI values, quantitative pyrolysis-gas chromatography confirms that the petroleum potential is primarily for gas or gas-condensate, with relatively little oil potential. Of the total paraffinic petroleum yield, gas ( n -C 1–5 ) contributes 80–100%, whilst total ( n -C 6+ ) and non-volatile ( n -C 15+ ) paraffinic oil contribute ≤20% and ≤6%, respectively. Despite the low paraffinic oil potentials, n -C 6+ and n -C 15+ correlate relatively strongly with HI, and less strongly with TOC. The overall gas-condensate-prone nature of the Waipawa organofacies at Taylor White reflects the predominance of woody phytoclasts, which are unlikely to produce paraffinic oil. The small paraffinic oil component is instead more likely to be associated with the marine algal component. Although this component is volumetrically subordinate, the higher paraffinic oil potential appears to result from the combination of high productivity of specific marine algae (i.e., pelagophytes), hypoxic depositional conditions and, most importantly, increased preservation of OM, including through sulfurisation. This raises the possibility of more algal-rich, oil-prone facies existing within the Waipawa Formation in areas with higher TOC or, alternatively, in areas beyond the direct reach of the terrestrial influxes, yet still sufficiently influenced by low water-column oxygen levels and high sulfur contents that promote enhanced OM preservation.