Depositional influences on the petroleum potential of the Waipawa Formation in the Orui-1A drillhole, Wairarapa

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Field, B.D.; Naeher, S.; Clowes, C.D.; Shepherd, C.L.; Hollis, C.J.; Sykes, R.; Ventura, G.T.; Pascher, K.M.; Griffin, A.G. 2018 Depositional influences on the petroleum potential of the Waipawa Formation in the Orui-1A drillhole, Wairarapa. Lower Hutt, N.Z.: GNS Science. GNS Science report 2017/49. 76 p.; doi: 10.21420/G2MM18

Abstract: The Orui-1A stratigraphic drillhole near Riversdale, coastal Wairarapa, was drilled in 2011 by East Coast Energy Ventures to assess the prospectivity of the Paleocene Waipawa Formation, a widespread marine source rock. The drillhole was cored from 30.35 m to TD at 117.3 m, and acoustic image logs were run. The cored interval spanned the basal few metres of Wanstead Formation then passed into Waipawa Formation to TD. In order to determine the depositional influences of petroleum potential of the Waipawa Formation, we have re-logged the core in detail and reviewed the stratigraphy, depositional setting and petroleum potential based on sedimentology, paleontology, elemental geochemistry (XRF), mineralogy (XRD) and source rock analysis (SRA). The Waipawa Formation in the Orui-1A drillhole is primarily an organic-rich, bioturbated mudstone. Its stratigraphic thickness, accounting for bedding dips but not fault offsets, is inferred to be ~65 m. Fourteen faults are recognised but their offsets are uncertain. The base of the overlying Wanstead Formation is marked by distinct changes in lithology consistent with a sedimentary break in deposition, although there is also evidence for shearing at or near the contact. Trace fossils are low diversity, generally parallel to bedding and vary in abundance through the core. Intervals of slumping occur throughout the core and a zone with brecciation is observed between 88 and 96 m. Thin greensand beds are observed at 66 and 80–86 m. Calcareous nannofossil and dinoflagellate biostratigraphy indicate a middle to late Paleocene age for the Waipawa Formation (nannofossil zone NP6 and dinoflagellate zone NZDP7 at the base) and a late Paleocene age for the overlying Wanstead Formation (nannofossil zone NP8). Palynofacies analysis of ten Waipawa Formation samples showed little variation, with all dominated by degraded phytoclasts (>85%). XRD analyses of 14 samples through the Waipawa Formation indicate that quartz is the dominant mineral (>50 wt%), with subordinate undifferentiated mica and clay (<40 wt%) and lesser amounts of plagioclase (~10 wt%) and calcite (<11 wt%). Calcite is absent from the upper Waipawa Formation (~40 to ~70 m) apart from the uppermost sample, which is also the only sample to contain significant dolomite (10% wt). An illite/mica complex is dominant with smectite typically a minor component of the clay fraction. SRA results of 83 samples through the Waipawa Formation indicate that Total Organic Carbon (TOC) ranges from 1–6 wt% with highest values in the upper part of the formation (37 to 61 m) and cyclical peaks and troughs in the lower part. As in other records of the Waipawa Formation, TOC is positively correlated with δ13CTOC, which ranges from -23 to -18‰. In this drillhole, TOC is also positively correlated with Hydrogen Index (HI) and Bitumen Index (BI) but is negatively correlated with Oxygen Index (OI) and Tmax values. The upper part of the formation is characterised by low Tmax values (~400°C); the mean for values below this interval is 426°C. The reasons for this decrease in Tmax is uncertain. The decrease coincides with a decrease in OI and increases in TOC, Hydrogen Index (HI) and bioturbation. The interval of high TOC and low Tmax is dominated by mudstone and is highly bioturbated whereas mixed sandstone and mudstone lithologies characterise the underlying interval. An accompanying increase in a Ba/Ti suggests an increase in marine productivity and, possibly, an increase in marine organic matter. However, palynofacies analysis indicates that terrestrial organic matter is uniformly dominant throughout the Waipawa Formation. Three samples near the top of the Waipawa Formation have high TOC and S2 values (>4 wt% and >10 mg HC/g, respectively), which are indicative of excellent source rock potential. Samples with lower TOC have good to very good source rock potential. Despite the dominance of terrestrial organic matter in palynofacies preparations, HI is relatively high and OI relatively low compared to other Waipawa Formation samples, indicating a mixture of Type II and III kerogen, and resulting in the Orui samples being slightly more oil-prone than average Waipawa Formation samples. (auth)