Palynofacies, organic geochemistry and depositional environment of the Tartan Formation (Late Paleocene), a potential source rock in the Great South Basin, New Zealand

Detailed palynofacies analysis of sidewall core samples taken from below, within and above the Tartan Formation (Thanetian, Late Paleocene, 58.7–55.8 Ma), a potential source rock in the epeiric Great South Basin, shows that the formation is characterised by very high percentages of degraded brown phytoclasts, rare marine algae and amorphous organic matter and thereby represents a mix of terrestrial and marine kerogen. The results indicate that the formation was deposited in a marginally marine (hyposaline), proximal environment under bottom conditions that varied from anoxic to oxic along a nearshore–offshore transect. Samples from the upper part of the underlying Wickliffe Formation indicate deposition in a marginal to normal marine, proximal environment under anoxic to oxic bottom environments. The lower part of the overlying Laing Formation was deposited in an open marine, relatively distal setting under anoxic to oxic bottom environments.The palaeodepositional changes observed through this sequence may be best explained as a result of base-level changes in a relatively shallow-water sea. The Tartan Formation was deposited in the central, southern and eastern parts of the Great South Basin during a peak regression in the Thanetian that terminated an overall mid- to Late Paleocene aggradational to regressive trend in the basin and gave way to a latest Paleocene–earliest Eocene transgression. Its earliest and thickest known occurrence is in the Pakaha-1 well, in the centre of the basin. The formation is absent from proximal offshore wells and absent to very thin in onshore Otago drill cores and outcrops, most probably due to sediment bypass or subaerial erosion.The Tartan Formation is characterised by a combination of high organic carbon contents and 13C enrichment relative to the enclosing formations; Corg and TOC values are mostly in the range 3.7–17.1% (mean 8.0%) and δ13C values, −21.4 to −15.8‰, up to c. 9‰ heavier than in the under- and overlying formations. For sidewall core samples, there is a strong correlation between TOC and δ13C values (R2 = 0.91). The heavy δ13C values can only partly be attributed to the global Paleocene Carbon Isotope Maximum, and indicate that other, organic matter source and/or regional depositional factors affected carbon fractionation in the Tartan kerogen. The Tartan Formation displays very good-excellent petroleum generative potential (up to 31 mg HC/g rock), but HI values presented herein range only from 157 to 268 mg HC/g TOC (mean 203 mg HC/g TOC), indicating the potential is primarily for gas, with only minor oil. The relatively poor HI values are attributed to the large influx of degraded woody material, rendering the Tartan Formation less oil-prone than previously inferred.The Tartan Formation is biostratigraphically coeval with the Waipawa Formation of the New Zealand East Coast Basin, which is a similar source rock unit also enriched in 13C. Samples from the Waipawa Formation taken 1000 km north of the Great South Basin share palynofacies and geochemical characteristics with the Tartan Formation, indicating that a base-level fall also controlled the deposition of the Waipawa Formation in the East Coast Basin. The size of the area affected by the base-level fall and the lack of evidence for contemporaneous large-scale tectonic uplift imply that the base-level fall was caused by eustasy. This suggests that the Tartan and Waipawa Formations may be more extensive in New Zealand offshore basins than previously thought.