Palynofacies, organic geochemistry and depositional environment of the Tartan Formation (Late Paleocene), a potential source rock in the Great South Basin, New Zealand Poul Schiøler a, * , Karyne Rogers b , Richard Sykes a , Chris J. Hollis a , Brad Ilg a , Dylan Meadows c , Lucia Roncaglia a , Chris Uruski a a GNS Science, 1 Fairway Drive, Avalon, P.O. Box 30368, Lower Hutt 5040, New Zealand b GNS Science, National Isotope Centre, 30 Graceﬁeld Road, Graceﬁeld, P.O. Box 31312, Lower Hutt 5040, New Zealand c School of Geography, Environment & Earth Sciences, Victoria University of Wellington, P.O. Box 600, Wellington 6140, New Zealand article info Article history: Received 27 April 2009 Received in revised form 17 August 2009 Accepted 21 September 2009 Available online 25 September 2009 Keywords: Tartan Formation Waipawa Formation Great South Basin Canterbury Basin Late Paleocene Petroleum source rock Palynofacies Geochemistry abstract 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 phyto- clasts, 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 (hypo- saline), proximal environment under bottom conditions that varied from anoxic to oxic along a near- shore–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 envi- ronments. 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 13 C enrichment relative to the enclosing formations; C org and TOC values are mostly in the range 3.7–17.1% (mean 8.0%) and d 13 C 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 d 13 C values (R 2 ¼ 0.91). The heavy d 13 C 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 13 C. Samples from the Waipawa Formation taken 1000 km north of the Great South Basin share palynofacies and geochemical charac- * Corresponding author. Tel.: þ64 4 5701444; fax: þ64 4 5704600. E-mail addresses: p.schioler@gns.cri.nz (P. Schiøler), k.rogers@gns.cri.nz (K. Rogers), r.sykes@gns.cri.nz (R. Sykes), c.hollis@gns.cri.nz (C.J. Hollis), b.ilg@gns.cri.nz (B. Ilg), meadowdyla@hotmail.com (D. Meadows), l.roncaglia@gns.cri.nz (L. Roncaglia), c.uruski@gns.cri.nz (C. Uruski). Contents lists available at ScienceDirect Marine and Petroleum Geology journal homepage: www.elsevier.com/locate/marpetgeo 0264-8172/$ – see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.marpetgeo.2009.09.006 Marine and Petroleum Geology 27 (2010) 351–369