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

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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 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.

Introduction

The Great South Basin (GSB) covers an area of c. 100,000 km2 and is one of the large Cretaceous-Cenozoic sedimentary basins on New Zealand continental crust (Fig. 1). Eight petroleum exploration wells were drilled in the basin in the period 1976–1984; four of them had oil and/or gas shows (Killops et al., 1997, Pearson, 1998, Cook et al., 1999). One of the wells, Kawau-1A (Fig. 1) tested 3.8–6.7 MMCFPD gas, with 7% CO2 and 24 BPD condensate. Contingent gas resources were estimated at 461 BCF (Hunt International, 1977). Although the Kawau-1A discovery was deemed uneconomic at the time, it proved the presence of an effective petroleum system in the basin. Exploration activity in the GSB has recently increased following the 2007 issue of exploration licences for five blocks covering a total of 81,000 km2 (see http://www.crownminerals.govt.nz/cms/petroleum, for further details).

The main source of petroleum in the GSB is thought to be non-marine, coaly sediments of the Hoiho Group (Late Cretaceous, Fig. 2; Pearson, 1998, Cook et al., 1999). However, with TOC values previously reported in the range 2–9%, the Thanetian (Late Paleocene) Tartan Formation is a potentially excellent petroleum source rock both in the GSB and in the adjacent Canterbury Basin (CB; Fig. 1; e.g., Killops et al., 1996, Killops et al., 1997, Killops et al., 2000, Cook et al., 1999, Sutherland et al., 2002). In the GSB wells, the Tartan Formation is a c. 28–72 m thick, dark brown, carbonaceous, slightly micaceous and glauconitic mudstone with a higher than normal GR response; it was formally established in the GSB with type section in the Pakaha-1 well in the interval 2503–2551 m (Cook et al., 1999; Fig. 1). Although considered immature over most of the Great South and Canterbury Basins (Killops et al., 1997), maturation modelling shows that the Tartan Formation has probably expelled oil since the Oligocene in the deepest, yet undrilled, part of the GSB (Sutherland et al., 2002).

The thickness, distribution, depositional environment and petroleum generative potential of the Tartan Formation and the correlative Waipawa Formation in the East Coast Basin (ECB, Fig. 1a) are subjects of ongoing scrutiny and debate with the focus being on interpretation of geochemical results (e.g., Killops et al., 1996, Killops et al., 1997, Killops et al., 2000, Cook et al., 1999, Rogers et al., 2001, Hollis et al., 2000, Hollis et al., 2006). Visual kerogen and palynofacies analysis has hitherto played very little role in the study of the formation. The present contribution engages in this debate by adding results from visual kerogen and palynofacies analysis of sidewall core samples from the Tartan Formation and adjacent strata from enclosing under- and overlying formations and presents a new predictive depositional model for the Tartan Formation that can be extended to cover coeval and similar, potential source rocks in other New Zealand basins. It builds on palynological, palynofacies and geochemical results recently presented as conference posters and abstracts (Rogers et al., 2008, Schiøler and Roncaglia, 2007, Schiøler and Roncaglia, 2008a, Schiøler and Roncaglia, 2008b).

Interpretations of depths to base and top of the Tartan Formation in individual GSB exploration wells vary somewhat in the literature (compare e.g., Killops et al., 1997, Killops et al., 2000, Cook et al., 1999 for the Kawau-1A and Pakaha-1 wells). This inconsistency has arisen because the original description of the formation implies coincidence of a high-GR response interval in the Upper Paleocene section (at and above 100 API units) and dark brown, organic-rich mudstone. However, only a part of this high-GR interval actually consists of dark brown carbonaceous mudstone and it is therefore difficult to unambiguously define the top and base of the formation in individual wells based on the GR log alone. For the same reason there is no consensus in the literature on the actual thickness variation of the formation and therefore the volume of potential source rock present is difficult to calculate. In order to resolve this, we have examined selected cuttings (ctgs) and sidewall core (swc) samples over a c. 200 m thick stratigraphic interval enveloping the Tartan Formation in the GSB wells and analysed an extended range of wire-line logs from the same wells. In order to test correlation of the Tartan Formation outside the GSB, we have further compared the study results with results from similar analyses of the CB wells Clipper-1, Endeavour-1 and Galleon-1 (Fig. 1b). In order to secure and constrain well correlations, we have undertaken detailed biostratigraphic analyses of sidewall core material from below, within and above the Tartan Formation in GSB and CB wells and determined dinoflagellate events useful for improved correlation of the Tartan and Waipawa Formations between New Zealand basins.

The main aims of this study are to: 1) determine the origin, composition and petroleum generative potential of the organic material in the Tartan Formation; 2) better understand the depositional environment of the Tartan Formation; and 3) identify base-level changes based on trends in palynofacies, lithology and wire-line logs. We test our findings by comparing with results from palynological analyses of well sections in the CB and an onshore exposure (Te Hoe section) of the coeval Waipawa Formation in the ECB (Fig. 1a).

Section snippets

Previous work

The geochemical characteristics of the Tartan and Waipawa Formations have previously been dealt with by Moore, 1988, Moore et al., 1987, Zumberge, 1990, Leckie et al., 1992, Leckie et al., 1995, Killops et al., 1996, Killops et al., 1997, Killops et al., 2000), Cook et al., 1999, Rogers et al., 1999, Rogers et al., 2001, Hollis et al., 2000, Hollis et al., 2005a, Hollis et al., 2005b, Hollis et al., 2006) and Hollis and Manzano-Kareah (2005). TOC contents of the Tartan Formation provided by

Basin history and petroleum systems

The structural and depositional history of the GSB and its petroleum systems are dealt with in detail by Beggs, 1993, Cook et al., 1999 and Sutherland et al. (2002); a summary is provided below. The allostratigraphy of onshore exposures of GSB sediments are dealt with in detail by McMillan and Wilson (1997).

The GSB is a mid-Cretaceous rift basin formed during the break-up of Gondwanaland and the subsequent separation of New Zealand from Australia and Antarctica (Beggs, 1993, Laird, 1993). The

Palynofacies

Thirty-one swc samples were studied from the GSB wells Toroa-1, Pakaha-1, Kawau-1A and Hoiho-1C. The four wells together constitute an N–S oriented, oblique, proximal–distal transect through the GSB (Fig. 1b). Cuttings samples were not used for palynofacies analysis due to the risk of contamination from cavings.

All samples were processed using standard palynological techniques (e.g., Batten, 1999) and 300 kerogen specimens >6 μm were counted in each sample including 12 major kerogen groups (

Definition, thickness and distribution of the Tartan Formation

In general, the brown, carbonaceous mudstone lithology characteristic of the Tartan Formation occurs in a high-GR interval with values at or above 100 API units. Typically, this interval is also characterised by low density and velocity readings and is delimited upwards and downwards by conspicuous increases on both these logs (Fig. 8). The most marked log shifts occur at the top of the formation indicating a sharp lithological break at that level. In basin-central wells (Hoiho-1C, Kawau-1A and

Sequence stratigraphy and distribution of the Tartan Formation

Restoration of the top Tartan seismic horizon to sea level in the back-stripped interpreted seismic section DUN06-13 (Fig. 10, lower panel) shows that Paleocene sediments draped an essentially flat sea floor topography in a continuous basin with a very low bottom gradient. It further shows that most or all extensional activity had ceased in the GSB by the end of the Cretaceous. The water depth in the basin was shallow (0–20 m) and hyposaline, based on the presence of a low-diversity foraminifera

Sea-level changes

The inferred base-level fall that led to the deposition of the Tartan Formation in the GSB and CB may have been caused either by localised or regional tectonic uplift or by eustasy. As regional tectonic activity in the basin had ceased by the Paleocene (see above and Fig. 10), a local tectonic cause is probably less likely. Large-scale tectonic uplift caused by flat-slab subduction or mantle plumes can affect even larger areas than that covered by the present study. However, these causes can

Conclusions

The Late Paleocene (Thanetian) Tartan Formation in the Great South Basin has been studied for palynofacies and bulk organic geochemistry in order to elucidate its depositional environment and petroleum source rock characteristics. Its kerogen is heavily dominated by degraded brown phytoclasts, with only minor proportions of other kerogen groups present. Based on its palynofacies characteristics, it may be deduced that the Tartan Formation was deposited in a marginally marine (hyposaline)

Acknowledgements

Drs Peter King, Rupert Sutherland (GNS Science, Lower Hutt), and Richard Cook (Crown Minerals, Wellington) are thanked for fruitful discussions about the Tartan and Waipawa Formations. Andrew Gray and Kitty Higbee are thanked for help with Fig. 1, Fig. 2. Per Erling Johansen (Applied Petroleum Technology, Norway) and Ross Stewart (Arctic Geochemical Consultants, Calgary) carried out the Rock-Eval analyses. The Journal referees Drs David Batten and Mac Beggs are thanked for constructive

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