Palynofacies, organic geochemistry and depositional environment of the Tartan Formation (Late Paleocene), a potential source rock in the Great South Basin, New Zealand
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
References (71)
Stable carbon isotope ratios of plankton carbon and sinking organic matter from the Atlantic sector of the Southern Ocean
Marine Chemistry
(1991)- et al.
The Paleocene–Eocene transition at Mead Stream, New Zealand: a southern Pacific record of early Cenozoic global change
Palaeogeography, Palaeoclimatology, Palaeoecology
(2005) - et al.
Paleoceanographic significance of Late Paleocene dysaerobia at the shelf/slope break around New Zealand
Palaeogeography, Palaeoclimatology, Palaeoecology
(2000) - et al.
Mid-Paleocene dropstones in the Whangai Formation, New Zealand – evidence of mid-Paleocene cold climate?
Sedimentary Geology
(1995) - et al.
A geochemical appraisal of oil seeps from the East Coast Basin, New Zealand
Organic Geochemistry
(1999) Paleocene stable isotope events
Palaeogeography, Palaeoclimatology, Palaeoecology
(1986)- et al.
Modern and ancient continental shelf anoxia: an overview
Small palynomorphs
- et al.
Basement geology of the Campbell Plateau: implications for correlation of the Campbell Magnetic Anomaly System
New Zealand Journal of Geology and Geophysics
(1990) Depositional and tectonic history of the Great South Basin
Cretaceous-Cenozoic geology and petroleum systems of the Great South Basin, New Zealand
Institute of Geological and Nuclear Sciences Monograph
Dinoflagellate cyst biostratigraphy across the Paleocene-Eocene transition of New Zealand
Hikurangi Plateau: crustal structure, rifted formation, and Gondwana subduction history
Geochemistry, Geophysics, Geosystems
Isotopic evidence for diminishing supply of available carbon during diatom bloom in the Black Sea
Nature
Sequence stratigraphy as a “concrete” discipline. Report of the ISSC task group on sequence stratigraphy
A Geological Time Scale
Mesozoic and Cenozoic sequence chronostratigraphic chart
Source Rock Potential of the East Coast Basin (Central and Northern Regions)
Age and Origin of the Waipawa (Black Shale) Formation
Biostratigraphy and carbon isotope stratigraphy of uppermost Cretaceous–lower Cenozoic Muzzle Group in middle Clarence Valley, New Zealand
Journal of the Royal Society of New Zealand
How good a source rock is the Waipawa (Black Shale) Formation beyond the East Coast Basin? an outcrop-based case study from Northland
Waipawa Black Shale (siltstone)
Final Report Kawau-1A
Structural modelling of interpreted DUN06 seismic data from offshore Great South Basin
Cretaceous and Cenozoic sedimentary basins of Northland, New Zealand
Institute of Geological and Nuclear Sciences Monograph
A geochemical appraisal of oil generation in the Taranaki Basin
AAPG Bulletin
The Waipawa Black Shale – a ubiquitous super source rock?
Petroleum potential and oil-source correlation in the Great South and Canterbury Basins
New Zealand Journal of Geology and Geophysics
Early Cenozoic decoupling of the global carbon and sulphur cycles
Paleoceanography
Cretaceous continental rifts: New Zealand region
Stratigraphic framework and source-rock potential of Maastrichtian to Paleocene marine shale, East Coast, North Island, New Zealand: hydrocarbon prospects
Institute of Geological and Nuclear Sciences Report
Wangaloa and Abbotsford Formations: Measley Beach drillhole, South Otago, New Zealand
Institute of Geological and Nuclear Sciences Report
Report of the Fairfield Estate drillhole (FE1) near Dunedin, Otago
Institute of Geological and Nuclear Sciences Report
Allostratigraphy of coastal south and east Otago: a stratigraphic framework for interpretation of the Great South Basin, New Zealand
New Zealand Journal of Geology and Geophysics
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2020, Chemical GeologyCitation Excerpt :There is, however, an unconformity in some sections where the Waipawa Fm is condensed (Wilson and Moore, 1988; Hollis et al., 2014). The top of the formation is commonly marked by an unconformity (Schiøler et al., 2010; Hollis et al., 2014). The Whangai Fm is a thick (typically 300–500 m), poorly bedded, variably calcareous and regionally extensive mudstone that consists of the Upper Calcareous, Rakauroa, Te Uri, Porangahau and Kirks Breccia members (Moore, 1988; Field et al., 1997).