Reservoir potential of Late Cretaceous terrestrial to shallow marine sandstones, Taranaki Basin, New Zealand

Marine and Petroleum Geology 27 (2010) 1849e1871

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Reservoir potential of Late Cretaceous terrestrial to shallow marine sandstones, Taranaki Basin, New Zealand K.E. Higgs*, M.J. Arnot 1, G.H. Browne 1, E.M. Kennedy 1 GNS Science, 1 Fairway Drive, Lower Hutt, New Zealand

a r t i c l e i n f o

a b s t r a c t

Article history: Received 6 April 2010 Received in revised form 18 June 2010 Accepted 13 August 2010 Available online 21 August 2010

Late Cretaceous coals and coaly source rocks are the main source of hydrocarbons in the Taranaki Basin, yet to date there have not been any hydrocarbon discoveries within Cretaceous strata, and sandstone distribution and reservoir quality for this interval have been poorly understood. The Late Cretaceous sediments were deposited in several sub-basins across Taranaki, with their distribution largely determined by sediment supply, subsidence, and sea level change. In this study, we describe potential reservoir facies in well penetrations of Cretaceous strata in Taranaki, as well as from outcrop in northwest Nelson, on the southern edge of the basin. Study of Cretaceous outcrop has shown that facies distribution is a major control on reservoir quality. Shoreface, intertidal, and estuarine depositional facies contain both excellent, well sorted sandstone reservoir facies as well as low net-to-gross, reservoir-poor facies. These contrast with deposits from generally less well sorted, and more heterolithic coastal plain environments. Much of the facies distribution is attributed to relative sea level fluctuations, which occurred throughout deposition. Late Cretaceous sandstones display a range of mean grain sizes and compositions, with the latter related to geographic location and sediment source. Burial depths are highly variable across the basin, ranging from <1 km to >6 km, and this has had a profound effect on reservoir quality. The main petrographic controls on reservoir quality are sandstone composition, volume of clay minerals, degree of mechanical compaction (related to maximum burial depth), and proximity to a source of acidic pore fluids for the generation of secondary porosity. Results from this study suggest that Cretaceous strata are a viable reservoir play over much of the western part of the Taranaki Basin and into parts of the south-eastern basin. Palaeogeographic maps and textural/compositional data can be used to high-grade regions for further exploration. Ó 2010 Elsevier Ltd. All rights reserved.

Keywords: Cretaceous New Zealand Petrography Facies Reservoir

1. Introduction Late Cretaceous sediments in the Taranaki Basin contain some of the most productive hydrocarbon source rocks in the region and are thus important economically for New Zealand (Johnston et al., 1990; King and Thrasher, 1996). Production in Taranaki is, however, exclusively from the PaleoceneePliocene succession. There are very few well penetrations into the older Cretaceous stratigraphic intervals, and the potential for reservoirs at this level is poorly understood. The paucity of data is due to the deep burial depths of Cretaceous strata in the onshore Taranaki peninsula region (>4.5 km, with maximum depths >5 km), which is where most exploration

* Corresponding author. Tel.: þ64 4 570 1444; fax: þ64 4 570 4600. E-mail address: [email protected] (K.E. Higgs). 1 Tel.: þ64 4 570 1444; fax: þ64 4 570 4600. 0264-8172/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.marpetgeo.2010.08.002

activity has taken place. However, in the offshore part of the basin, Cretaceous rocks have typically undergone less burial and occur at present-day depths between 1 and 5 km. Further south, the Late Cretaceous crops out, with strata exposed on the northern part of the South Island in northwest Nelson (Fig. 1). Potential Late Cretaceous reservoirs have been identified in the Taranaki Basin from the limited offshore well penetrations, and comprise sandstones within the Rakopi and North Cape Formations (Bal, 1994; Baillie and Uruski, 2004; Browne et al., 2008). These Late Cretaceous plays have seen increasing interest from industry, as the primary target in some recently drilled wells and also as planned targets for drilling in 2010 and beyond. In addition, the North Cape Formation is seen as a key hydrocarbon target in the deepwater exploration blocks located along the far western side of the offshore basin (Stagpoole et al., 2001; Uruski, 2007). This paper provides a preliminary assessment of the potential for Late Cretaceous reservoirs in the Taranaki Basin, using both well data and outcrop studies. Some moderately well exposed transects

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Fig. 1. Taranaki Basin map showing the main structural elements, Cretaceous sub-basins, outcrop and study wells.

are described and reservoir potential is assessed based on new information from sedimentological and petrographic studies. We use existing reconstructions plus data from new wells drilled in the basin since the last regional palaeogeographic maps were published (King and Thrasher, 1996), to better define the Late Cretaceous paleogeography of the basin, and draw conclusions about reservoir potential. 2. Geological background The Taranaki Basin is located along the west coast of New Zealand’s North Island and northern South Island. The basin is bounded on the east by the buried Taranaki Fault and related WaimeaeFlaxmore faults. Though the western boundary is commonly placed at the present day shelf break, the stratigraphy

extends west into what has been called the Deepwater Taranaki Basin, and beyond that into the New Caledonia Basin (Uruski, 2008). The Late Cretaceous marked the early phase of basin evolution and was dominated by extensional faulting and syn-rift deposits associated with the breakup of Gondwana and the formation of the Tasman Sea (Bal, 1994; King and Thrasher, 1996; King et al., 1999; Thrasher, 1990). Late Cretaceous sediments were deposited in numerous sub-basins, with some of the thickest deposits in the Pakawau, Manaia, and Maui sub-basins in the south, and the Moa and Te Ranga sub-basins in the north (Fig. 1). Late Cretaceous sediments also form a cover over the relatively unfaulted Western Platform region in western parts of the basin. Late Cretaceous sediments are overlain by a Tertiary late-rift and post-rift transgressive succession (King and Thrasher, 1996).

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3. Cretaceous stratigraphy Late Cretaceous strata in the Taranaki Basin belong to the Pakawau Group (Thrasher, 1992a), rest unconformably on Paleozoic metasedimentary and Cretaceous plutonic basement rocks (Bishop, 1971; Thrasher, 1992a; Rattenbury et al., 1998), and are overlain by Tertiary marine and non-marine sediments of the Kapuni and Moa Groups (for details see King and Thrasher, 1996). Over most of its mapped distribution the Pakawau Group is at least 500 m thick and is up to 4.2 km thick in parts of the basin (King and Thrasher, 1996). The Late Cretaceous Pakawau Group is subdivided into the Rakopi and North Cape Formations (Fig. 2), defined from seismic reflection mapping offshore, and from lithofacies character in wells and outcrop (Thrasher, 1992a). Thrasher (1992a) recognised a disconformity between the Rakopi and North Cape Formations, a contact that is “remarkably uniform throughout the Taranaki Basin” (Thrasher, 1992a), and that is a regionally identifiable seismic horizon (e.g. Thrasher, 1988; Thrasher and Cahill, 1990). In places this surface is an unconformity with local loss of section, and in other places it represents a transgressive surface. The Rakopi and North Cape Formations are CampanianeMaastrichtian (Haumurian) in age, miospore zone PM2 (Raine, 1984; J.I. Raine pers comm. 2008). The lowermost Rakopi Formation is the earliest widely distributed CretaceouseCenozoic stratigraphic unit in the Taranaki Basin, extending throughout the basin, and towards the New Caledonia Basin in the north (Baillie and Uruski, 2004; Uruski, 2008). It is recognised in outcrop, but has been penetrated by only six wells, all in the southern or western parts of the basin (Browne et al., 2008). On seismic sections, the Rakopi Formation is characterised by high amplitude, hummocky, and laterally discontinuous reflectors, whilst wireline logs indicate a strongly interbedded and heterolithic character (King and Thrasher, 1996). The formation was deposited within a series of fluvial floodplains, transgressed periodically by marine incursions (Browne et al., 2008). It includes a basal conglomeratic member (Otimataura Conglomerate), an

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alluvial fan to braided river succession developed locally adjacent to contemporaneous faults (Wizevich, 1994). The more typical lithologies include sandstone, often cyclically interbedded with carbonaceous siltstone (where greater than half of the composition is silt-sized particles), mudstone (where greater than half of the composition is clay-sized particles) and thin coal seams. Conglomerate is rare higher in the sequence. The North Cape Formation overlies the Rakopi Formation and is typically distinguished from the latter by its greater marine depositional influence and a more featureless seismic reflection character (King and Thrasher, 1996). The North Cape Formation is widespread in the Taranaki Basin, yet has been penetrated by fewer than 20 wells to date. Siltstone and sandstone are the predominant lithologies in both outcrop and wells, but coals and conglomerate are locally prominent (Wainui and Puponga Members, Titheridge, 1977; Bussell, 1985; Wizevich et al., 1992; Bal and Lewis, 1994; King and Thrasher, 1996; Stark, 1996, and Fresne Member, Thrasher, 1992a). These deposits represent shallow marine, paralic, and terrestrial environments formed during a major south-directed transgression at the end of the Cretaceous. Further north, toward the New Caledonia Basin, the North Cape Formation becomes fully marine (King and Thrasher, 1996; Uruski, 2008), and the uppermost North Cape strata comprise mud-rich facies that are effectively the older equivalents of the overlying PaleoceneeEocene Turi Formation (King and Thrasher, 1996). 4. Methodology In this study we have reviewed the already substantial literature on Late Cretaceous stratigraphy (Titheridge, 1977; Bussell, 1985; Wizevich et al., 1992; Bal and Lewis, 1994; Stark, 1996; Browne et al., 2008), paleontology (Mildenhall and Wilson, 1982; Mildenhall et al., 1982; Crosbie and Clowes, 1984; Raine, 1984; Hayward, 1985; Hayward and Raine, 1985; Pocknall et al., 1989; Crundwell et al., 1992; Kennedy et al., 2002; Higgs et al., 2004; Strong et al., 2004), and source rock potential (Thompson, 1982;

Fig. 2. Late CretaceousePaleocene Taranaki Basin chronostratigraphy; not to scale. New Zealand stage abbreviations are Cn ¼ Ngaterian (Cenomanian), Mh ¼ Haumurian (CampanianeMaastrichtian), Dt ¼ Teurian (Paleocene).

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Cook, 1987; Johnston et al., 1990; Thrasher, 1992b; Sykes and Dow, 2000; Sykes and Raine, 2008) for well and outcrop data. We include results from new outcrop studies, and petrographic analyses of selected outcrop and subcrop samples to illustrate the range of textures and compositions of Cretaceous sedimentary rocks present in the basin. Methods used in the outcrop studies followed standard field and laboratory techniques. Outcrop sections were measured by tape and compass traverses, combined with hand-held GPS locations. Gamma logs were recorded with a spectral gamma logger. Transmitted light petrographic samples were impregnated with bluedyed resin, and stained for K-feldspar using sodium cobaltinitrite and for carbonates using potassium ferricyanide and alizarin red. The samples were point counted for mineralogy (300 counts per sample) and for grain size (100 counts per sample), and samples classified using the scheme of Folk et al. (1970). Mineralogical percentage compositions quoted in this paper refer to the proportion of the bulk sample volume rounded to the nearest percent.

5. Outcrop facies analysis Late Cretaceous strata are widespread throughout the Taranaki Basin but are only exposed in the south in the area of Whanganui Inlet and Cape Farewell, northwest Nelson (part of the Pakawau sub-basin; Figs. 1 and 3). The authors of this paper have undertaken detailed sedimentological and paleontological outcrop-based studies of both Rakopi and North Cape Formations in the region. Outcrop work on

the Rakopi Formation is summarised in Browne et al. (2008) who present stratigraphic logs, facies descriptions, and interpretations of the environments of deposition (see also Titheridge, 1977; Wizevich et al., 1992, 1994; Sykes et al., 2004). Therefore, in this current paper we summarise only the major facies and depositional interpretations of the Rakopi Formation (based on Browne et al., 2008). New work has been undertaken as part of the current study on the younger North Cape Formation outcrops and results from that work are presented here together with several measured stratigraphic sections that characterise lithologies and facies.

5.1. Facies analysis: Rakopi Formation Browne et al. (2008) recognised seven major lithologies within the Rakopi Formation: 1. Conglomerate e decimetre- to metre-thick beds comprising rounded to well rounded, greywacke-dominated clasts up to 20 cm diameter supported by a fine- to medium-grained sandy matrix. Clasts lack grading or imbrication and are crudely trough cross-stratified with cross-sets 5e50 cm thick. 2. Cross-bedded sandstone e light creamegrey to greenegrey, moderately sorted, medium to pebbly (mostly mediumgrained), quartzose, micaceous, trough cross-bedded sandstone. Finely disseminated carbonaceous debris is common throughout. Rounded to well-rounded quartz and greywacke basement clasts up to 5 cm in diameter may occur scattered

Fig. 3. Outcrop locality map, Northwest Nelson, New Zealand.

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Fig. 4. Measured outcrop sections from the North Cape Formation, Whanganui Inlet, South Island, New Zealand. Facies interpretations follow the scheme of Bal and Lewis (1994).

throughout. Rare centimetre-thick siltstone lenses may also occur. 3. Planar-laminated sandstone e light greenegrey to brownegrey, well sorted, fine- to medium-grained, carbonaceous-rich, micaceous, planar-bedded sandstone forming beds up to 80 cm thick. 4. Interbedded sandstone and siltstone e centimetre-thick, interbedded sandstone and siltstone, the sandstones being medium blueegrey, well sorted, very fine- to medium-grained

(mostly fine-grained), massive, planar- to trough cross-bedded units, 5e50 cm thick. The intercalated fine-grained beds are medium grey, very fine sandy siltstone up to 20 cm thick. 5. Siltstone e centimetre-thick medium to dark grey, carbonaceous-rich generally massive siltstone, but may display faint planar lamination. 6. Coaly Mudstone e up to 45 cm thick coaly mudstone associated with coal seams.

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7. Coal e up to 80 cm thick coal with dispersed subhorizontal lenses of bright vitrain. Typically these lithologies occur in metre to decimetre-thick fining-upward successions, the base consisting of conglomerate (1 above) or cross-bedded sandstone (2 above), passing upward into the finer grained lithofacies (3e7 above). The coarse-grained lithologies (1e2 above) were interpreted by Browne et al. (2008) as subaqueous dunes deposited within channels. The unit 3, planar-laminated sandstones, were interpreted as channel fill sands, and the finer grained lithologies (4e7 above) as progressively waning channel fill and overbank deposits. Notably, although the Rakopi Formation has been interpreted as a terrestrial deposit, evidence was presented for periodic marine influence during deposition, suggesting that marine incursions were much more significant than had previously been thought (for more details see Browne et al., 2008). 5.2. Facies analysis: North Cape Formation The North Cape Formation has been examined from several outcrop localities in northwest Nelson (Fig. 3), and the range of lithologies and facies identified can be illustrated from three measured sections that comprise parts of the upper, middle, and lower units of the formation (Fig. 4). Facies interpretations have largely followed the outcrop stratigraphic framework of Bal and Lewis (1994; Table 1), who recognised three lithofacies through the North Cape Formation as follows: A1 e thick-bedded, fine- to coarse-grained sandstone, often cross-bedded with mud drapes; A2 e thinly bedded and laminated (pin-striped), very fine- to fine-grained sandstone with mudstone, claystone, and coal; A3 e thinly bedded fine-grained sandstone with alternating mudstone, rare conglomerate and coal seams. Lithofacies A1 forms approximately 15e20% of the North Cape Formation in outcrop. It is more common toward the northern and western parts of the Whanganui Inlet area, at Melbourne Point, Oyster Point, Maori Point, and Pecks Point, where the lithofacies includes pebble- to cobble -size material (Fig. 5C). This lithofacies is also present in the south at Mangarakau (Figs. 3 and 4 and Bal and Lewis, 1994, fig. 4) where some 30 m is exposed. Bal and Lewis (1994) interpreted lithofacies A1 to represent deposition in intertidal/subtidal channels or sand shoals, dominated by migrating sinuous dune bedforms or tabular straight crested dunes. Bidirectional and herringbone cross-lamination,

reactivation surfaces and mud drapes are all common in this lithofacies, and are consistent with deposition within a tidal setting. At Pecks Point, some of the cross-beds attain thicknesses of >2 m, indicating corresponding channel depths during deposition and hence demonstrate the large size of these channel systems (Fig. 5A and B). Also at Pecks Point are abundant Ophiomorpha burrows (c. 2e3 cm in diameter) and pebbly, sand-filled, vertical burrows that cut stratification (Fig. 5C). These trace fossils represent an opportunistic fauna within the high-energy environment and are further evidence for brackish-water conditions. We infer a fan-delta depositional setting for the beds at this location, with sediment being derived from a local point source probably related to a faulted and uplifted region adjacent to the depocentre. Lithofacies A2 comprises approximately 35e40% of the North Cape Formation outcrop sections and occurs both in the south (Mangarakau) and west (Oyster and Moki points). This lithofacies has a low overall sand content (Bal and Lewis, 1994 estimate net:gross 30%) and consists of fine-grained, laminated, often pinstripe laminated, heterolithic sandstones, mudstones, and claystones with coals (Fig. 6). Horizontal and small-scale ripple laminae are common in this lithofacies (Fig. 6), whilst small rootlets and bioturbation have locally been observed. The rhythmic bedding that characterises the lithofacies is interpreted to represent the repetitive variations in flow regime of intertidal and supratidal mudflats, salt marshes, and paralic low-lying mires. Coals are only thinly developed, which is thought to be a response to repeated inundation of saline water within this marginal marine environment. Isolated lenticular sandstones were interpreted by Bal and Lewis (1994) as small tidal channels that eroded into the surrounding sediments. Lithofacies A3 forms approximately 40e45% of the North Cape Formation outcrop and is characterised by wavy and wispy sandstone and mudstone with sparse conglomerate and coal seams (Fig. 7). The bedded lithofacies has been identified at Oyster Point, Mangarakau, and Melbourne Point, and includes both well-developed channel forms and sheet beds. Sedimentary cycles generally thicken upwards, whilst beds may either fine-upwards or coarsenupwards and display variably deformed, wavy cross-lamination. Overall, there is less evidence for marine influence in this lithofacies (i.e. fewer burrows, bidirectional cross-beds, tidal couplets, reactivation surfaces) compared to lithofacies A1 and A2, and therefore the interpreted depositional setting is non-marine to intertidal, upper delta plain. Facies are dominated by channels and associated crevasse channels, splays, and splay-mouth bars into interdistributary bays. Fining-upwards individual splay deposits were interpreted by Bal and Lewis (1994) to reflect waning flows within an overall thickening-up sequence or progradational sequence. We would also place the Rakopi Formation outcrop

Table 1 Lithofacies characteristics and inferred depositional settings. Modified from Bal and Lewis (1994). Lithofacies

Sedimentary Structures

Inferred Depositional Setting

B

Thick (m) sheet sands; thin bedded mudstones; broad complex stacked channels; unidirectional cross-bedding

A3

Metre-scale beds; alternating sandstone/siltstone units; fining and coarsening upward beds; wavy lamination; thickening upward cycles; well-developed channel forms; sheet beds; large rootlets; quasi-liquid deformation Decimetre-scale beds; small channels; rhythmic bedding, micro-ripples; bioturbation; small rootlets; micro faulting/slumping

Alluvial Valley Braided or coarse-grained meandering river and floodplain deposits Upper Delta Plain Distributary/crevasse channel splays; levees, mouth bars into freshwater bays; mires

A2

A1

Metre-scale bedding; bidirectional and herringbone cross-bedding; tidal bundles; sand waves; reactivation surfaces; nested trough and stacked tabular cross-beds; climbing ripples

Tidal Mudflats Intertidal and supratidal mud & sand flats; salt marsh and mires Tidally Influenced Tidal channels and intertidal sand shoals; local fan deltas

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Fig. 5. Outcrop photographs of facies A1; (A) fine- to coarse-gained planar and trough cross-bedded sandstones, cross-bed sets >2 m. Ruler is 1 m with 10 cm dimensions. (B) Trough cross-bedded fine to pebbly sandstone with mud drapes along some trough lamina (thin resistant lamina). (C) Ophiomorpha burrow in medium- to coarse-gained sandstone of facies A1.

lithofacies (described above) into lithofacies A3 of Bal and Lewis (1994). The northwest Nelson outcrop studies demonstrate the occurrence of reservoir facies throughout the North Cape Formation, with the best sandstones developed within lithofacies A1. Deposits of the shoreface and intertidal estuarine paralic depositional settings include both well sorted sandstone likely to have reservoir potential, and lower net to gross more interbedded facies, which are likely to have lower reservoir potential. This contrasts with generally less well sorted, and more heterolithic coastal plain environments.

5.3. Distribution of Coal-bearing rocks in the North Cape Formation It is likely that the outcrop in northwest Nelson reflects all or a sizeable proportion of the stratigraphic section of North Cape Formation across the basin. Current stratigraphy suggests a lower (Wainui) and upper (Puponga) member distribution for coalbearing rocks (e.g. Titheridge, 1977; Bussell, 1985; Wizevich et al., 1992), but from the outcrop studies it is apparent that coals and coaly mudstones are distributed throughout the stratigraphic thickness of the formation. The lower Wainui Member, as present at Wainui-1 well, is likely to be a local facies variant, but from an

Fig. 6. Outcrop photographs of lithofacies A2; (A) 2.5 m section of thinly laminated or pin-strip laminated, heterolithic sandstone and mudstone which is characteristic of lithofacies A2 (Ruler is 0.5 m with 10 cm dimensions). (B) close-up photo of thinly laminate sandstone and mudstone with horizontal and ripple cross lamination (10 cm increments on ruler). (C) Photo showing thinly laminated lithofacies A2 sandstone and mudstone with coaly horizon at base of photo (lens cap for scale).

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Fig. 7. Outcrop photographs of lithofacies A3; (A)e2 m thick unit of wavy and wispy sandstone and mudstone with thin organic stringers, overlain by coarse-grained (lithofacies A1) unit. (B) Wavy and wispy sandstone unit with coaly beds above and below. (C) Close-up photo showing wavy laminated lithofacies A2 sandstone with coaly stringers and coal horizon at base (Ruler is 0.5 m with 10 cm increments).

outcrop and seismic perspective across the basin, we see no justification to retain a lower Wainui Member interval. Similarly, if we assume the outcrop around Whanganui Inlet includes sediment of comparable age as in the adjacent Puponga area, coals currently included in the Puponga Member are likely to be no different from coals observed throughout the thickness of the formation. These arguments can also be applied to data from wells and seismic surveys in many parts of the basin (see Discussion below) and it appears unlikely that either the Wainui or Puponga members can be correlated any distance across the basin. In summary, from our field-observations, we suggest that coalbearing rocks are likely to be present throughout the interval thickness of the North Cape Formation, and not restricted to the upper and lower portions. We suggest therefore the Wainui and Puponga Members should no longer be used in a lithostratigraphic or chronostratigraphic sense. The presence of coals likely reflects the periodic sea level changes through time; coals were deposited further out in the basin during periods when sea level was relatively low. Given the presumed low gradient of the coastal plain during the Late Cretaceous, even a relatively small rise in sea level would result in shoreline migration south during relative highstand periods, and covered these same areas with shallow seas.

some cases the entire Cretaceous section has been drilled, with well TD in the underlying basement or mid-Cretaceous Taniwha Formation. Examples include the relatively thin penetrations of North Cape Formation at Amokura-1 (<60 m), Kiwa-1 (c. 151 m), Kiwi-1 (c. 185 m), Te Ranga-1 (c. 50 m) and Wainui-1 (c. 157 m), of which all are located close to regions of non-deposition/erosion. In other wells, mostly from the southern part of the basin, well TD is within the Late Cretaceous (Cape Farewell-1, Cook-1, Fresne-1, Kupe South-4, Pukeko-1, Tahi-1). Thick Cretaceous sequences in Cook-1 and Cape Farewell-1 are within the fault-controlled Pakawau subbasin, close to the outcrop area (Fig. 1). Stratigraphic correlation of Cretaceous strata is complicated by the wide thickness variations across the basin (Fig. 8), by the geographic discontinuity between various Pakawau Group depocentres, and by the large distances between many of the wells used to calibrate seismic reflection mapping. Correlation of the Late Cretaceous is further complicated by the lack of biostratigraphic distinction between and within the Rakopi and North Cape Formations, which are CampanianeMaastrichtian in age (Haumurian on the New Zealand timescale; PM2 miospore zone).

6. Subsurface data

6.2.1. Conventional core Very little conventional core has been cut through Cretaceous strata. From the Rakopi Formation, one c. 6.7 m core was taken in the Pakawau sub-basin (Cook-1) and 1.5 m core recovered from the Manaia sub-basin (Tahi-1). These cores comprise pebbly to coarsegrained sandstone or conglomerate interbedded with carbonaceous shale, which is consistent with deposition from high-energy channelised environments with local coastal plain marsh and swamp development (cf. Browne et al., 2008). Coarse grain-size suggests proximity to sediment source. Two spot cores have been cut through the North Cape Formation on the western platform well Tane-1 (c. 1.1 m and 9.2 m respectively). The uppermost core comprises clean, well sorted, fine- to medium-grained, glauconitic sandstones with only local vague lamination, interpreted to represent deposition as a shallow marine

6.1. Stratigraphic distribution Late Cretaceous strata have been penetrated by 17 open-file petroleum wells in the Taranaki Basin (Table 2, Fig. 1). With the exception of Cape Farewell-1 in the south, all wells are located offshore, and most were drilled pre-1990. However, there has been recent interest in the Cretaceous play, with several new wells targeting Cretaceous strata on the Western Platform (e.g. Kiwi-1 by Transworld and Takapou-1 by STOS in 2004 and Hoki-1 drilled by AWE in 2010). Wells drilled into Late Cretaceous strata mostly penetrate the younger North Cape Formation (17 wells), with only six of these wells penetrating the underlying Rakopi Formation (Table 2). In

6.2. Subsurface facies analysis

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Table 2 Petroleum wells penetrating Late Cretaceous strata, Taranaki Basin, New Zealand. Well

Well Completion Report

Year Drilled

Cretaceous Penetration (depths along hole BKB)

Cretaceous Reservoir Facies

Amokura-1

Stroud et al. (2004)

2004

x

Ariki-1

Shell BP Todd Oil Services (1984)

1983

Cape Farewell-1

Carter and Kintanar (1987)

1985

Cook-1

Van Oyen and Branger (1970)

1970

Fresne-1

NZ Aquitaine Petroleum (1976)

1976

Kiwa-1

Shell Todd Oil Services (1982a)

1981

Kiwi-1

NZ Overseas Petroleum (2004a)

2004

Kupe South-4

Crowley et al. (1989)

1989

Maui-4

Shell BP Todd Oil Services (1970)

1970

North Tasman-1

Brophy and Falloon (1979)

1978

Pukeko-1

NZ Overseas Petroleum (2004b)

2004

Takapou-1

Shell Todd Oil Services (2004)

2004

Tahi-1

Palmer (1984)

1984

Tane-1

Shell BP Todd Oil Services (1976)

1976

Taranga-1

Shell Todd Oil Services (1992)

1992

Te Ranga-1

Shell BP Todd Oil Services (1986)

1986

Wainui-1

Shell BP Todd Oil Services (1982b)

1981

?3935-? m North Cape Fmn Total <60 m overlying basement (3) 4319e4762 m North Cape Fmn Total c. 443 m overlying basement1 178e238 m Puponga Member 238e671 m North Cape Fmn 671e1159 m Pillar Member 1159e2694 m Rakopi Fmn 2694e2817 m Oimataura Conglomerate Total c. 2639 m to TD3 2200e2251 m North Cape Fmn 2251e2688 m Rakopi Fmn Total c. 488 m to TD2 923e1230 m Puponga Member 1230e2447 m North Cape Fmn 2447e2504 m Rakopi Fmn Total c. 1,581 m to TD1 3688e3839 m North Cape Fmn Total c. 151 m overlying basement (2) 4030e4215 m North Cape Fmn Total c. 185 m overlying basement (3) 3500e3660 m Puponga Member 3660e3795 m North Cape Fmn Total c. 295 m to TD1 2789e3235 m North Cape Fmn 3235e3839 m Rakopi Fmn Total c. 1,050 m overlying basement1 2273e2430 m North Cape Fmn 2430e2669 m Wainui Fmn Total c. 396 m overlying basement (2) 4018e4177 m North Cape Fmn Total c. 159 m to TD3 4012e4183 m North Cape Fmn Total 171 m overlying basement 1167e1391 m North Cape Fmn 1391e1776 m Rakopi Fmn Total c. 609 m to TD2 3492e3638 m North Cape Fmn 3638e4000 Wainui Member 4000e4475 m Rakopi Fmn Total c. 983 m overlying basement (2) 3729e4035 m Tane Member 4035e4109 m undiff. Pakawau 4109e4179 m North Cape Fmn Total c. 450 m overlying basement (2) 3667e3717 m Total c. 50 m overlying mid-Cretaceous Taniwha Fmn (2) 3718e3762 m North Cape Fmn 3762e3875 m Wainui Member Total c. 157 m overlying basement (2)

U U

U

U

U U U

U

U

U U U

U

U

x U

Key to references: (1) stratigraphy from King and Thrasher (1996); (2) stratigraphy from GNS wellsheets; (3) stratigraphy from well completion report.

sand body. The lower core comprises reddened, slightly argillaceous, cross-laminated to ripple-laminated and fine-grained sandstones interbedded at the top with carbonaceous mudstone and coal. These deposits are interpreted as channel, crevasse, floodplain, and peat mire deposits. 6.2.2. Wireline logs and biostratigraphy The paucity of conventional core has meant that the depositional environment of Cretaceous strata has largely been interpreted from wireline log profiles and available paleoenvironmental interpretations from biostratigraphy. Logs through coal-bearing intervals (in both North Cape and Rakopi Formations) are highly serrate, demonstrating the heterolithic nature of interbedded sandstone, siltstone, coal, and local conglomerate lithofacies (e.g. Fig. 8A, Tane-1). By comparison, the marine parts of the North Cape Formation are characterised by a uniform gamma, resistivity, and density log profile with an abrupt transition from the coal-bearing facies.

The Rakopi Formation has long been considered to be entirely non-marine in origin, and recent outcrop studies have confirmed that the majority of the Rakopi Formation can be described as a series of fluvial channel sandstones and associated overbank/ floodplain deposits (Browne et al., 2008). The highly serrate log profile with abundant coals identified from logs through the Rakopi Formation is consistent with this interpretation. However, Browne et al. (2008) have demonstrated some marine influence from the presence of dinoflagellates, glauconite, and elevated coal seam sulphur contents as far south as the present-day outcrop area in northwest Nelson. The stratigraphically younger North Cape Formation was originally defined and distinguished from the Rakopi Formation on the basis of its marginal marine facies (Thrasher, 1991; Wizevich et al., 1992). Locally, the entire section of North Cape Formation is inferred to be marine from the presence of abundant marine palynomorphs (e.g. Ariki-1, Crosbie and Clowes, 1984). These are low net-to-gross shelfal deposits that are dominated by non-reservoir

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K.E. Higgs et al. / Marine and Petroleum Geology 27 (2010) 1849e1871

Fig. 8. Cross sections showing distribution of Cretaceous deposits, Taranaki Basin, New Zealand. (A) Tane-1, Western Platform to Te Ranga-1, eastern margin; (B) Amokura-1, Western Platform to Tahi-1, Manaia Sub-basin; (C) Cook-1, Pakawau Sub-basin to Amokura-1, Western Platform. Cross sections are flattened on the top Cretaceous and areal distribution of the Rakopi and North Cape Formations is taken from King and Thrasher (1996).

K.E. Higgs et al. / Marine and Petroleum Geology 27 (2010) 1849e1871

mudstone and siltstone. Elsewhere, the North Cape Formation comprises significant sandstone with locally interbedded coals and mudstones; biostratigraphic data confirms the variable marine influence through this latest Cretaceous interval. Shallow or marginal marine paleoenvironments have been inferred through parts of the North Cape Formation at many well sites from palynology and/or foraminiferal analysis (e.g. Wainui-1, Taranga-1, Kiwa-1, Pukeko-1, and Cook-1; Mildenhall and Wilson, 1982; Crundwell et al., 1992; Mildenhall et al., 1982; Higgs et al., 2004; GNS in-house data respectively). In the Manaia sub-basin (Kupe South-4 and Tahi-1), high energy inner shelf to shallow marine settings have been suggested for the basal part of the formation (Pocknall et al., 1989). However, overall, the marine signature of the formation increases uphole, with the greatest marine influence in the northern part of the basin. Coals are relatively common in the lowermost part of the drilled North Cape Formation (Fig. 8), yet dinoflagellates have been recorded from several wells within these facies (e.g. Wainui-1, Pukeko-1; Mildenhall and Wilson, 1982; Higgs et al., 2004 respectively), and are interpreted as evidence for marine influence. The most proximal facies are recorded in the eastern part of the Pakawau sub-basin at Cape Farewell-1 and Fresne-1, where a lowermost conglomerate/sand-rich interval occurs with a predominantly non-marine palynology (Hayward and Raine, 1985). These conglomerates have been interpreted by King and Thrasher (1996) to represent alluvial fan deposition against steep contemporaneous fault scarps. 7. Late Cretaceous paleogeography Maps for the Late Cretaceous (Rakopi Formation) and latest Cretaceous (North Cape Formation) presented by King and Thrasher (1996) are based on 1980s well data and seismic interpretations. The

1859

Rakopi paleogeography of King and Thrasher (1996) has been used in this study, given the absence of any new well data through the formation. Slight modifications have been suggested for North Cape Formation paleogeography to include recent data from wells Kiwi-1, Amokura-1, Takapou-1 and Pukeko-1. Well penetrations through the Rakopi Formation support deposition on a fluvial floodplain with common development of swamp and overbank facies. Browne et al. (2008) have demonstrated local marine influence within the Rakopi Formation in the southernmost Pakawau sub-basin, which suggests periodic inundation of the coastal plain regions. King and Thrasher (1996) have suggested a transgressive shoreline with shelfal conditions prevailing in the northwest, where the coastal plain fringed the southern margin of the New Caledonia Basin. Large areas of nondeposition are evident within the Taranaki Basin, shown from seismic as either onlaps around basement highs or deposition within the faulted sub-basins (Fig. 9A). The main reservoir facies are considered to be channel sandstones and more locally occurring alluvial fan deposits; reservoir facies occur in all six well penetrations through the Rakopi Formation. Coal-bearing intervals are inferred from well and/or seismic across much of the central-western basin during deposition of the lowermost North Cape Formation. Thin coals reach as far north as Wainui-1, on the margin of the western platform, and south to crop out at northwest Nelson in the Pakawau sub-basin. A marginal to shallow marine embayment is suggested, with deposition of marine-influenced strata at Cook-1, Kiwa-1, Pukeko-1, and Maui-4, covering much of the Pakawau, Kiwa, and Maui sub-basin areas. There is some uncertainty about the correlation between the Pakawau and Manaia sub-basins, but using the existing top Rakopi pick, the lower part of the North Cape Formation is represented by shallow marine facies at both Manaia wells (Kupe South-4 and Tahi-1; Figs. 1, 8B and 9B). These data are consistent with open

Fig. 9. Palaeogeographic maps of the Taranaki Basin during deposition of the (A) Rakopi Formation; (B) Loweremid North Cape Formation; (C) upper North Cape Formation. Maps are modified from King and Thrasher (1996).

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K.E. Higgs et al. / Marine and Petroleum Geology 27 (2010) 1849e1871

marine conditions further north beneath much of the Taranaki peninsula region, although this cannot be proven given the lack of well penetration. Marginal to shallow marine strata of the North Cape Formation overlie many of the North Cape coal-bearing rocks and suggest gradual inundation of the coastal plain region from the north (Fig. 9C). Marine-influenced sandstones were still being deposited in some parts of the Pakawau sub-basin and across parts of the western platform, whilst further coastal plain deposits developed at the top of the North Cape as localised pockets fringing the marine deposits. In the latest Cretaceous, the sea is likely to have encroached across most of the basin, resulting in a locally thin marine deposit; very fine and argillaceous sediments at Amokura-1 and Te Ranga-1 are interpreted as shelfal siltstones and mudstones that were deposited during this latest Cretaceous transgression. 8. Petrographic characterisation of reservoir facies Forty-two samples from potential reservoir facies were petrographically analysed from Late Cretaceous strata as part of this study and integrated with existing data from 16 other samples (Table 3). Of the new data, 16 samples are from outcrop and 26 are from Taranaki wells (core or sidewall core e SWC-samples). The majority are from the North Cape Formation, with eight subcrop samples from the Rakopi Formation. Pre-existing data comprises six outcrop samples from the Rakopi Formation (Browne et al., 2008) and ten samples analysed from the recently released Pukeko-1 well (Pollock and Funnell, 2004; Table 3). All petrographic data are summarised in Table 4 and have been integrated in order to compare sandstone texture, mineralogy, provenance and diagenesis both geographically and stratigraphically. 8.1. Texture and detrital composition The analysed sandstones range from very fine-grained and locally argillaceous, to very coarse-grained with typically little or no detrital clay matrix. Three siltstones were also analysed. The range of textures observed is related to facies, with the coarsest sandstones taken from probable Lithofacies A1 (nomenclature after Bal and Lewis, 1994; Table 1) environments (high energy tidally influenced channels/shoals), and many finer grained sandstones

taken from Lithofacies A3 environments (upper delta plain). Typically, the coarsest sandstones display the poorest sorting characteristics, whilst good to moderately good sorting occurs in many of the very fine- and fine-grained sandstones. The majority of Late Cretaceous samples classify as feldsarenites or lithic feldsarenites with rare feldspathic litharenites and litharenites (using the classification scheme of Folk et al., 1970). There is no clear differentiation in detrital sandstone composition between the Rakopi and younger North Cape Formations (Fig. 10), although there does appear to be some geographic variability. Overall, the data indicate that sandstones from Western Platform margin wells (Wainui-1 and Pukeko-1) are the most quartz-rich. Samples taken from further west on the platform (Tane-1) and from the Manaia sub-basin (Kupe South-4 and Tahi-1) are relatively feldspathic and locally lithic (Tahi-1). Outcrop samples from the Pakawau sub-basin are most comparable to samples from Kupe South-4, Tahi-1, and Tane-1. Feldspar content is highly variable in the Cretaceous samples. It occurs as K-feldspar and plagioclase, both of which show signs of dissolution. K-feldspar is often more abundant than plagioclase, but notably, plagioclase is the dominant feldspar type in samples from the Manaia sub-basin (Kupe South-4 and Tahi-1). In addition, samples from the North Cape Formation, show an overall negative correlation between proportion of total feldspar and the K-feldspar:plagioclase ratio (Fig. 11); this relationship is not clear for Rakopi Formation samples. The proportion of lithic fragments is also highly variable in the study samples, and varies with grain size, geographic location and source area. In both the Rakopi and North Cape Formations, lithics are relatively abundant in the Manaia sub-basin well Tahi-1, with highest concentrations occurring within the coarsest samples. Similarly, lithics are relatively abundant at Paturau River and Maori Point outcrops (Pakawau sub-basin, Rakopi and North Cape Formations respectively), where an increase in lithics corresponds to an increase in grain size (Fig. 12). However, it is noticeable that even very coarse-grained samples from other outcrop or subcrop samples (Mangarakau, Pukeko-1 and Cook-1) contain relatively few lithic grains. The type of lithic fragment also varies with sample locality. Metamorphic fragments are dominant in many outcrop samples (Pakawau sub-basin) and in the subcrop at Wainui-1; plutonic or

Table 3 Petrographic samples from the Late Cretaceous. Location/Well

Region

Number/Type of Samples

Formation

Forest Sectiona Paturau Rivera Cook-1 Tahi-1 Tane-1 McDonalds Farm Paturau River Mangarakau Point Muddy Creek-sth of bridge (upstream) Muddy Creek-nth of bridge Maori Point Oyster Point nth side Oyster Point sth side Kupe South-4

Pakawau Sub-basin

3 outcrop 3 outcrop 4 core 2 SWC 2 SWC 1 outcrop 1 outcrop 3 outcrop 3 outcrop 1 outcrop 3 outcrop 2 outcrop 2 outcrop 3 SWC coal-bearing rocks 3 SWC 3 SWC 10 rotary SWC 1 core, 1 SWC 2 core coal-bearing rocks 1 SWC 4 SWC coal-bearing rocks

Rakopi outcrop samples (Browne et al., 2008) Rakopi subcrop samples

Tahi-1 Pukeko-1 * Tane-1 Taranga-1 Wainui-1 a

Samples analysed by Pollock and Funnell (2004).

Pakawau Manaia W platform Pakawau Sub-basin

Manaia Sub-basin

Western Platform

North Cape outcrop samples

North Cape subcrop samples

Table 4 Summary of petrographic point count data from the Late Cretaceous. Formation

Location

North Cape Mangarakau Swamp

Data Source

This Study

Sample Sample ID Type

Outcrop Outcrop Outcrop Outcrop Outcrop Outcrop Outcrop Outcrop Outcrop Outcrop Outcrop Outcrop Outcrop Outcrop Outcrop Outcrop SWC SWC SWC SWC SWC SWC SWC SWC SWC SWC SWC SWC SWC SWC SWC SWC SWC SWC SWC Core SWC Core Core SWC SWC SWC SWC SWC

N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A N/A 3527 3619 3626 3714 3723 3758 4060.8 4063 4067 4083 4097.3 4099 4130 4137.5 4142.4 4152.8 1270 1320.5 1381.9 3515 3600 3691.45 3694.25 4138.5 3776.5 3785 3804 3867

mm

class

129 767 152 1282 468 224 121 35 51 131 105 90 79 145 91 236 274 250 217 228 122 72 354 227 121 237 216 213 293 619 625 639 1050 310 149 292 99 156 225 257 211 181 212 135

fL cU fL vcL mU fU vfU silt silt fL vfU vfU vfL fL vfU fU mL mL fU fU vfU vfL mU fU vfU fU fU fU mL cL cL cL vcL mL fL mL vfU fL fU mL fU fU fU fL

Classification (Folk et al., 1970)

MP (%) DM (%) Detrital Grains (%)

Lithic feldsarenite Feldsarenite Feldsarenite Feldspathic litharenite Lithic feldsarenite Lithic feldsarenite Feldsarenite Feldsarenite Feldsarenite Feldsarenite Feldsarenite Feldsarenite Feldsarenite Lithic feldsarenite Lithic feldsarenite Feldsarenite Feldsarenite Lithic feldsarenite Feldsarenite Lithic feldsarenite Lithic feldsarenite Feldsarenite Feldsarenite Feldsarenite Feldsarenite Feldsarenite Feldsarenite Feldsarenite Feldsarenite Feldsarenite Feldsarenite Feldsarenite Feldspathic litharenite Lithic feldsarenite Feldsarenite Feldsarenite Feldsarenite Lithic feldsarenite Lithic feldsarenite Feldsarenite Feldsarenite Lithic feldsarenite Lithic feldsarenite Lithic feldsarenite

5.3 13.0 Tr 10.4 13.4 15.6 5.3

3.7 0.7 1.3 2.4 0.6 0.3 4.3 0.3 0.6 0.3 1.3

0.3 0.3 1.7 0.3

1.3 12.7 3.7 1.0 15.0 0.7 1.7 2.7 7.0 2.3 5.0 4.7 2.3 1.3 17.0 11.0

7.5 6.4 7.0 6.4 3.7 0.4 3.5 5.5 9.1 Tr 7.7 6.0 16.3 1.4 0.7 Tr 2.3 0.7 0.6 0.7

0.3 2.3 4.3 5.7 16.3 0.3 4.3 Tr 9.7 1.0 1.0 0.3 1.3 2.0 3.3

Authigenic Clay Minerals (%)

Q

KF

PF

L/D

DM

O

27.6 34.7 21.7 24.7 29.0 21.7 25.4 30.0 23.6 27.3 20.7 25.7 16.4 28.0 25.0 17.3 18.0 22.4 23.0 23.3 21.0 30.0 34.3 28.3 35.7 37.3 38.0 41.0 39.0 43.0 49.3 39.3 8.6 22.4 22.0 30.3 19.0 27.0 27.7 20.0 34.0 37.4 29.1 25.0

12.0 16.0 9.3 11.7 12.0 11.0 10.3 15.0 13.3 15.3 10.7 14.0 12.0 13.7 10.0 9.3 11.0 11.7 6.7 7.7 7.0 12.5 16.0 14.7 10.7 14.0 11.3 13.3 12.0 11.7 14.3 13.7 2.7 10.7 9.3 20.0 13.3 17.3 17.7 10.0 14.0 19.7 15.3 13.0

11.0 7.3 21.3 12.3 17.0 20.0 6.7 8.3 11.7 14.0 15.7 12.0 19.0 10.7 13.3 20.0 21.3 28.3 31.7 22.3 19.0 11.5 11.7 10.0 8.7 11.7 8.7 6.0 6.7 5.0 3.3 8.3 22.3 24.0 21.0 12.0 24.0 17.3 16.7 7.3 1.0 1.7 2.7 3.0

9.3 7.9 6.7 25.6 16.7 16.7 3.6 4.9 4.6 9.3 6.6 7.1 4.6 13.7 11.0 8.0 6.0 11.8 9.3 12.4 8.6 3.0 1.7 1.7 3.7 1.0 3.0 2.0 3.3 4.7 4.0 3.0 33.7 16.7 5.9 9.0 6.3 11.2 11.9 8.3 6.6 10.9 11.5 11.6

6.3 7.0 7.3 1.7 1.6 4.0 12.0 12.0 9.3 10.0 8.3 16.6 23.0 10.0 14.0 22.3 6.0 4.7 8.3 5.3 11.0 10.8 0.3 2.3 3.7 0.3 0.3 0.7 0.7 0.7

4.9 1.0 3.6 0.7 6.0 1.3 3.3 12.6 3.3 2.6 2.3 3.6 6.7 2.6 3.3 4.4 4.0 3.4 4.7 2.6 3.7 1.9 Tr 3.1 3.3 3.3 4.7 4.3 16.0 3.7 1.5 1.0 0.3 6.6 4.0 5.7 6.7 4.0 0.3 4.7 4.0 1.7 4.0 4.7 6.3 0.7 1.7 1.0 2.0 9.7 4.7 0.7 9.3 0.6 3.3 4.0 2.0 1.0 2.0 1.0 2.0 2.4 3.2

0.7 2.7 2.7 9.6 0.7 6.3 9.7 5.0 1.6 Tr 1.3 1.6 2.3

Ch

Ka 4.7 2.3 1.7 1.4 0.3 0.3 8.7

IK

Other Authigenic Minerals (%) I/S

Q

Cc

D

S

15.6 0.3 5.7 0.7 0.3 6.6 18.0 0.3 3.4 0.3 4.0 3.0 1.7 0.7 7.7 0.7 4.3 0.3 1.3 9.7 13.4 2.3 10.0 0.7 1.0 3.4 5.7 Tr 3.0 12.7 0.7 1.3 9.6 0.3 0.3 4.3 11.3 0.7 1.0 13.3 1.0 4.0 1.3 1.0 3.3 4.6 0.3 16.7 5.0 3.4 5.3 0.7 2.3 3.3 1.3 1.7 0.3 2.0 0.3 2.0 Tr 1.0 8.0 0.3 6.0 10.2 0.5 4.0 4.3 9.7 6.3 8.0 6.3 6.3 9.3 2.7 5.3 10.3 4.7 2.7 11.3 3.7 7.0 11.3 2.7 6.7 10.0 1.3 4.0 5.7 5.7 1.3 Tr 3.7 2.7 6.3 5.0 11.7 0.3 3.3 9.7 0.3 4.3 0.3 6.0 0.3 1.0 1.3 1.3 4.0 3.7 4.7 5.0 0.6 0.3 6.3 4.6 1.3 11.7 4.3 1.6 2.9 2.0 45.4 21.0 0.3 2.7 3.7 9.3 3.0 0.3 17.3 0.3 1.9 2.0 2.3 0.7 12.3 2.6 1.7 11.7 4.6 13.6 0.7 3.3 2.0 8.6 4.0 0.3

O 2.4 3.6 0.9 2.3 4.6 2.0 2.3 9.7 6.7 1.6 3.7 2.6 0.7 0.9 0.3 2.7 0.3 Tr 1.6 1.3 3.3 2.8 3.3 0.7 3.3 0.7 0.7 1.0 2.0 5.7 2.0 3.3 0.3 Tr 0.9 0.6 1.3 1.4 1.3 Tr 0.6 0.3 1.7 5.2

K.E. Higgs et al. / Marine and Petroleum Geology 27 (2010) 1849e1871

P75972 P75973 P75974 Maori Point P75979 P75983 P75984 McDonalds Farm P75970 Muddy Creek P75975 P75976 P75977 P75978 Oyster Point P75985 P75986 P75987 P75988 Paturau River P75971 Kupe South-4 P76552 P76553 P76554 P76555 P76557 P76556 Pukeko-1 Pollock and ND Funnell (2004) ND ND ND ND ND ND ND ND ND Tahi-1 This Study P76511 P76513 P76512 Tane-1 P76522 P76517 P76520 P76521 Taranga-1 P76527 Wainui-1 P76523 P76524 P76525 P76526

Sample Mean Depth (m) Grain Size

(continued on next page)

1861

2.3 1.0

0.3 1.3 3.0 0.3

26.7

16.4

0.3

3.3

2.6 0.3 13.3 3.0 35.0 4.0 24.3 6.7 20.0 5.0 41.0 5.3 3.3 3.3 8.0 19.7 19.3 23.0 19.7 19.7 1.3 2.4 15.7 1.0 1.3 2.6 4.0 0.7 1.7 0.7 Tr 0.3

7.3 5.7 2.3 2.3 2.3

Tane-1

Tahi-1

This Study Cook-1

Paturau River

Sand Grain Size: vfL e very fine lower; vfU e very fine upper; fL e fine lower; fU e fine upper; mL e medium lower; mU e medium upper; cL e coarse lower; cU e coarse upper; vcL e very coarse lower; vcU e very coarse upper. Minerals: MP e macroporosity; DM e detrital matrix; Q e quartz; KF e K-feldspar; PF e plagioclase feldspar; L/D e llithic fragments and degraded grains; DM e detrital mica; O e other; Ch e chlorite; Ka e kaolin; IK e illitised kaolin; I/S e other illite and illiteesmectite; Cc e calcite; D e dolomite; S e siderite.

S D Cc Q

6.7 1.0 6.0 5.7 12.3 43.2 1.0 13.2 13.0 2.4 4.0 4.6 6.3 4.3 2.6 4.7 5.4 10.3 1.3 7.4 7.3 0.6 4.4 5.7 0.3 6.7

I/S IK Ka Ch

2.0 2.3 6.0

2.0 2.4 2.7 1.3 0.7 Tr 1.3 1.3 3.0 3.4 9.0 5.9 1.9 2.0

O DM

6.3 1.7 2.3 1.0 1.0 5.3 10.0 10.9 3.6 8.9 Tr 4.0 8.9 6.7 4.7 8.3 6.7 34.9 34.6 7.6 9.7 6.7 11.3 9.0 66.7 31.3 7.0 10.4

L/D PF

0.7 1.3 3.3 9.6 11.6 10.9

KF Q

36.3 35.7 35.0 12.6 40.4 26.7 29.9 34.1 42.3 18.0 2.0 4.7 23.1 25.7 17.7

Lithic Feldsarenite Lithic Feldsarenite Lithic Feldsarenite Litharenite Litharenite Lithic Feldsarenite Feldsarenite Feldsarenite Feldsarenite Feldsarenite Litharenite Feldspathic litharenite Feldsarenite Feldsarenite silt vfU vfUefL vfU mL fUemL mU mU cU mL vcL fU fL mL

class

Collingwood Rakopi

mm

ND ND ND ND ND ND 410 466 922 331 1173 187 173 317 N/A N/A N/A N/A N/A N/A 2338.40 2339.03 2341.23 2343.24 1478.9 1736 4052 4177 Outcrop Outcrop Outcrop Outcrop Outcrop Outcrop Core Core Core Core SWC SWC SWC SWC P69079 P69080 P69081 P69083 P69084 P69086 P76516 P76508 P76509 P76510 P76515 P76514 P76518 P76519

Authigenic Clay Minerals (%) MP (%) DM (%) Detrital Grains (%) Classification (Folk et al., 1970) Sample Mean Depth (m) Grain Size Sample Sample ID Type Data Source Location Formation

Table 4 (continued )

Browne et al. (2008)

Other Authigenic Minerals (%)

O

K.E. Higgs et al. / Marine and Petroleum Geology 27 (2010) 1849e1871

14.0 Tr 11.0 2.3 2.3 5.5 1.3 0.6 2.1 Tr 1.0 1.0 0.3 1.7 0.3 0.9 1.0 1.0 0.3

1862

volcanic lithics are dominant in the samples from Tahi-1 (Manaia sub-basin), whilst significant metamorphic and plutonic fragments occur in most other subcrop samples (e.g. Cook-1, Kupe South-4, Tane-1, Taranga-1). Mica is variably abundant in the Cretaceous sandstones, and is thought to have originally been derived from a granitic source. However, mica content is also related to facies, commonly occurring as aligned laminations in the rock and with an overall negative correlation between mica content and grain size. Other detrital grains are minor in the Cretaceous sandstones and may include degraded and heavy minerals, rare organic debris, mudstone intraclasts and, locally, minor glauconite. Skeletal bioclasts have only been identified in one well (Taranga-1). 8.2. Provenance Basement rocks have sourced much of the thick sedimentary pile that has been deposited within New Zealand basins, and basement maps have been used here to help to identify potential source areas for the Late Cretaceous sediments of Taranaki. Published work suggests that basement in the Taranaki Basin can be subdivided into several terranes based upon age, origin and rock type (e.g. Mortimer et al., 1997; Mortimer, 2004). Eastern terranes (Torlesse, Brook Street, Murihiku, Caples, Waipapa, and Maitai) are generally lithic/volcanic-rich, quartz- and K-feldspar-poor, whilst the western part of the basin is dominated by igneous rocks (Median Batholith) that intruded into typically metasedimentary Western Province terranes (Buller and Takaka). Buller Terrane rocks are generally quartz-rich clastics (Cooper and Tulloch, 1992; Rattenbury et al., 1998), while the Karamea and Separation Point Suites (Median Batholith) comprise K-biotite granite rocks (Rattenbury et al., 1998; Allibone et al., 2009). Petrographic work presented here illustrates a quartzefeldspar dominated mineralogy for Late Cretaceous sandstones in western wells (Wainui-1, Cook-1, Pukeko-1, Tane-1; Figs. 10 and 13C, E, G, H) and outcrop sections, generally consistent with a dominant granitic source. Likely source areas are the Median Batholith plutons of the Separation Point Suite (Cretaceous), and further south (Pakawau sub-basin, Cook-1 and outcrop) the Karamea Suite (DevonianeCarboniferous). A Separation Point Granite provenance seems likely, in view of its extensive occurrence in the Western Platform basement (Smale, 1996). The variable detrital mineralogy observed in outcrop samples from northwest Nelson (e.g. Fig. 13A and B) is indicative of provenance from several different terranes and granitoid suites around the areas of Cretaceous outcrop. Variably abundant metamorphic lithic fragments and relatively common plagioclase feldspar grains occur in some western Taranaki samples. The former indicates derivation from a metamorphic province, probably from the Takaka Terrane, which has experienced possible contact metamorphism and extensive regional metamorphism (up to greenschist facies; Rattenbury et al., 1998). Plagioclase is inferred to have been derived from granodiorites or gabbros of the Median Batholith. The mineralogy of Cretaceous sandstones from the Manaia subbasin in the southeast of the Taranaki Basin (Kupe South-4 and Tahi-1) is significantly different from that of their western equivalents. Plagioclase feldspar and volcanic lithics are common (e.g. Fig. 13D and F), and consistent with derivation from the Median Batholith and Brook Street Terrane volcanics respectively. This may imply a dominant southerly sediment source with subordinate input from the western basin. However, there is also the possibility that the volcanic fragments could have been derived from volcanics formerly present above the now-eroded granitoids. Wizevich et al., (1992) noted an upward trend from (metamorphic) litharenites to lithic feldsarenites to feldsarenites through the outcrop sequence, Otimatara Conglomerate to Farewell

K.E. Higgs et al. / Marine and Petroleum Geology 27 (2010) 1849e1871

1863 Q

Q 100

Quartz Arenite

0

100

Sublitharenite

Subfeldsarenite

0

Sublitharenite

Subfeldsarenite

20

20

80

80

40

60

80

F

100

Lithic Feldsarenite

80

Feldspathic Litharenite

60

40

Litharenite

Feldsarenite 100

20

Cook-1

Tahi-1

80

20

0

R Outcrop

60

40

20

Feldsarenite

40

60

60

40

0

Quartz Arenite

Tane-1

0

F

100

Lithic Feldsarenite

80

Feldspathic Litharenite

60

Litharenite

40

100 20

0

R Outcrop

Kupe South-4

Tahi-1

Tane-1

Wainui-1

Pukeko-1

Fig. 10. Ternary diagrams showing classification of sandstones from the (A) Rakopi Formation; (B) North Cape Formation. Q ¼ quartz, F ¼ feldspar, R ¼ lithic fragments. Rakopi Formation outcrop samples from Browne et al. (2008), North Cape Formation Pukeko-1 samples from Pollock and Funnell (2004).

Formation (Late CretaceousePaleocene), which might suggest a change in sediment source through time. Our data does not support this. In particular, samples from the Rakopi Formation outcrop (Forestry section) are similar to samples from the middleetop North Cape Formation outcrop (Mangarakau and Oyster Point); samples from the midetop of the North Cape Formation outcrop are commonly more lithic than samples from the basal sections (Muddy Creek and McDonalds Farm); samples from the Rakopi and North Cape Formations within a single well display comparable ranges of QeFeL (such as Tane-1, Tahi-1).

8.3. Compaction and burial history Cretaceous sandstones in the Taranaki Basin appear to have undergone variably advanced compaction. Outcrop samples from both the Rakopi and North Cape Formations typically display tangential or straight grain contacts, which suggest that they have undergone limited mechanical compaction. Labile lithics are locally bent or deformed around more rigid grain types, which again can be attributed to mechanical compaction. Signs of chemical compaction include concavoeconvex to locally slightly sutured grain contacts, and these are restricted to some of the fairly deeply 100 Outcrop

buried subcrop samples, including samples from the Western Platform (Tane-1, Pukeko-1, and Wainui-1). Variations in the degree of compaction are related to burial history. On the Western Platform, temperatures and burial depths are currently at a maximum due to a prolonged history of semicontinuous subsidence and progressive sedimentation (King and Thrasher, 1996). In contrast, wells from the southern sub-basins experienced rapid subsidence and sediment accumulation in the Late CretaceousePaleocene and Miocene, followed by significant inversion (King and Thrasher, 1996). Using exhumation patterns in Taranaki Basin (Armstrong et al., 1998), Cretaceous samples from wells Tane-1, Taranga-1, Wainui-1, Kupe South-4 and Pukeko-1 have undergone the greatest maximum burial (c. 3.5e4.2 km), with slightly less burial in the Pakawau sub-basin well Cook-1 (c. 3.1e3.2 km), and relatively shallow burial in the Pakawau outcrop samples and Manaia sub-basin well Tahi-1 (1.6e2.1 km). 8.4. Authigenic mineralogy and diagenesis Authigenic mineralogy of the Cretaceous sandstones is highly variable between samples reflecting differences in their original texture and composition and differences in burial history. Overall, clays are the most significant authigenic minerals, with other mineral cements generally occurring as a volumetrically minor component.

Kupe South-4 Tahi-1

K-feldspar:plagioclase

Tane-1 Taranga-1 Wainui-1

10

Pukeko-1

1

0 0

10

20

30

40

50

Feldspar (%)

Fig. 11. Scatter plot showing K-feldspar:plagioclase ratio versus total feldspar volume, North Cape Formation, Taranaki Basin.

8.4.1. Authigenic clay minerals Authigenic clay minerals are most abundant in many outcrop samples (Pakawau sub-basin) and subcrop samples from Kupe South-4 (Manaia sub-basin), Wainui-1, Tane-1, and Pukeko-1 (Western Platform). Overall, they are more abundant in samples from the Rakopi Formation than the North Cape Formation. Kaolinite is the dominant clay type in several petrographic samples (from McDonalds Farm, Tane-1 and Wainui-1) and is volumetrically significant in many other samples (from Cook-1, Kupe South-4, and Pukeko-1). It can occur as kaolinitised mica (e.g. Fig. 14A), but more commonly has been observed as kaolinite books and plates. The kaolinite books mostly occlude secondary grain dissolution pores and sometimes enclose remnant detrital feldspar (Fig. 14B, E and G). Traces of illitised kaolinite have been identified in only a few, relatively deeply buried samples from the Western Platform (Wainui-1, Tane-1, Pukeko-1).

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A 70 60

Outcrop Cook-1 Tahi-1 Tane-1

50 40 High lithic volume in both fineand coarse-grained sandstones from well Tahi-1 and outcrop at Patarau River

30 20

line of 15% lithics by volume

10

Outcrop Kupe South-4 Tahi-1 Tane-1 Taranga-1 inui-1 Pukeko-1

60

Lithic Fragments (%)

Lithic Fragments (%)

B 70

N.B. estimated grain size for outcrop samples

High lithic volume in both fineand coarse-grained sandstones from well Tahi-1 and outcrop at t

50 40 30 20

line of 15% lithics by volume

10 0

0 0

200

400

600

800

1000

1200

1400

Mean Grain Size (microns)

0

200

400

600

800

1000

1200

1400

Mean Grain Size (microns)

Fig. 12. Scatter plots showing lithic fragment volume versus mean grain size for the (A) Rakopi Formation; (B) North Cape Formation.

Mixed-layer clays are dominant in many samples, particularly in the Manaia sub-basin (Tahi-1 and Kupe South-4). These clays have an illite-rich composition. They mostly replace labile lithic and feldspar grains, complicating the distinction between lithics and mixed-layer clays (Fig. 14C). Authigenic chlorite has locally been identified from thin section and occurs in samples from several southern regions (Kupe South4, Tahi-1, Pukeko-1, Pakawau outcrop). The clays commonly occur as a grain-coating phase (Fig. 14D), but also occur as minor porefilling and grain-replacive phases. Oxidation effects have been observed in some outcrop samples, occurring as a locally thick grain-coating, pore-lining phase (Fig. 14E) or a grain/mud replacement phase. It is possible that some of the oxide grain coats observed in outcrop may represent modern weathering of chloritic clays. 8.4.2. Clay diagenesis The two main morphologies of kaolinite observed in Cretaceous sandstones suggest different origins for the authigenic clays. Kaolinitised mica has clearly replaced detrital mica grains, whilst books and plates of kaolinite within secondary grain dissolution pores are thought to have been largely sourced from detrital feldspar. Kaolinitised mica is often interpreted as an early diagenetic phase (e.g. Boyd and Lewis, 1995; Wilkinson et al., 2006) and it is therefore possible that this phase formed in acidic pore waters at relatively shallow burial depths. The reaction could have been related to local influxes of meteoric water within fluvial/marginal depositional environments that characterised the Late Cretaceous in Taranaki. Reaction of detrital feldspars with acidic pore fluids can also result in precipitation of authigenic kaolinite, with fluids sourced either from meteoric flux or during burial diagenesis (e.g. Bjørlykke, 1984; Ehrenberg and Jakobsen, 2001). In this study, the remnants of detrital feldspar observed within patches of kaolinite books are consistent with a feldspathic source for the kaolinite. In addition, very high kaolinite volumes are associated with low plagioclase volumes (Table 4, Wainui-1), and this suggests a plagioclase source for some of the kaolinite. However, there is wide variability in feldspar volume, particularly where kaolinite content is relatively low (<5%), and it is likely that at least some of this variability is related to differences in the original sandstone mineralogy. The dominant illitic composition of mixed-layer clay minerals, many of which replace detrital grains, is interpreted here to represent gradual transformation during burial diagenesis over a fairly long period of time. Expandable clays (smectite) will gradually transform to mixed-layer clays (e.g. 55e200þ C, Boggs, 1992) and finally to illite with temperatures above c.120  C

(Boggs, 1992), which corresponds to burial depths in excess of 3 km in the Taranaki Basin (King and Thrasher, 1996). Kaolinite will also transform to illite in the presence of potassium at high temperatures and pressures, and the presence of illitised kaolinite in only a few, relatively deeply buried samples is consistent with illitisation of kaolinite occurring at relatively high temperatures and pressures (i.e. deep maximum burial). Overall, illitisation of feldspar and degraded lithics is thought to start at an earlier stage of diagenesis than illitisation of kaolinite. Ions will have been sourced from the unstable, dissolving grains, which is consistent with the higher concentration of grain-replacive clay minerals in regions where lithic grains are abundant (Table 4). The distribution of chlorite may also be partly related to original sandstone mineralogy and/or facies. Grain-coating chlorite is often given as evidence for recrystallisation from grain-coating green marine clays (e.g. Odin, 1988; Aagaard et al., 2000), and in the case of the Cretaceous sandstones may therefore be related to specific depositional environments where fluvial input meets saline marine waters. Transformation to chlorite will have occurred during burial diagenesis at temperatures and pressures that generally correspond to burial depths in excess of c. 1.5 km (i.e. c. 55  C, Boggs, 1992, c. 90  C, Aagaard et al., 2000). Pore-filling chlorite may represent a later phase of precipitation. The abundance of chlorite in the Manaia sub-basin is probably related to relatively common iron-rich volcanic lithics, which likely provide the ions for reaction. 8.4.3. Other authigenic minerals Authigenic minerals, other than clays, are relatively minor in most of the Cretaceous samples, although several sandstones contain significant volumes of cement occurring as pore-filling carbonate (mostly calcite, Table 4). Much of the carbonate occludes secondary dissolution pores, and locally is observed to enclose authigenic quartz and kaolinite (Fig. 14B, C and E). Other authigenic cement phases in the Cretaceous sandstones include minor quartz, feldspar, leucoxene, pyrite, and local siderite. Quartz is the most ubiquitous of these and occurs in most samples as thin, partial overgrowths on detrital grains (Fig. 14A, B, C, G), with local enclosure of authigenic clay minerals within the cement (Fig. 14G). Moderately high volumes of quartz cement have been reported in only one Late Cretaceous sample (11.7% at 4152.8 m Pukeko-1, Fig. 14F; Pollock and Funnell, 2004). Some of the earliest diagenetic phases to precipitate are likely to be the minor anatase/leucoxene, pyrite and siderite cements. They are generally considered to precipitate during early diagenesis, close to the sedimentewater interface and within the iron and sulphate reduction zone (e.g. Raiswell, 1987; Wilkinson et al., 2000; Laenen and De Craen, 2003).

K.E. Higgs et al. / Marine and Petroleum Geology 27 (2010) 1849e1871

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Fig. 13. Selected photomicrographs showing geographic and stratigraphic variability in mineralogy of the Cretaceous sandstones, Taranaki Basin. (A) Maori Point, 75984, North Cape Fmn, fU sandstone, quartzefeldsparelithic composition, oxide grain-coating, few authigenic clays, good visible porosity. (B) Oyster Point, 75987, North Cape Fmn, fL sandstone, quartzefeldsparelithicemica composition, compacted, abundant authigenic clays, little visible porosity. (C) Cook-1, 2343 m, Rakopi Fmn, mL sandstone, quartzefeldsparemica composition, compacted, moderately common authigenic clays, little visible porosity. (D) Tahi-1, 1736 m, Rakopi Fmn, fU sandstone, quartzefeldsparelithic composition, compacted, moderately common authigenic clays, little visible porosity. (E) Pukeko-1, 4061 m, North Cape Fmn, mU sandstone, quartzefeldspathic composition, few authigenic clays, good visible porosity. (F) Kupe South-4, 3714 m, North Cape Fmn, fU sandstone, quartzefeldsparemica composition, abundant authigenic clays (chlorite-rich), little visible porosity. (G) Tane-1, 3515 m, North Cape Fmn, mL sandstone, quartzefeldspathic composition, undercompacted, few authigenic clays, good visible porosity. (H) Tane-1, 4052 m, Rakopi Fmn, mL sandstone, quartzefeldspathic composition, compacted, moderately common authigenic clays, little visible porosity. Scale e 450 mm.

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Fig. 14. Selected photomicrographs showing the authigenic mineral phases of Cretaceous sandstones, Taranaki Basin. (A) Cook-1, 2339 m, Rakopi Fmn showing variably kaolinitised mica and partial quartz overgrowth cements. Scale e 280 mm. (B) Wainui-1, 3776 m, coal-bearing rocks of North Cape Fmn showing secondary pore-filling kaolinite books and carbonate cement; overgrowths locally enclosed by carbonate (arrow). Scale e 280 mm. (C) Tahi-1, 1382 m, North Cape Fmn. showing variably recrystallised metasedimentary lithic; localised quartz overgrowths are in places enclosed by later carbonate cement. Scale e 140 mm. (D) Kupe South-4, 3714 m, North Cape Fmn. showing abundant grain-coating chlorite. Scale e 280 mm. (E) Wainui-1, 3776 m, coal-bearing rocks of North Cape Fmn. showing intergranular and secondary pore-filling carbonate cement; quartz grains have been locally corroded by the alkaline fluids (arrow). Scale e 280 mm. (F) Pukeko-1, 4153 m, North Cape Fmn. showing the development of thick quartz overgrowth cements. Scale e 340 mm. (G) Kupe South-4, 3619 m, coal-bearing rocks of North Cape Fmn. showing the enclosure of kaolinite books in quartz cement (arrow). Scale e 140 mm. (H) Maori Pt, 75979, North Cape Fmn. showing the occurrence of locally thick oxide grain rims. Scale e 280 mm. M ¼ muscovite, KM ¼ kaolinitised mica, KB ¼ kaolinite books, Ch ¼ chlorite, MC ¼ grainreplacive mixed-layer clays, Q ¼ quartz overgrowths, C ¼ carbonate cement, Ox ¼ oxide grain-coating.

K.E. Higgs et al. / Marine and Petroleum Geology 27 (2010) 1849e1871

Quartz cements are interpreted to have formed during burial diagenesis, mostly as a product of the reaction between detrital feldspar and acidic pore fluids. Enclosure of authigenic clay minerals within the cement suggests that the quartz formed later than locally occurring grain-coating clay and that it either postdated or formed concurrently with authigenic kaolinite books. The association between authigenic quartz and kaolinite supports a feldspar origin of ions (cf. Bjørlykke, 1984; Land, 1984; Siebert et al., 1984). Further support comes from the rare presence of quartz occluding secondary dissolution porosity, which suggests that quartz precipitation occurred after, or concurrently with feldspar dissolution. However, the small volumes of quartz cement, even within sandstones that have been buried to depths in excess of 4 km, is unusual and suggests that burial pore fluids were not saturated with respect to silica for a long period of time. Carbonate cements are interpreted to represent one of the latest authigenic phases. The location of carbonate within secondary dissolution pores suggests that in all examples the present carbonate phase precipitated after feldspar dissolution, whilst local enclosure of authigenic quartz and kaolinite is consistent with late development (post-quartz and -kaolinite). The carbonate cement would have precipitated in response to a combination of factors, such as the presence of carbonate ions, circulation of alkaline pore waters, and temperature/pressure conditions. The effect of alkaline pore fluids on some quartz grains is locally evident from the presence of corroded grain margins (Fig. 14E).

8.5. Macroporosity 8.5.1. Macroporosity distribution Macropores (pores >16 mm long) have been counted from thin section, and observed in many samples. However, the volume of macroporosity is highly variable between samples, with locally abundant pores occurring in some outcrop and subcrop samples (Table 4). Intergranular, isolated grain dissolution, and oversized hybrid pores have been recognised, with intergranular and hybrid pore types being the most common. Overall, the volume of macroporosity shows a negative correlation with volume of clay (Fig. 15), suggesting that these pores are only abundant in relatively clay-poor sandstones. Grain size and cement volume do not show a clear relationship with macropore volume, although macropores are rare or absent in samples with pervasive carbonate cement. Notably, some of the highest macropore volumes occur within some of the deepest-buried samples of

40 Locally higher macroporosity volume than expected for given clay volume due to a combination of reasons including: little intergranular clay, dominant kaolin over illite, few labiles and low compaction, little cement.

Authigenic + Detrital Clay (%)

35 30 25

Outcrop Cook-1 Kupe South-4 Pukeko-1 Tahi-1 Tane-1 Taranga-1 Wainui-1

20 15 General trend of increasing macropore volume with decreasing

10 5 0 0

5

10

15

20

Macropores (%)

Fig. 15. Scatter plot showing total point-counted clay volume (including microporosity) versus macroporosity, Cretaceous sandstones, Taranaki Basin.

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the marginal or shallow marine sand body facies (e.g. 9% at 4152.8 m in Pukeko-1, 16% at 3515 m in Tane-1). 8.5.2. Generation of secondary porosity The presence of grain dissolution and hybrid macropores, together with signs of detrital feldspar leaching, is evidence for formation of secondary porosity in the Cretaceous sandstones. Notably, variations recorded in the detrital composition of the Cretaceous sandstones are largely due to differences in the proportion of total feldspar, and the K-feldspar:plagioclase ratio. It is likely that some of these differences are related to different sediment provenance. However, it is also possible that the relatively low feldspar content and high K-feldspar:plagioclase ratio in some samples (e.g. Wainui-1, Fig. 11) is a result of preferential dissolution of plagioclase during diagenesis, resulting in large, oversize hybrid pores. Secondary porosity generated from carbonate cement dissolution may also be important in some samples, but is not easily proven. It is postulated that the timing of grain dissolution porosity generation is linked to authigenic kaolinite and quartz cementation during burial as a result of reaction between detrital feldspar and acidic burial pore fluids (cf. Bjørlykke, 1984). However, an early phase of dissolution resulting from leaching in meteoric water during shallow burial cannot be ruled out. 9. Reservoir quality Open-file core analysis data for Cretaceous reservoirs is limited to sparse data from four offshore wells (Cook-1, Pukeko-1, Tahi-1, Tane-1). These data demonstrate the presence of good reservoir quality locally, with samples >100 mD occurring at Tahi-1, Tane-1, and Pukeko-1 (Table 5). There is no strong trend of porosity versus permeability, although most samples with >0.1 mD permeability have measured porosity values >10% (Fig. 16); notably four samples from Cook-1 have permeability in excess of 10 mD but porosity between c. 7 and 9%. These data suggest that reservoir potential may occur in Cretaceous sandstones with total porosities as low as 7%. However, most good quality sandstones will have porosities in excess of 10e12%, and permeabilities of >100 mD have only been measured for sandstones with porosity >16%. Core analysis data from Cook-1 in the Pakawau sub-basin (Table 5, Fig. 16) shows that reservoir quality of the Rakopi Formation in that area (Cook-1) is fair at best (Kh < 20 mD), despite the presence of reservoir facies and the moderate burial depth (max. c. 3.1e3.2 km). It is suggested that relatively low porosity and permeability values of these medium- and coarse-grained sandstones may partly be a response to their high feldspar and mica contents (e.g. Fig. 14C). High feldspar content could indicate that there was relatively little reaction with acidic pore fluids. This is consistent with burial models from the region, which show that the Pakawau sub-basin is immature for oil expulsion (e.g. King and Thrasher, 1996). It is therefore likely that large volumes of acidic fluid from maturation of coaly source rocks have not have passed through the reservoir, resulting in limited potential for secondary porosity and associated authigenic phases. Observed compaction of the labiles (e.g. mica), clay minerals, and the moderate-poor sorting have also probably contributed to low porosity and permeability. In contrast, rotary SWC sample data from the North Cape Formation on the Western Platform margin at Pukeko-1 demonstrate the presence of good reservoir quality sandstones at depths in excess of 4 km (Kh up to 303 mD). Reservoir facies are considered to be marginal marine channel and shallow marine shoreline sandstones (Lithofacies A1 and to a lesser extent A3, after Bal and Lewis, 1994), and the best quality sandstones occur where there are relatively low clay volumes and high quartz cement volumes.

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Table 5 Core analysis data for the Late Cretaceous, Taranaki Basin (from well completion reports). Well

Well Completion Report

Sample

Depth (m)

Helium Porosity (%)

Horizontal Permeability (mD)

Cook-1

Van Oyen and Branger (1970)

Core

Pukeko-1

NZ Overseas Petroleum (2004b)

Rotary SWC

Tahi-1

Palmer (1984)

SWC and Core

Tane-1

Shell BP Todd Oil Services (1976)

Core

2338.27 2338.33 2338.82 2338.94 2339.13 2341.87 2342.54 2343.45 3601.70 3732.10 3892.60 3908.00 3913.80 3914.80 3917.10 4056.10 4060.80 4063.00 4067.00 4083.00 4092.00 4097.30 4099.00 4130.00 4137.50 4142.40 4145.80 4153.80 1250.00 1320.50 1381.90 1476.65 1495.90 3514.60 3514.78 3691.15 3692.32 3693.42 3694.08 3695.15

14.3 9.0 0.4 6.8 2.9 9.4 9.4 1.1 15.6 2.0 2.9 11.7 14.1 12.8 4.3 4.4 17.2 12.6 10.0 12.0 3.9 14.6 12.3 9.4 2.6 12.2 8.8 16.7 20.2 30.5 28.5 5.3 21.4 26.9 26.1 11.4 10.6 11.4 11.6 11.9

8 10 0.02 16.4 0.07 10.9 10.9 0.06 14.1 0.01 0.005 1.65 37.6 34 0.17 0.02 46.6 14 0.07 4.56 0.06 1.09 0.88 0.07 0.006 76 0.006 303 8.2 1055 54 0.05 8.4 816 477 0.75 1.8 2.6 4.3 2.6

Earliest oil generation was modelled to have occurred at Pukeko-1 in the Late Miocene (c. 9 Ma), with much of the hydrocarbons thought to have been sourced from deeper depocentres in the Kahurangi Trough (Pollock and Funnell, 2004). The early-mature region is consistent with high concentrations of CO2 passing through the reservoir in Middle and Late Miocene time, resulting in feldspar reactions, precipitation of clays and cement, and generation of secondary porosity. Core analysis data from the Western Platform well Tane-1 are highly variable, with some excellent reservoir quality for burial depth noted in uppermost North Cape Formation samples (Kh 477e816 mD), and much poorer quality recorded in samples from the underlying beds (Kh < 5 mD). Point-count data from the upper samples suggest that much of the macroporosity is intergranular, which is consistent with an earlier study by Van der Lingen and Smale (2000). These authors suggested that high preserved primary porosity is due to the hydrologic isolation of the reservoir, the relatively recent deposition (Plio-Pleistocene) of the top 2 km of the sediment pile, and the relatively stable tectonic history of the Western Platform. However, from the present study it is clear that samples buried just 100e700 m deeper do not display the same high porosity of the marine North Cape Formation despite similar grain size and mineralogy. We therefore suggest that high porosities in the upper samples are unlikely to be due solely to primary pore preservation. Collen and Newman (1991) stated that high porosity at Tane-1 is due to secondary porosity generation from carbonate cement dissolution and this concept is retained as a possibility here.

A. Mean 6.7%

A. Mean 10.0%

A. Mean 21.2%

A. Mean 26.5% A. Mean 11.4%

G. Mean 1.4 mD

G. Mean 34.3 mD

G. Mean 11.6 mD

G. Mean 624 mD G. Mean 2.1 mD

Additional factors could include 1) slightly higher plagioclase dissolution in upper samples (K-feldspar/plagioclase ratio is the highest in the porous sample), 2) slightly lower compactional effects (labiles e mica is much less common in the upper sample), 3) fewer clays (authigenic clay minerals are minor in the upper sample (c. 2%) but much more common in lower samples (c. 8e16%). In the Manaia sub-basin, moderate volumes of macropores were counted from some samples from Tahi-1 (up to 7.7%), which is consistent with some good porosity and permeability measured from core analysis data (Table 5, Fig. 16); the Rakopi is only slightly less porous than the North Cape Formation. Note, however, that Tahi-1 data are based on percussion SWC; fine fractures have been recorded and therefore the high reported permeability should be treated with caution. Reservoir facies are clearly present, although the high volume of labile lithics (and associated clays) in the area will be detrimental to reservoir quality. Reservoir quality has been preserved in several samples due to relatively shallow burial (c. 1.6e2.1 km) and due to the proximity of mature coals that have been expelling hydrocarbons and associated acidic fluids since Late Miocene time. Cretaceous reservoirs at Kupe South-4 may have experienced similar fluids, but the deeper burial in this well (c. 3.5e4.2 km) will result in poorer overall reservoir quality. 10. Discussion Late Cretaceous reservoir facies have been proven in the Taranaki Basin from both outcrop and the limited subcrop data. In the

K.E. Higgs et al. / Marine and Petroleum Geology 27 (2010) 1849e1871

Funnell et al., 2001; Higgs et al., 2007). Significant CO2 will have been expelled from these coaly source rocks, starting earlier than hydrocarbon migration, and preferential migration into relatively coarse grained and quartzefeldspar sandstones could have resulted in the generation of secondary porosity (from the dissolution of feldspar and possibly carbonate). Reservoir quality is therefore also likely to be related to the proximity to mature hydrocarbon regions as a source for acidic pore fluids and detailed petrographic, diagenetic and geohistory modelling may therefore help to prioritise areas for exploration. Cretaceous plays are increasingly being considered as targets in the Taranaki Basin, and they are also potential targets for frontier areas such as Deepwater Taranaki and the Great South Basin. Work presented here provides evidence for both the presence of reservoir facies and reservoir quality at this stratigraphic level. In addition, it is suggested that Cretaceous deposits in Taranaki may be used as analogues for the frontier areas, where at least some Cretaceous strata are likely to have a granitic provenance (Lindqvist, 1997; Cook et al., 1999). Given the proximity of the Great South and Taranaki basin areas in the Cretaceous (Mortimer, 2008), such a comparison seems all the more valid.

10000

Horizontal Permeability (mD)

1000

100

10

1 Most samples with > 0.1 mD permeability have measured porosity > 01%.

0.1

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Cook-1 (Rakopi) Pukeko-1 (N Cape)

11. Conclusions

Tahi-1 (N Cape) Tahi-1 (Rakopi) Tane-1 (N Cape)

0.01 0

5

10

15

20

25

30

35

Measured Porosity (%) Fig. 16. Scatter plot showing horizontal permeability versus measured porosity data, Cretaceous sandstones, Taranaki Basin.

older Rakopi Formation, these facies are likely to be channel sandstones, whilst the overlying North Cape Formation will contain both channel and shoreline sand bodies. Palaeogeographic reconstructions have been modified slightly for the stratigraphically younger North Cape Formation, and suggest that the best reservoir facies are likely to occur in the western part of the basin. Results from petrographic studies suggest that reservoir quality of the Cretaceous sandstones in Taranaki is controlled by several factors, many of which are related to geographic position in the basin. Sandstone quality is partly dependent upon detrital composition, specifically the volume of labile lithics, detrital mica, and feldspar. Abundant labile grains are detrimental to reservoir quality and have been shown to be common in parts of the Manaia sub-basin. In contrast, much of the western part of the basin has a granitic source, with few labiles and more quartz, and these deposits are likely to retain a greater proportion of macroporosity during burial. The amount of compaction (and hence porosity reduction) of the sediment is related to maximum burial depth, with Cretaceous strata from the onshore Taranaki Basin considered to have been too deeply buried to preserve reservoir quality. In addition, the volume of detrital and authigenic clay minerals is related to macropore volume and is therefore interpreted to be one of the principal controls on reservoir quality. Abundant authigenic clays are often associated with the high volume of labile grains (i.e. related to source and geographic location). It has been suggested here that reservoir quality of the Cretaceous sandstones may also be related to regions where there has been CO2-flux through the reservoir facies. Late Cretaceous coals are well known for their significant source potential (e.g. Thompson, 1982; Cook, 1987; Johnston et al., 1990; Thrasher, 1992b; Sykes and Dow, 2000; Sykes and Raine, 2008) and several studies have shown that in many parts of the basin these reached maturation in the Late Miocene-present (Armstrong et al., 1996;

 Outcrop studies have demonstrated the presence of potential reservoir facies in both the Rakopi Formation and the overlying North Cape Formation.  Channel sandstones are considered the best reservoir facies in the Rakopi Formation, whilst both channel and high energy shoreface sandstones are potential targets in the North Cape Formation.  Well data are consistent with the presence of potential reservoir facies (channel and alluvial fan) over the region of Rakopi deposition, whilst new penetrations into the overlying North Cape Formation suggest the development of more extensive reservoir sandstones than previously predicted (shoreline and shallow marine).  Coal seams and coaly mudstones are common throughout the stratigraphic thickness of the formation, both in outcrop, and in seismic and logs. Recognition of lower and upper coaly members as Wainui and Puponga members may have local application but on a regional scale coaly sediments occur throughout the interval thickness of the formation. We therefore suggest these members no longer continue to be used.  Petrographic data confirm the presence of fine- to coarsegrained reservoir sandstones within the Late Cretaceous succession.  Relatively quartz- and feldspar-rich compositions are dominant over the western part of the basin, derived largely from granitic sources, and are likely to represent some of the best quality sandstones. Relatively lithic-rich compositions are dominant in the Manaia sub-basin and locally in some of the southeastern parts of the Pakawau sub-basin, derived from mixed sources including Takaka Terrane metasedimentary rocks and Brook Street Terrane volcanics, with a contribution from other Eastern terranes.  The limited core analysis data suggests that reservoir quality of Late Cretaceous sandstones varies significantly across the basin; in most cases, potential reservoirs will have measured porosity in excess of 10%; permeabilities >100 mD have only been measured for sandstones with porosity >16%.  It is likely that Cretaceous deposits along the entire eastern margin of the basin have been too deeply buried to preserve reservoir quality, although there are no well penetrations to test this.

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 Clay minerals are proven to be related to macropore volume and hence are detrimental to reservoir quality.  Authigenic minerals, other than clays, are generally not volumetrically significant, even within deeply buried samples, and therefore are not considered to have had a major impact on reservoir quality (with the exception of localised pervasive carbonate).  Grain size, which is often quoted to have an important control on reservoir quality, does not appear to be a major factor controlling the quality of Cretaceous reservoirs.  It is suggested here that the main factors affecting reservoir quality of Cretaceous reservoir facies are detrital composition, volume of detrital and authigenic clay minerals, maximum burial depth and proximity to mature hydrocarbon regions. Notably, these factors have locally resulted in some of the highest macropore volumes occurring within some of the deepest-buried samples of marginal or shallow marine facies. Acknowledgments This project has been funded by the Foundation for Research, Science and Technology under contract No. CO5X0302. The authors would like to thank Neville Orr for preparation of thin sections; Brad Field and Mike Isaac are thanked for their helpful comments on the manuscript. The project benefited from useful discussions with colleagues at GNS Science. Bruno and Monica Stompe of Westhaven Retreat assisted our studies over a number of years. Thanks are also extended to the Department of Conservation, Takaka, who granted permission to work in the wildlife reserves of Whanganui Inlet. Wireline log data, core analysis data and other technical data/reports have been mined from the Ministry of Economic Development, New Zealand. References Aagaard, P., Jahren, J.S., Harstad, A.O., Nilsen, O., Ramm, M., 2000. Formation of grain-coating chlorite in sandstones. Laboratory synthesized vs. natural occurrences. Clay Minerals 35, 261e269. Allibone, A.H., Jongens, R., Scott, J.M., Tulloch, A.J., Turnbull, I.M., Cooper, A.F., Powell, N.G., Ladley, E.B., King, R.P., Rattenbury, M.S., 2009. Plutonic rocks of the Median Batholith in eastern and central Fiordland: field relations, geochemistry, correlation, and nomenclature. New Zealand Journal of Geology and Geophysics 52, 101e148. Armstrong, P.A., Allis, R.G., Funnell, R.H., Chapman, D.S., 1998. Late Neogene exhumation patterns in Taranaki Basin (New Zealand): evidence from offset porosity-depth trends. Journal of Geophysical Research 103, 30269e30282. Armstrong, P.A., Chapman, D.S., Funnell, R.H., Allis, R.G., Kamp, P.J.J., 1996. Thermal modelling and hydrocarbon generation in an active-margin setting: Taranaki Basin, New Zealand. American Association of Petroleum Geologists Bulletin 80,1216e1241. Baillie P., Uruski C., 2004. Petroleum prospectivity of Cretaceous strata in the deepwater Taranaki Basin, New Zealand. In: PESA Eastern Australian Basins Symposium II, Adelaide, 19e22 September 2004. Unpaginated DVD. Bal, A.A., 1994. Disparate hydrocarbon generation potential and maturation profiles of Pakawau Group source coals: implications for Taranaki Basin exploration. In: New Zealand Petroleum Exploration Conference Proceedings. Ministry of Economic Development, Wellington, New Zealand, pp. 322e337. Bal, A.A., Lewis, D.W., 1994. A Cretaceous-early Tertiary macrotidal estuarine-fluvial succession: Puponga Coal Measures in Whanganui Inlet, onshore Pakawau subbasin, northwest Nelson, New Zealand. New Zealand Journal of Geology and Geophysics 37, 287e307. Bishop D.G., 1971. Sheet S1 and S3 e Farewell and Collingwood. Geological map of New Zealand 1:63 360. Department of Scientific and Industrial Research, Wellington, New Zealand. Bjørlykke, K., 1984. Formation of secondary porosity: how important is it? In: Surdam, R.C., McDonald, D.A. (Eds.), Clastic Diagenesis American Association of Petroleum Geologists, Memoir 37, 277e286. Boggs, S., 1992. Petrology of Sedimentary Rocks. Macmillan Publishing Company, New York. Boyd, R.J., Lewis, D.W., 1995. Sandstone diagenesis related to varying burial depth and temperature in Greymouth Coalfield, South Island, New Zealand. New Zealand Journal of Geology and Geophysics 38, 333e348. Brophy, F., Falloon, A., 1979. North Tasman-1 Well Completion Report. Unpublished Petroleum Report 736. Ministry of Economic Development, Wellington.

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