Abstract

Conoco (now Conoco-Phillips) relinquished PEP 38602 on April 1st 2003 after acquiring 7000 km of high fold seismic data and drilling two wells, Wakanui-1 and Karewa-1. Wakanui-1 set a record for drilling in New Zealand, being drilled in just less than 1500 m of water and as New Zealand's most expensive well at $US37 million. Although this well was dry, it encountered Jurassic coal measures similar to the Huriwai Beds of onshore Northland. The coal measures are part of the Murihiku Supergroup. Both of these facts will result in re-assessments of New Zealand's petroleum geology. Previously, sedimentary rocks of Jurassic age were considered to be part of the economic basement but they have never been metamorphosed and, on the contrary, are immature for generation and expulsion of petroleum. Jurassic sedimentary rocks deposited away from the plate boundary must now be considered to have petroleum potential.

The second well, Karewa-1, also set a record, being drilled in only seven days. The results from this well are encouraging enough that Todd has taken an exploration block around the well site. In consequence, the petroleum potential of the northern Taranaki region must now be upgraded.

Six distinct source rocks are now known to be present in the general Northland region and others can be inferred. This paper describes the regional geology of offshore Northland and examines consequences of the Wakanui-1 drilling results for the region's petroleum potential. With the encouraging results of Karewa-1, perhaps the exploration of the basin can now begin in earnest.

Introduction

The Northland 'Basin' is that part of the greater Taranaki Basin (Fig. 1) that lies mainly offshore and to the west of the Northland Peninsula. It has no meaningful boundary with the area traditionally associated with the Taranaki Basin as many of the features and sedimentary systems of the Taranaki Basin are continuous with those of Northland (Fig. 2). There may be an argument to suggest that the Northland depocentre is separated from the deep water part of the Taranaki Basin by an extension of the West Norfolk Ridge, but the lack of data in this region is the main boundary at present.

The following sections introduce the geology of the Northland Basin, covering stratigraphy and structural elements. Wakanui-1 is analysed to suggest reasons for its exploration failure and the remaining petroleum potential of the basin is discussed.

Stratigraphy

Economic Basement

In common with most New Zealand sedimentary basins, economic basement in Northland was thought to be any rocks deposited prior to the Late Cretaceous extension leading up to, and accompanying, rifting of the New Zealand mini-continent from Antarctic and Australia. In northwest New Zealand, this basement consists of a series of Palaeozoic and Mesozoic terranes grouped into two provinces, the Western and the Eastern provinces (Mortimer et al, 1999). The Western Province includes pre-Permian rocks of the Gondwana margin, while the Eastern Province is composed mostly of Permian and Mesozoic metasedimentary rocks. Between the two provinces lies the Median Batholith (previously the Median Tectonic Zone) which includes subduction-related plutonic, volcanic and sedimentary rocks of Mesozoic and older ages (Mortimer et al, 1999). In Taranaki, the Taranaki Fault is the boundary between the Eastern Province and the Median Batholith to the west. Similarly, most of the offshore Northland region is underlain by Western Province terranes.

In New Zealand, Jurassic rocks are represented in the Murihiku Supergroup, which was deposited in shallow water at shelf depths, or just above, for the non-marine units. It is thought to have formed as the fill of a forearc basin near the Gondwana margin (eg Ballance, 1988). Although it is often referred to as the Murihiku 'terrane' and is classed as part of the Eastern Province, the Murihiku Supergroup forms the fill of a large sedimentary basin and is not a classic terrane (Cook et al, 1999). The Murihiku Supergroup is characterised as a thick sequence of Permian to Early Cretaceous marine and non-marine sedimentary rocks, gently folded and little metamorphosed. It is seen in outcrop in the Kawhia syncline near Auckland and on seismic data onshore and close to the coast of North Taranaki.

In the Kawhia Syncline (Fig. 1), the Jurassic part of the Murihiku is more than 5000 m thick (Kamp and Liddell, 2000) and represents almost the entire Jurassic period, ranging in age from 195 to 147 Ma (Pleinsbachian to Tithonian). Although it is dominated by marine sediments, non-marine units are episodically present from the Middle Jurassic upwards. Much of the clastic material comprising this succession is derived from volcanic rocks, generally from the adjacent Brook Street terrane (Mortimer et al, 1997, 1999). The youngest member of the Murihiku Supergroup is the Huriwai Formation of Late Jurassic age, which was deposited in braid plain and delta environments (Ballance, 1988) and includes thin coal beds of high-volatile bituminous rank (Suggate, 1990).

The Murihiku Supergroup also crops out in the South Island, where it forms the Southland Syncline and extends offshore into the Great South and Canterbury basins (Cook et al, 1999). The two main outcrops are separated today by the Alpine fault. Seismically defined layered rocks, which are probably equivalent to units of the Murihiku Supergroup, are also present in 'basement' units below the onshore Canterbury Basin (Bennett et al, 2000).

Volcanic Rocks

Volcanic rocks are common components of New Zealand's basement terranes and have been extruded at various times during development of the petroleum basins.

An Early Cretaceous volcanic arc is believed to have been situated in the New Zealand region (Muir et al., 1995; Sutherland et al., 2001). Late Cretaceous rhyolites were sampled on Lord Howe Rise by DSDP well 207 (DSDP, 1973) and evidence for volcanism of Middle and Late Cretaceous ages (~100 Ma and ~75 Ma) in the deep water part of the Taranaki Basin was reported by Uruski and Baillie (2002), Uruski et al. (2003) and Baillie and Uruski (2004, this volume).

Eocene volcanic rocks were dredged and drilled in Challenger Plateau (Nelson et al, 1986; Carey et al, 1991) and also imaged on seismic data along the northeast flank of the Challenger Plateau (Uruski and Wood, 1991).

Miocene volcanic rocks are well known from Taranaki (King et al, 1996) and Northland (Isaac et al, 1994) and Arco made a sub-commercial discovery in the flank of Kora, a Miocene shield volcano in northern Taranaki (Bergman et al, 1992). The Aotea Seamount on the northeast flank of the deep water Taranaki Basin is a very large Miocene volcano extending approximately 100 km in an east-west direction and is 40 km wide (Uruski and Wood, 1991).

Pliocene and Recent volcanoes form part of the New Zealand landscape today, and New Zealand's petroleum industry has the Recent Mount Egmont in the Taranaki Peninsula as its "centre of gravity". Recently, a hydrocarbon play involving drape across the crest of a buried volcanic body appears to have been proven by the Karewa-1 discovery.

Cretaceous Units

The oldest Cretaceous unit in the region is probably the upper part of the Syn-rift Megasequence of Uruski and Baillie (2003) in the deep water Taranaki Basin. By analogy with the Strzelecki Group of Gippsland Basin (Norvick et al, 2001), and by its palaeogeographic and stratigraphic position, the unit is likely to consist mainly of terrestrial deposits with a high proportion of coarse clastic material adjacent to faults. In Gippsland, the Strzelecki Group ranges in age from 140 to 105 Ma (Berriasian to Albian) and a similar age is likely in Taranaki.

In northern Taranaki and Northland, the oldest Cretaceous unit known is the 100 to 90 Ma Taniwha Formation of late Albian to early Cenomanian age (= Ngaterian). This formation has only been sampled in one well, Te Ranga-1 (Fig. 1) drilled in 1986 by Shell, BP and Todd (SBPT, 1986). The well drilled through a complex thrust zone that carried Murihiku Supergroup rocks of Triassic age over Tertiary and Cretaceous units. The well reached TD at 3882 m (RKB) within the Taniwha Formation, which consists of siltstone, sandstone, coal and carbonaceous shale. Seismic evidence shows that basement lies more than 2000 m below the well's TD (King et al, 1996). In Northland, the presence of Taniwha Formation rocks has been suggested by Isaac et al (1994) and by Gage and Kurata (1996), both based on seismic evidence and comparisons with Te Ranga-1.

The Rakopi Formation is of Campanian age (~75 Ma, Haumurian) and is described as 'the lowest stratigraphic unit with widespread distribution in Taranaki Basin' (King et al, 1996). It is the older of two formations that together comprise the Pakawau Group and is seen onshore in outcrop in northern South Island. The Rakopi Formation has been encountered in six wells and is present at the shelf edge in Tane-1 (Fig.1). Below the Taranaki Peninsula and shelf, the Rakopi Formation is a coal measure unit, defined seismically by high-amplitude, hummocky discontinuous reflectors. These same reflectors extend continuously across the shelf edge into the deep water where they form the topset beds of a large Late Cretaceous delta (Fig. 2; Uruski and Baillie, 2002). The basal units of the Taranaki delta may include equivalents of the Taniwha Formation. The Rakopi Formation itself marks the culmination of delta building and was followed by transgression. It is the source rock for oil in the Maui field. As far as is known, the Rakopi Formation may or may not extend into the Northland region. The uncertainty is due to a lack of data.

The North Cape Formation overlying both the Rakopi and Taniwha formations is of late Campanian and Maastrichtian age (75 to 65 Ma, late Haumurian) and is a transgressive marine succession representing deposition during the first part of the drift phase of opening of the Tasman Sea (King et al, 1999). The North Cape Formation is almost ubiquitous, being present from northern South Island across the Taranaki shelf, into deep water and across Northland. The exceptions are that the crests of structural highs remained emergent through the Late Cretaceous (King et al, 1996) and are the only regions known within the basin that were not covered by North Cape Formation sediments. Facies are variable. Although, volumetrically, marine mudstones are by far the most dominant rock type, coal measures and sandstones are also present. Sandstone units fringe many of the structural highs and the basin margins, while coal measures of the Wainui Member, which crosses the present shelf edge, bear evidence of broad, periodically emergent land areas. Following the establishment of the Taranaki delta, the shelf and coastal plain area was very large, extending some 200 km from the present coastline (Baillie and Uruski, 2004, this volume). Small changes of relative sea level caused large lateral shifts of depositional environment and facies. The coast probably migrated landwards and seawards by a hundred kilometres or so several times during deposition of the North Cape Formation. Onshore in Northland, the Waiari Formation is the closest equivalent to the North Cape Formation, being of similar age. Lithologically, however, the closest equivalent to the Waiari Formation is the Whangai Formation marine siltstones of the East Coast Basin.

Paleogene Units

Although local names have been given to many units in the Northland region, they are essentially similar in facies to those of Taranaki. Paleocene and Eocene rocks are subdivided into the mainly terrestrial Kapuni Group and the marine Moa Group. The latter is dominated by marine mudstones of the Turi Formation and include a siltstone member, the Tane Member near the present shelf edge. The Kapuni Group includes fluvial sediments, coal measures and the marginal marine McKee sandstone and is the reservoir for many of the hydrocarbon fields in Taranaki.

Facies variations within the Kapuni and Moa groups are a result of palaeogeography inherited from the Late Cretaceous, when a broad shelf and coastal plain system developed. Throughout the Paleocene and Eocene, small relative changes in sea level continued to produce large shifts in facies belts. In Northland, Kapuni Group equivalents are known as the Kamo coal measures. They extend northwards almost the length of the Northland Peninsula where they are preserved in graben, before merging into a marine equivalent, the Mangapa Mudstone. Offshore from Kaipara Harbour, a large lobate area of sediments has been interpreted as deltaic and shallow marine.

Oligocene rocks record gradual subsidence of the region. In Taranaki, the background mudstones become more calcareous and eventually merge into the Tikorangi Formation limestone. In Northland, the equivalent is the Whangarei Limestone. The differences in lithology reflect relative water depths and the proportion of fine clastic sediment supply.

Miocene Units

One of the earliest events in the Miocene was the emplacement of the Northland Allochthon, a set of thrust sheets emplaced from the north (Fig. 2). The allochthon includes rocks as old as Early Cretaceous (Albian, Motuan,100+ Ma) and as young as earliest Miocene (Waitakian, Aquitanian, 23 Ma). Six nappes are recognised in northern Northland (Isaac et al, 1994) and they carried a variety of rocks to the south across younger formations. Rocks incorporated in the thrust sheets consist mainly of sedimentary rocks deposited across the ocean-facing Gondwana margin although volcanic rocks, interpreted as ocean crust material, are also present. Repeated sequences have been proven by onshore wells such as Waimamaku-2 (Fig. 1; New Zealand Petroleum Exploration Company, 1972).

Volcanism started as the Northland Allochthon was emplaced, both events resulting from the onset of subduction of the Pacific Plate from the northeast. Volcaniclastic rocks dominate the Early Miocene sediments. The locus of volcanism moved south into the Taranaki Basin by the Middle Miocene and from 15 Ma the dominant setting for deposition has been that of a passive margin. Rock units below the shelf and slope are dominated by clastic sediments grading to carbonates in the distal offshore regions.

Structural Elements

Six structural elements are recognised in the Northland part of the greater Taranaki Basin (Fig. 2). The North Taranaki Graben continues northwards into the Northland area. The eastern margin of the basin is formed by a continuation of the Taranaki Fault which extends below the Northland shelf until it is overthrust by the third element, the younger Northland Allochthon. The Northland Ridge is the fourth structural element, and this high basement ridge is a large horst striking approximately parallel with the coast extending from northern Taranaki, where it merges with the western flank of the North Taranaki Graben, at least as far north as Wakanui-1. The main Cretaceous depocentre, the Northland Graben, occupies the region between the Taranaki Fault extension and the Northland Ridge. A further depocentre is seen on the few seismic lines that extend west of the Northland Ridge, but its extent is unknown.

North Taranaki Graben

The North Taranaki Graben is a late Neogene feature bounded to the west by the Cape Egmont Fault Zone and to the east by the Turi Fault Zone. The Cape Egmont Fault zone is a series of east-dipping normal faults in a zone approximately 30 km wide, while the Turi Fault Zone dips westwards and appears to terminate at the Northland coast, possibly at the north-dipping Waikato Fault (Hochstein and Nunns, 1976), although there is no seismic evidence for a large fault in the offshore region (Stagpoole, 1997). Some faults are reactivated Cretaceous structures, but all were active during the Late Miocene and Pliocene as a result of rotational stresses caused by realignment of the plate boundary to the present configuration.

The sedimentary fill of the North Taranaki Graben includes 2000 m of Cretaceous and Paleogene rocks and 5000 m of Neogene rocks (King et al, 1996). Source rocks include the marine Waipawa Formation black shale that sourced a proportion of the oil tested in the Kora discovery of 1988. Kora is a small strato-volcano on the western flank of the North Taranaki Graben. The original drilling target was the Kapuni Group, which was locally domed by the passage of magma, but oil was discovered in the volcaniclastic apron of the volcanic cone. The Kora discovery proved to be sub-commercial, possibly due to problems with seal rather than reservoir quality (Cook, pers. comm.). Biomarkers from the Kora oil showed a mixed source, with characteristics typical of a Cretaceous component in addition to the Paleocene Waipawa Formation, so a Cretaceous source is also expected to be present in the North Taranaki Graben.

Reservoir rocks have been intercepted by several wells, but perhaps the most attractive in this region is the Mangaa sands of early Pliocene age. As discussed later, we infer that the Karewa well drilled by Conoco, Inpex and Todd in 2003 made a small discovery in this interval.

The Taranaki/Northland Fault

The Taranaki Fault is a major fault forming the eastern boundary of the Taranaki Basin (Nicol et al, this publication). It trends northwards across the Taranaki Peninsula, crosses the north Taranaki shelf parallel to the coast and swings to the northwest at about 38.5oS. From that point it runs almost parallel to the Northland coast and about 25 km offshore. It is interrupted by several major Early Miocene volcanic centres, so most maps show the faults as being discontinuous. Seismic line cnl94a-32 (Fig. 3) shows this reverse fault system to great effect.

An easterly-thickening wedge of sedimentary rocks is folded and thrust westwards along a flat-lying fault. This wedge is as much as three seconds (TWT) thick and appears to be composed of two main sequences. The older sequence onlaps the fault plane and the younger downlaps onto the top surface of the older sequence. The older sequence is interpreted as Murihiku Supergroup rocks and the younger as Cretaceous sediments, probably equivalents to the Taniwha Formation further south. The Cretaceous sediments appear to have prograded across the older Murihuki units and may be part of a preserved Early Cretaceous delta. The overall pattern is of the fill of a sedimentary basin or half-graben that thickens to the east. Early Eocene sediments are deformed by the thrusting and Late Eocene and Oligocene sediments appear to onlap the thrust wedge. Total horizontal movement on the thrust was not large and may be only several kilometres.

Northland Allochthon

Offshore from the northern part of the Northland Peninsula, the thrust sheets of the Northland Allochthon at first appear to be a continuation of the basin margin fault. However, the thrust sheets are shallower than the Taranaki and Northland boundary fault and involve younger sediments. Although elements of petroleum systems are present within the allochthon, its structure is complex and poorly imaged on seismic data. It is not possible with existing data to locate good reservoir facies as drilling targets within the thrust sheets. One of the main effects of emplacement of the allochthon was tectonic thickening leading to maturation of the succession below. Another effect was to buckle the underlying rocks to create large-scale folds (Isaac et al, 1994) which may have trapped hydrocarbons expelled from Jurassic, Cretaceous and Paleogene source rocks.

Northland Ridge

The Northland Ridge (Gage and Kurata, 1996) trends approximately parallel with the Northland Peninsula and merges to the southeast with the western margin of the North Taranaki Graben. Wakanui-1 (see below) has shown that this basement high incorporates Murihiku Supergroup sediments that may contain significant source rock facies, although good quality reservoir rocks are unlikely to be present. The Northland Ridge may still have considerable petroleum potential, particularly southeast of Wakanui-1 where overlying facies may include good quality clastic reservoirs such as Cretaceous and Paleogene shoreface and turbidite sands.

Depocentres

Cretaceous and Paleogene depocentres are present both east and west of the Northland Ridge. The eastern depocentre, the main Northland Graben, lies between the west-verging boundary fault and the Northland Ridge and appears to be the largest depocentre in the Northland region. Gage and Kurata (1996) referred to this depocentre as the Northland Basin. It differs from the North Taranaki Graben mainly because of the absence of Neogene extension. The main Northland Graben may be an analogue for the North Taranaki Graben prior to the Neogene extension. Few seismic lines exist to the west of the Northland Ridge, although they indicate the presence of a western depocentre along at least a part of the region. Further west, basement climbs through a series of smaller graben to the crest of the West Norfolk Ridge within the deep water Taranaki Basin. Basement rocks within the main Northland Graben are rifted, forming a variety of large structures across which Cretaceous and younger sediments are draped. The resulting anticlines are large and may have significant petroleum potential. Modelling shows that source rocks within the depocentres should be expelling hydrocarbons.

Wakanui-1

Wakanui-1 was drilled by Conoco, Inpex and Todd in a water depth of 1455 m approximately 150 km northwest of Auckland in 1999 using the drillship 'Deep Water Frontier' (Milne and Quick, 1999). The structure is a large tilted fault block with good syn-tectonic sedimentary reflectors overlying the basement surface.

Wakanui-1 was drilled to a total depth of 3681 m relative to the kelly bushing (all following depths in wells are given relative to the kelly bushing). It is believed to have spudded in Pleistocene sediments, although no returns were available until the 20" casing had been cemented at 2138 m. Marls dominate the Late and Middle Miocene succession and the Early Miocene, from 2347 m, is dominated by volcaniclastic sediments and volcanic tuffs (Fig. 4). The Miocene-Oligocene boundary is marked by an abrupt transition to limestone at 2579 m. The limestone is assigned to the widespread Tikorangi Formation and is much thicker than expected at 167 m. Its base is marked by a transition to marls and limestones of Eocene age. The base of the Eocene is at 2951 m and a 504 m succession of Paleocene marine claystone follows. The Paleocene Turi Formation includes a 26 m thick section of the Waipawa Black Shale, a unit that has known source rock potential elsewhere in the Greater Taranaki Basin. The Turi Formation claystone sits above an Early Paleocene sand and conglomerate unit (Fig. 4) assigned by Conoco to the Kapuni Group The topmost sand was interpreted as a transgressive sand. A major unconformity was encountered at 3544.5 m.

No Cretaceous rocks were discovered in this well. The unit prognosed as Cretaceous was the thick marine mudstone of Paleocene age which, instead of overlying Cretaceous coal measures, covered a Middle Jurassic coal measure succession, dated radiometrically (Folland, 1999) by the sill at 3602 m and by pollen (Strong et al., 1999). The well took 79 days to drill and the only logs acquired were a measure while drilling (MWD) suite of gamma, resistivity and density. This is one of the few wells drilled in recent times in New Zealand without a sonic log, or at least a checkshot survey, which means ties to seismic data are uncertain.

In their post-drill analysis of Wakanui-1, Dolan and Istadi (2003) showed that the Middle Jurassic coal measures had good source potential, that a Paleocene transgressive sand has some reservoir potential and that the thick Paleocene mudstones and Tertiary marls encountered were excellent seal rocks.

Source rock characterisation studies were carried out on nine sidewall cores (West, 1999) between 3415 m and 3624 m. Although somewhat compromised by the Soltex drilling mud used, total organic carbon (TOC) was successfully measured for four samples. At 3515 and 3433 m, TOC values were 0.35% and 0.56% respectively, suggesting low source potential. The deeper samples, at 3584 and 3612 m, were washed with detergent to remove contamination by drilling mud and they yielded TOC values of 0.78% and 9.31%, respectively. Rock-eval data for the sample from 3584 metres show that it is fully mature with a Tmax of 450oC, has a moderate level of free hydrocarbons (S1: 0.27 mg/g) and moderate source potential for oil (HI: 322) with some gas. The 3612 m sample is organically rich and fully mature with TOC of 9.31% and Tmax of 452oC. The sample has excellent source potential (S2: 15.6 mg/g) for gas (HI: 168), although the high S1 value of 1.12 mg/g may have been due to residual contamination by drilling mud. The sample from 3612 m was submitted for pyrolysis GC analysis and the results are consistent with mature kerogen from a coally origin that would generate mostly gas.

Most vitrinite reflectance values were below 0.7% with the exception of a set of values from around the volcanic sill intrusion (Hegarty, 1999), dated at 158 Ma and encountered at 3602 m. Below that level, vitrinite reflectance decreases to less than 0.7% which does not support the conclusion that the Middle Jurassic coal measures had been buried to great enough depths to generate hydrocarbons prior to the Cretaceous. Killops et al (1994) showed that oil expulsion from typical New Zealand coaly source rocks occurs at vitrinite reflectance levels of 0.8% or greater.

The Paleocene transgressive sand flowed some fresh water into the wellbore when penetrated (Dolan and Istadi, 2003), but no salinity data have yet been made available. Dolan and Istadi (2003) also suggested that this sandstone is 'plumbed into' the deeper basin, presumably to the west, and should therefore have provided a viable migration pathway for hydrocarbons migrating from that kitchen.

The freshness of the water may be significant. It is difficult to see how this reservoir could have been flushed by meteoric water from the land as it would have had to travel nearly 90 km across a major thrust system, a deep, rifted Cretaceous and Tertiary sedimentary basin, to the westwards-dipping Wakanui rotated fault block. Alternatively, if water of low salinity was trapped in this unit and has remained there since burial, it may be that the sand body is isolated from the hydrocarbon kitchen, which might explain the almost negligible hydrocarbon indications from the well, comprising three grains of sand that held oil in fluid inclusions (Lisk, 1999).

Conoco wrote off virtually the entire basin on the basis of this well, but went on to drill a second well, Karewa-1, in the extension of the North Taranaki graben (Figs. 1 & 2). This second well is believed to have targeted early Pliocene sands that yielded a strong seismic bright spot anomaly. The results of that well are not known, but Conoco relinquished the licence block as it expired on April 1st 2003. However, Todd Petroleum extended the licence in a small block around the well, leading to speculation that a small discovery has indeed been made.

The main conclusions from the Wakanui-1 well are that the Murihiku Supergroup extends much further offshore than was previously believed and that the Wakanui block appears to have originated as a tilted fault block following a Middle Jurassic rifting episode. The age of the faulting that formed the Wakanui block is ambiguous. Some sections appear to show westwards-diverging reflectors within the Murihiku package, which suggests that deposition of the Murihiku sediments was syn-tectonic. Geological relationships are complicated by erosional truncation of the dipping reflectors across the crest of the block. A Late Cretaceous rifting episode also affected the region and further complicated the Wakanui structure.

The Wakanui block and other highs along the Northland Ridge remained as islands for the entire Cretaceous and until the transgressive Paleocene sea eventually inundated it. The Paleocene succession is thicker than was expected and no Cretaceous sediments were encountered in the well. Despite the good sealing properties of the Paleocene succession, the validity of the Wakanui structure as a trap must also be doubted as it relies on no less than three seals; a top seal, a fault seal and an bottom seal as the transgressive unit onlaps the underlying Murihiku rocks leaving a large region with no reservoir cap. Wakanui-1 is therefore not a good test of an active petroleum system.

Although Cretaceous sediments are not present across the crest of the Wakanui block, their presence is highly likely in the depocentres to the west and to the east of the block. Cretaceous source rocks, particularly in a very large depocentre to the east of the block, are likely to be mature and expelling hydrocarbons that are probably trapped by the very good seal provided by the anomalously-thick Turi Formation marine claystone. Rather than down-grading the basin, the presence of Jurassic coal measures suggests that a new petroleum system may also be present in the region.

Petroleum Systems

Active petroleum systems are indicated by several gas seeps and by gas shows in wells Waimamaku-1 and Waimamaku-2. Several oil shales have been recorded and analysed, mostly during the early twentieth century. Among these are examples of the Waipawa Formation black shale, which has been documented in widespread basins to the east of New Zealand. Between 1910 and 1972, 17 wells were drilled onshore in Northland, eight of which reported shows of gas and two of oil. As in Taranaki, coaly source rocks are abundant in the Cretaceous and Paleogene succession. The discovery of coal-bearing Murihiku Supergroup sediments some 90 km offshore in the Greater Taranaki Basin suggests that at least one Jurassic petroleum system is also present.

Jurassic sediments, including those with source potential, are present in several basins around the Tasman Sea, notably in the Gippsland Basin (Holdgate and McNicol, 1992) and the Clarence-Moreton Basin in New South Wales (Wells and O'Brien, 1994, Shaw et al., 2001). The latter basin contains the MiddleJurassic Walloon Coal measures which may be similar to those discovered by Wakanui-1.

The basal sedimentary unit imaged by the Astrolabe Survey in the head of the New Caledonia Basin (Uruski and Baillie, 2004) may also consist of sedimentary rocks of Jurassic age. The most obvious mechanism for creating an extensional sedimentary basin during a period of active subduction is formation of a back-arc basin, as suggested by many previous authors (eg Uruski and Wood, 1991). This possible back-arc basin may have occupied the present site of the New Caledonia Basin, continuing from the present Coral Sea, through New Zealand to the Bounty Trough (Fig. 1). Its existence leads to further speculation on the nature of the sedimentary systems being deposited during the Jurassic. Evidence is growing, in the New Zealand region, that un-metamorphosed Jurassic rocks are more widely distributed than previously thought (Cook et al, 1999). It may be that the Jurassic coal measures in Wakanui-1 are a link between the mainly marine rocks of the Murihiku Supergroup and the fill of a marine back-arc basin below the New Caledonia Trough. In the next few years, seismic data from the region will be closely examined for further signs of Jurassic sedimentary systems.

Two potential Jurassic source rocks are therefore known, the Huriwai Beds of Late Jurassic age and the Middle Jurassic coal measures penetrated by Wakanui-1. It may be that coal is not the only Jurassic source rock, but that an Australasian Kimmeridge Clay equivalent may be discovered. Reservoir facies for these potential source rocks may be sandstones of almost any age from Cretaceous to Neogene.

Cretaceous source rocks in the region include the Early Cretaceous Taniwha Formation coal measures, the Late Cretaceous Rakopi Formation coal measures and coaly units within the mainly marine latest Cretaceous North Cape Formation. The Rakopi Formation is known to be the source of oil in the Maui Field. Marine mudstones are also likely to have source potential. Reservoir rocks for these source rocks are likely to be sandstones within the coal measure sequences, as well as younger turbidite and shoreface sands.

Finally, the Waipawa Formation black shale is known to be present and widely distributed across the Northland offshore region as it was intersected by Wakanui-1 and was a source for oil in the Kora discovery. This source rock is known to be mature within the North Taranaki Graben where reservoir targets are likely to be Miocene and Pliocene turbidite sands.

Six units with source potential are therefore known to be present: the Middle Jurassic Wakanui-1 coal measures, the Late Jurassic Huriwai Beds, the Early Cretaceous Taniwha Formation coal measures, the Late Cretaceous Rakopi Formation coal measures which charged the Maui Field with oil, the latest Cretaceous North Cape Formation coal measures and the Paleocene Waipawa Formation black shale. In addition, marine source rocks of Jurassic and Cretaceous ages may be present, although their existence is purely speculative at present.

Conclusions

1. The Northland Basin is part of the Greater Taranaki Basin with many features in common with surrounding areas.
   
2. Wakanui-1 was not a valid test of petroleum prospectivity. Apart from the Paleocene sand, no reservoir facies were discovered, the structure relied on three seals which, assuming they were all unbreached, isolated the only known reservoir from migrating petroleum.
   
3. Six petroleum systems are known to be present in the Northland region and two are known to be active. Additionally, Jurassic and Cretaceous marine mudstones may be contributing to petroleum charge.
   
4. Structures and reservoir rocks are present throughout the region.
   
5. In the north, large anticlines were formed in the Early Miocene when the allochthon was emplaced, and were present to accept charge from migration stimulated by the addition of the thrust sheets to overburden.
   
6. The central depocentre contains rocks of Jurassic and Cretaceous age with several large rift structures. Drape across these rift blocks created traps that were in place prior too expulsion of petroleum from the coaly source rocks.
   
7. Within the North Taranaki Graben, the black marine shales of the Paleocene Waipawa Formation are known to be mature and to have expelled oil. Neogene turbidites provide excellent quality reservoirs, and traps are provided by the numerous fault blocks and by drape across Miocene volcanoes, such as at Karewa.

References

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