New Zealand Potential - 2: Numerous play types evident in Taranaki basin

July 23, 2001
After almost 50 years of utilizing "modern" techniques to search for hydrocarbons in New Zealand's Taranaki basin, explorationists are only now beginning to appreciate its multiplicity of plays.

After almost 50 years of utilizing "modern" techniques to search for hydrocarbons in New Zealand's Taranaki basin, explorationists are only now beginning to appreciate its multiplicity of plays.

Horst, tilted fault block, and thrust anticlines have been the traditional targets, but companies are showing increased interest in relatively more difficult plays that involve turbidite, volcaniclastic, and diagenetic reservoirs.

Penetrated by fewer than 50 offshore wildcat wells, the Taranaki basin remains lightly explored. The central part of the Northern Taranaki graben has never been drilled and no lithology older than Eocene tested.

Explorers are unlocking the Taranaki basin's complex tectonic and sedimentary history, as summarized in the first part of this two-part article (OGJ, July 16, 2001, p. 38). Cretaceous-Paleogene extensional tectonism has been overprinted by Neogene compressional events along the eastern and southern margins of the basin, effectively positioning repeated reservoir sections over deeply buried source rocks.

In the 1950-60s, poor seismic imaging of these relationships due to thrusted "basement" metasediments contributed to drilling failures along the Tarata reverse fault zone. However, current exploration programs are employing sophisticated geophysical acquisition and processing techniques and drilling practices to lower risk in this tectonic regime.

Acoustic scattering, diffraction, and absorption within a chain of buried Miocene volcanic edifices (50-1,500 m in thickness) in the Northern Taranaki graben inhibit seismic energy from descending into the older sequences.

Modern reprocessing of existing seismic data is assisting exploration efforts with these volcaniclastic rocks. Deepwater exploration is heating up with spec seismic acquisition programs planned later this year in the western platform. In addition, new players in the basin are applying geological methodology learned in a global environment to achieve elevated success rates.

Taranaki basin

The Taranaki basin is located along the central west coast of New Zealand's North Island.

Taranaki covers 100,000 sq km, mostly offshore, including all of New Zealand's producing fields. Depocenters containing as much as 9 km of Cretaceous-Cenozoic sediments occur in the region. The basin possesses a north-northeast structural trend and is locally overlain by Quaternary volcanic rocks of the onshore Taranaki Peninsula.

Geological setting

Numerous superimposed sub-basins, depocenters, areas of uplift, interbedded volcanic edifices and recent volcanism contribute to the Taranaki basin's complicated morphology.

The initial structural phase of the basin occurred in the Late Cretaceous. Post-rift sagging resulted in compaction and drape of the section over pre-existing structure through Paleogene time. By early Neogene the basin had infilled with sediment and lost expression as a rift. Miocene volcanic activity along the Cape Egmont fault zone provided localized uplift and heating events in the central and northern parts of the basin.

Compressional events associated with the formation of the Taranaki thrust fault gave rise to deformation resulting in west-directed thrust anticlines, generally confined to the eastern and southern parts of the basin.

The stratigraphy of the Taranaki basin consists of a Cretaceous-Cenozoic succession of terrestrial and marine sedimentary and volcanic rocks overlying basement composed of Paleozoic-Mesozoic granites, basalts, andesites, and metasediments. King and Thrasher divided the Cretaceous-Cenozoic section into four megasequences that are based on seismic mapping, which are in part lithostratigraphic, chronostratigraphic and tectonostratigraphic:

  1. Late Cretaceous synrift sequence (Pakawau Group);
  2. Paleocene-Eocene late-rift and post-rift transgressive sequence (Kapuni and Moa groups);
  3. Oligocene-Miocene foredeep and distal sediment-starved shelf and slope sequence (Ngatoro Group) and Miocene regressive sequence (Wai-iti Group); and
  4. Plio-Pleistocene regressive sequence (Rotokare Group).

Hydrocarbon elements

The Taranaki basin contains a complicated system of petroleum elements due to the complex stratigraphic and tectonic history of the region.

Source-generation-migration

All hydrocarbons currently produced in the Taranaki basin are derived from terrestrial source rocks with shallow marine influences of Cretaceous-Eocene age. Additionally, oils recovered from the Tangaroa-1 and Kora wells in the Northern Taranaki graben were derived from a Paleocene organic-rich marine mudstone.

In the main, Late Cretaceous source rocks entered the peak oil generative phase during the Eocene-Miocene; the stratigraphic section currently buried deeper than 5,000 m is generally considered to be in the mature to overmature generative phase.

Paleogene source rocks probably reached maturity in the Late Miocene-Early Pliocene after formation of the Miocene inversion structures. However, individual sub-basins in the Taranaki basin experienced different hydrocarbon generation and migration histories due complex Neogene tectonism. Gas chimneys and shallow gas anomalies indicate periodic movement of deep basinal faults thought to provide the primary conduits for hydrocarbon migration from source kitchens to younger reservoirs.

Reservoirs

Hydrocarbons have been encountered in a commercial scale in every chronostratigraphic level in the Taranaki basin except the Cretaceous. The majority of petroleum resources found to date in the basin are contained in sandstone and subsidiary carbonate reservoirs of Eocene to Miocene age.

Coastal and tidally influenced estuarine sandstones of the Eocene Kaimiro, Mangahewa, and McKee formations contain much of the basin's petroleum resources in Maui, Kapuni, McKee, Mangahewa, Stratford, and Pohokura fields. An extensive system of Eocene basin floor turbidite fan deposits has been penetrated in the Northern Taranaki graben.

Oligocene Otaraoa formation reservoirs include glauconitic sandstones of the Matapo member and shelf turbidites of the Tariki member that are the primary reservoirs in the Tariki and Ahuroa fields and in the recent Rimu discovery. In addition, fractured carbonates of the Tikorangi formation form prolific reservoirs in Waihapa/Ngaere field.

The regressive sedimentary system that dominated the Neogene formed at least two turbidite reservoir systems, the Moki and Mount Messenger formations. The former contains hydrocarbons in the Maari accumulation, and the latter comprises the main reservoirs in the Kaimiro and Ngatoro fields and the recent Goldie-1 and Windsor-1 discoveries.

Play concepts

The Taranaki basin possesses many play types, mostly structural in nature. Although the majority of wildcats targeted anticlinal or four-way dip closures, the mechanisms that produced these features may be radically different due to the basin's complex tectonic history. A Miocene compressional phase following Late Cretaceous rifting produced most of the structures that currently trap hydrocarbons. Other play concepts include half-graben fill, submarine fan systems, buried volcanic edifices, and diagenetic traps.

Thrust features

Traps along the Tarata reverse fault zone are composed of relatively thin sheets of detached allochthonous material thrust in a westward direction. McKee, Tariki, Ahuroa, Waihapa, and Ngaere fields and the newly found Rimu accumulation may be assigned to this play category.

Thrust traps have been the primary target of several recent exploration programs in the eastern Taranaki basin, including wells Rimu-1, Huinga-1, and Tuihu-1. Additional wells in the thrust zone are planned this year at Kauri-1 and Kahili-1.

Inversion structures

Click here to enlarge image

The largest fields delimited thus far in the Taranaki basin may be classified as inversion structures (Fig. 4).

Crustal shortening during the Miocene uplifted and inverted sub-basins in the eastern and southern parts of the Taranaki basin, causing reactivation and reversal of movement along many of the basin's extensional faults. The resulting features are generally asymmetric with more steeply dipping beds to the west. The discoveries at Kapuni, Maui, Mangahewa, Kupe, Maari, and Pohokura fall into this category.

Due to the initial success in drilling this type of trap, most of the obvious inversion structures in the basin have been penetrated during the past five decades. Future exploration of this play concept will involve step-out wells on these features to delimit any traps with stratigraphic or diagenetic components.

Extensional structures

A post-compressional relaxation established a second extensional phase in the Taranaki basin. Some normal faults that formed during Cretaceous-Paleocene rifting and experienced reverse movement in the Miocene were again subjected to a normal sense of travel in the Pliocene. Miocene uplift and subsequent Pliocene extensional tilting and block faulting created the traps at Kaimiro and Ngatoro fields and the recent Goldie-1 and Windsor discoveries.

Click here to enlarge image

The inversion phase of regional tectonism had little effect in the Northern Taranaki graben where structures have been subjected primarily to extensional stresses since Late Cretaceous (Fig. 5). Many basement-cored horst and tilted fault blocks adjacent or in the sourcing depocenters have not been penetrated by the drill bit in this region. Compaction and drape of younger sediments over these highs may play an important role in trapping hydrocarbons in this region.

Volcanic edifices

During the Miocene, a belt of submarine stratovolcanoes developed along the axis of the Northern Taranaki graben in a northerly direction from the Taranaki Peninsula.

Click here to enlarge image

Occupying about 20% of the area of the graben, the edifices are primarily andesitic in composition and consist of volcanic and lapilli tuffs and volcanic slump breccias. Pore geometry is complex and highly variable, and reservoir distribution is vertically and laterally discontinuous. Many stratovolcanoes are overlain by 1.5-2 km of Pliocene sediments and have been displaced by later normal faulting (Fig. 6).

Previously avoided by most exploration programs, the volcaniclastics achieved notoriety in 1988 when Arco tested 668 b/d of 35° gravity oil from Kora-1, a well penetrating the flank of the Kora stratovolcano enroute to a deeper Eocene target.

Although not deemed economic at the time, Kora-1 demonstrated reservoir porosities and permeabilities as high 30% and 300 md, respectively, in the volcaniclastics. Lack of adequate top seal was thought to be the primary reason for the sub-economic accumulation.

The stratovolcanoes along the axis of the Northern Taranaki graben are more deeply buried by Pliocene sediments, with possibly improved sealing capabilities, than the Kora edifice. In addition, the intrusive nature of the volcanic pipes that supplied magma to the stratovolcanoes has uplifted the underlying Eocene and Early Miocene reservoirs into large domes. Both of these parameters enhance the trapping capabilities in and beneath the volcanic edifices.

Half-graben fill

Many half-grabens developed in the Taranaki basin during the Cretaceous-Paleocene rifting episode.

Click here to enlarge image

Although formed during extensional tectonism, half-graben traps generally differ from rotated fault blocks in the basin as they contain more unconformities and stratigraphic elements. Uplift and rotation of basement-cored blocks with synrift sedimentation and episodic erosional and nondepositional events produced wedges of Cretaceous fluvial and marginal marine lithologies on the dip slopes (Fig. 7).

Although petroleum accumulations have not yet been found in Cretaceous rocks, the proximity of source to reservoir in these sequences is promising.

Submarine fans

As demonstrated on a global scale for the past three decades, submarine fan reservoirs are significant contributors to petroleum production in many basins.

Neogene shelf and basin mass-emplaced sandstones are seen in outcrop along the Taranaki coastline and have been penetrated by numerous wells in the Taranaki basin. Miocene turbidites are the primary reservoirs in Kaimiro and Ngatoro fields, Windsor-1, and the new pool discovery at Goldie-1. Past exploration has concentrated on drilling anticlinal or fault block traps, and thoughts concerning the stratigraphic trapping capabilities of the Miocene turbidite sandstones were ancillary.

Eocene turbidite systems have been penetrated by several wells in the Northern Taranaki graben. Kora-1 recovered oil on test from the Tangaroa formation, a relatively large fan complex north of the Kora edifice. Thin turbidite sandstones of the Eocene Mangahewa formation in Turi-1 exhibited good gas shows.

These wells indicate that slope and basinal fan sandstones are valid targets in the Northern Taranaki graben north of the Eocene paleocoast sediments that run on a line through Maui field to the Pohokura discovery. Post-rift deposition of low-seeking Eocene sands is likely along the axis of the graben; however, the depocenter has never been drilled. This play concept provides the most promise of large undiscovered commercial resources in the Taranaki basin.

Diagenetic traps

Some of the most difficult plays to define in the Northern Taranaki graben are associated with diagenetically altered sandstones.

The massive volumes of hot fluids generated by the Miocene eruptive cycle altered reservoir characteristics of Cenozoic rocks proximal to the intrusive centers. In Kora-1, located within 2 km of the igneous pipe complex that supplied magma to the Kora stratovolcano, the Eocene Tangaroa formation exhibits much lower porosity and permeability values than that penetrated by Kora-4 (20% porosity and 150 md permeability) situated approximately 5 km from the pipe.

This relationship demonstrates that a circlet of tight reservoir rock may exist near the igneous pipes that fed the stratovolcanoes, creating possible diagenetic traps for hydrocarbons in the more porous reservoir downdip of the pipes. Unfortunately, determining the location of the transition between seal and reservoir in the same rock unit is difficult in the Northern Taranaki graben due the resolution of available seismic data, overlying volcanic edifices, and lack of well control.

Acknowledgments

This article was initiated and funded by Crown Minerals, Ministry of Economic Development, which administers petroleum exploration in New Zealand. The author is grateful to EEX Corp., Houston, for allowing publication of newly reprocessed seismic data and to the directors, Mac Beggs and Glenn Thrasher, and staff of GeoSphere Exploration Ltd. for critically reading this manuscript.

Bibliography

Crown Minerals, "Explore New Zealand petroleum," Ministry of Economic Development, Wellington, 2000, 48 p.

King, P.R., and Thrasher, G.P., "Cretaceous-Cenozoic geology and petroleum systems of the Taranaki basin, New Zealand," Institute of Geological and Nuclear Sciences Monograph 13, 1996, 244 p.

Stagpoole, V., "A geophysical study of the North Taranaki basin," unpublished PhD thesis, Victoria University of Wellington, 1997, 245 p.

Wood, R.A., Funnell, R.H., King, P.R., Matthews, E.R., Thrasher, G.P., Killops, S.D., and Scadden, P.G., "Evolution of the Taranaki basin-hydrocarbon maturation and migration with time,"in 1998 New Zealand Petroleum Conference Proceedings, Crown Minerals, Ministry of Economic Development, Wellington, 1998, pp. 307-316.