Overview of New Zealand's petroleum systems, potential

Richard Cook
Institute of Geological & Nuclear Sciences
Lower Hutt, N.Z.

Roger Gregg
New Zealand Crown Minerals
Wellington

Hydrocarbons are found in most New Zealand basins, but it is only since the 1960s that hydrocarbon production has been significant in terms of New Zealand's economy and fuel needs. Currently New Zealand's total estimated remaining reserves are some 145 million bbl of oil and 2.4 tcf of gas. Initial reserves were 335 million bbl and 5.14 tcf. A large proportion of these reserves, 93 million bbl and 1.44 tcf, are in offshore Maui field. As these proved reserves continue to deplete, there is a desire to focus on the prospectivity of all the basins.

Petroleum systems

The fundamentals of the New Zealand petroleum geology and prospectivity are currently best described by the use of the petroleum system concept published in 1994 by Magoon and Dow. A petroleum system identifies the active source rocks in a basin and combines them with the essential elements and processes needed for oil and gas accumulations to exist. These elements are the source rock, reservoir rock, seal, and overburden. The processes include trap formation, generation, migration, and accumulation. These are placed in time and space so that the mechanism required to form a petroleum accumulation can be followed. This system covers the key parameters for developing exploration concepts and risks for a basin.

New Zealand has at least 10 sedimentary basins with potential to produce economic levels of hydrocarbons. They have some diversity and in considering their petroleum systems can usefully be grouped into three provinces-western, southern, and eastern. The best known basin in each province, the Taranaki, Great South, and East Coast basins, respectively, provides an understanding of each province's petroleum system and indicates its exploration potential.

Western province

The continental shelf west of New Zealand and its onshore extensions contain the Northland, Taranaki, and the less definitive West Coast and Wanganui basins. For 100 million years, the region has been located along the western margin of the proto-New Zealand landmass, hence the similarity of the fundamentals over all these basins.

Since the 1959 discovery of Kapuni gas field onshore Taranaki, there have been eight further commercial oil and gas fields discovered with a number of stratigraphically diverse reservoirs. Numerically most of these discoveries have occurred in the last decade, providing an ever-changing insight to basin prospectivity and use of exploration paradigms. The discovery of oil in a variety of reservoirs has established the Taranaki basin as a gas-oil province rather than just a gas-condensate province. Exploration momentum has been maintained by the proving of reservoir potential in sequences younger than the traditional play target of the Paleocene-Eocene Kapuni Group.

The potential of the Northland basin has yet to be tested, with the first drilling in the larger, offshore part of the basin not scheduled until early to mid 1998. The West Coast has many oil seeps, and several small oil discoveries have been tested but no commercial reservoir has been found.

Geologic setting

The stratigraphic framework of the Taranaki basin has been described in recent work by the Institute of Geological and Nuclear Sciences (Fig. 4 [34672 bytes]). The sedimentary and tectonic history of the Taranaki basin is complex, but it can be divided into three phases of plate-boundary kinematic development:

1. Late Cretaceous to Paleocene intra-continental rift transform, characterized by nonmarine sedimentation in restricted, fault-controlled basins. These sediments include important coal measure source rocks.

2. Eocene to Early Oligocene passive margin, associated with post-rift thermal contraction and regional subsidence. Basinwide sedimentation patterns were characterized by comparative tectonic quiescence, peneplanation, and marine transgression. Coal measures and well developed sandstones in the marginal marine setting contain proven source and reservoir rocks.

3. Oligocene to Recent pericratonic basin, straddling the outer limit of broad-scale deformation associated with the evolution of the Australia-Pacific convergent plate boundary through New Zealand. Throughout the Neogene, the basin has been part of the Australian Plate, and evolved with two tectonic settings: active margin (Eastern mobile belt) and passive margin (Western stable platform).

While the other western basins have some variation, they all reflect the initial rifting of each basin with terrestrial sediments deposited in mid to late Cretaceous. Through the Paleogene a gradual transgression saw a change in sedimentation to marine, mostly siliciclastic deposits culminating with an Oligocene limestone deposition. In the Miocene there were changes in tectonic pattern; to compression uplift and erosion in the South Island and extensive volcanics to the north. The basins responded to the tectonic change with reversals developing on existing faults, reverse faulting, and thrust and fold structures.

Source

Primary source rocks in the basin are hydrogen rich coals and carbonaceous mudstones of Late Cretaceous-Eocene age that accumulated in thick sequences in synrift/drift sub-basins and on the post-drift passive margin. The coal measures' average hydrocarbon yield exceeds 10 kg/tonne and is often greater than 100 kg/tonne, with hydrogen indices in the 200-400 range.

The Upper Paleocene marine "black shale" in the northern Taranaki basin is widespread through other areas of New Zealand and is easily extrapolated into the Northland basin. This shale could well be a significant source rock for Northland where it is buried deep enough to be mature. With TOCs of 1-2%, hydrogen indices of 500+, and yields up to 30 kg/tonne, these are valuable source rocks.

Maturation

Recent thermal and maturation modeling in Taranaki shows much of the hydrocarbon generation and migration into known reservoirs took place relatively late in the basin's history. The trapping structures are Miocene or Pliocene, and some of the proven reservoir rocks are Late Miocene and Early Pliocene.

Hydrocarbons may have been generated and expelled much earlier in some places, particularly where Late Cretaceous-Eocene successions coincided with high initial heat flow. Many of these early-formed hydrocarbons have been lost from the basin, although some could have remigrated into the younger reservoirs after the Neogene structural reorganization.

It is estimated that hydrocarbon volumes discovered so far are less than 1% of those potentially generated from all source rocks in the eastern part of the basin.

Reservoir rocks

Proven reservoir rocks were deposited throughout much of the basin's history (Fig. 5 [74524 bytes]). Most known petroleum reserves are reservoired in Paleocene-Eocene shoreline and coastal plain sandstones, which were deposited in a late- or post-drift passive margin. Younger reservoirs have varied origins, including fractured Oligocene and earliest Miocene foredeep limestones, Miocene slope and submarine fan sandstones, Miocene volcaniclastics, and Pliocene shelf sandstones. Producible hydrocarbons are not known from Late Cretaceous strata, although these have reservoir potential. Porosity is 15-25% in siliciclastic sediments, and while permeabilities average 50-100 md, Maui field has permeability of several Darcies.

Seals

Cap rocks are generally mudstones deposited during marine incursions and eventual transgression over the coastal plain in the Late Cretaceous and Paleogene. Some inter- and intra-reservoir seals in Paleogene successions are terrestrial mudstones. Neogene seal is mainly deep marine mudstone. With thick marine sequences dominating, sedimentation seals are seals are not considered to be a limiting factor of the system.

Structures

Phases of convergent tectonics and later subsidence were critical to developing petroleum accumulations in the Taranaki basin. The majority of Taranaki fields occurs in the Miocene fold/thrust belt. Deposition of thick Miocene-Pliocene overburden in the eastern foredeep and northern and central grabens was instrumental in triggering or enhancing hydrocarbon maturation after trapping structures had formed. Structural traps have provided most of the known leads, but little exploration has taken place on the stratigraphic variations that developed in the initial basin infill and from the complex tectonics that have prevailed since the Miocene.

Potential

Nearly three times as many fields have been discovered onshore than offshore. Most producing fields are onshore, and although numerically greater they are smaller and account for about 30% of reserves. The size variation is attributed to different trap styles, reservoir levels, and stratigraphic conditions.

The bulk of New Zealand's production and reserves is from offshore Maui field, which had estimated reserves of 173 million bbl of oil/condensate and 3.4 tcf of gas. By January 1996, more than half of this had been produced.

The stacked nature of reservoirs and the complexity of petroleum systems in Taranaki suggests that there is potential for further discoveries at most stratigraphic levels throughout the Western province, particularly in the untested offshore Northland basin.

Next: Overview of New Zealand's southern and eastern provinces.

Adapted and updated from Petroleum Exploration in New Zealand News, November 1996.

The Authors