Petroleum systems of New Zealand
There are eight sedimentary basins with known or
potential hydrocarbons onshore and underlying the continental shelf of New Zealand, as
well as several deepwater basins within its Exclusive Economic Zone. Commercial and
sub-commercial discoveries, abundant potential source rocks, thick Cretaceous-Cenozoic
sediments, and numerous hydrocarbon seeps and shows in exploration wells all indicate that
petroleum generation and migration has and is taking place in many of these basins.
The petroleum systems hosted in several of the country’s basins have many common
characteristics because the stratigraphic distribution of potential source, reservoir, and
seal rocks generally follows a similar pattern. However, the diversity of tectonic
environments in New Zealand has produced differences in maturation and migration history.
Detailed information on the petroleum systems of individual basins is given in Introduction to New Zealand's Petroleum
Systems, a CD-ROM published by Crown Minerals in February 2000 and available on
request at no charge. Contact Crown Minerals for a
copy.
Tectonic development of New Zealand basins
All New Zealand’s prospective sedimentary basins are younger than Early Cretaceous
and most are composite basins, exhibiting multiple evolutionary phases and sediment
deposition. Basin evolution can generally be divided (from oldest to youngest) into rifted
margin, passive margin, and convergent margin episodes that reflect the broad plate
tectonic development of the New Zealand sub-continent.
The break-up of Gondwana from the mid-Cretaceous, and spreading in the Tasman Sea in
the Late Cretaceous and Paleocene, initially led to the formation of rift basins and the
deposition of terrestrial sediments that now form the predominant source rocks in many
basins. The early Paleogene was a period of comparative tectonic quiescence, basin
development was characterised by regional post-rift thermal subsidence, and widespread
marine transgression leading to the deposition of shoreline sands followed by marine silts
and muds. These transgressive shoreline systems contain important productive and potential
reservoir rocks, while overlying silts, muds and carbonates form good seals.
In the Middle and Late Eocene, sea-floor spreading to the southwest of New Zealand
marked the initial stages in the development of the present-day Australian-Pacific plate
boundary in the region. Pacific Plate subduction began to impinge upon northern New
Zealand from the mid-Oligocene. As subduction progressed through the Early Miocene,
regional subsidence was accompanied by reverse faulting along the eastern margin of the
Northland and Taranaki basins. Back-arc extension and local volcanism ensued, and has
continued to the present day.
During the Neogene, the Alpine Fault evolved to become the primary focus of dislocation
between the Pacific and Australian plates in the New Zealand region, with more than 450 km
of dextral strike-slip motion on the fault since the Early Miocene. Since the Middle
Miocene, relative plate motion has become more oblique and the rate of convergence has
accelerated, resulting in increasingly rapid uplift, erosion and sedimentation, and
widespread basin inversions. Deformation associated with this tectonism has produced many
structural traps.
Source rocks
Most of New Zealand’s current hydrocarbon production is sourced from terrestrial
and marginal marine rocks of Late Cretaceous to Eocene in age within both syn-rift and
transgressive coastal plain settings. Biomarkers indicate marine influences in Cretaceous
and Paleocene rocks, and there are marine rocks with source potential in laterally
equivalent basinal settings.
Reservoirs
Potential reservoir rocks are present throughout the stratigraphic record. The most
productive reservoir rocks are in Paleogene transgressive shoreline systems and a variety
of facies belts within Neogene clastic depocentres. Oligocene and Early Miocene limestone
with high fracture permeability is a prolific oil reservoir in the Taranaki Basin.
Hydrocarbons are also produced from Miocene and younger deepwater sandstone turbidites in
Taranaki, and similar Neogene slope and basin floor fans are found in many other New
Zealand basins. Late Cretaceous transgressive shoreline sands are viable but untested
reservoirs. In the East Coast region, Pliocene-Pleistocene coquina limestone has
favourable reservoir characteristics.
Seals
Seals are abundant in all potential petroleum basins and overpressures, indicative of
fluid confinement within the stratigraphic sequence, are often encountered during drilling
operations. Seal rocks in most basins are widespread marine mudstones that were deposited
during both the passive margin transgressive phase and the regressive convergent margin
phase. Oligocene and Early Miocene limestones, that were not fractured during Neogene
tectonism, may also provide seals in many basins.
Overburden
In general, there are two important periods of sediment deposition that resulted in
substantial burial. Late Cretaceous rifting produced grabens several kilometres deep that
rapidly filled with sediments. Hydrocarbon generation began in the deepest of these basins
during the late Cretaceous and early Tertiary.
A second phase of rapid sedimentation occurred in many basins during the Neogene when
the development of the plate boundary through New Zealand caused rapid uplift and erosion.
Very high Neogene sedimentation rates, in excess of 1000 m per million years, have
occurred in some basins. For example, deposition of the Giant Foresets Formation in the
Taranaki Basin increased the overburden thickness by as much as 2000 m over much of the
basin in just 3 million years. The volume of the Giant Foresets Formation is greater than
the present day volume (above sea level) of the New Zealand landmass.
Traps
All commercial hydrocarbon accumulations encountered in New Zealand to date are in
structural traps formed in the Neogene. In the Taranaki and East Coast basins the most
prospective traps include thrust-controlled anticlines, overthrusts, inversion structures
and normal fault-bounded blocks. Other leads include pinch-outs and drape-folds on high
standing basement blocks, which are common in the Northland and Great South basins. In the
East Coast region, where smectitic mudstones are common, daipiric structures may also form
traps.
Generation, migration and accumulations
Thermal modelling studies predict that over most of New Zealand the present day depth
of source rocks for oil expulsion requires burial of more than about 4000 m. Some
hydrocarbons were possibly expelled in the deepest parts of basins as early as the Late
Cretaceous, but in many basins the present day mature kitchen areas are essentially
confined to areas where there has been significant Neogene deposition. In general, deepest
source rocks comprise type III terrestrial sources, and younger overlying rocks are type
II marine sources.
Migration paths from source rocks to reservoirs are poorly understood in New Zealand
basins. In basins where there has been a lot of deformation, there is uncertainty about
the importance of faults as pathways for hydrocarbons.
In most basins, generation, migration and entrapment of hydrocarbons took place in the
Tertiary. The critical moment for the petroleum system of various accumulations, when all
prerequisite geological factors for the charging of traps begins, is predicted to have
been during the Eocene and Oligocene for the Great South, West Coast and Western Southland
basins, and during the Neogene for all other basins.
Prospectivity
All of New Zealand’s production so far has been from the Taranaki Basin, the
country’s most explored and commercially successful hydrocarbon province. However,
the basin is only moderately explored compared with basins world-wide, and there is
considerable scope for further commercial discoveries as demonstrated by recent
exploration successes.
The rest of New Zealand is severely under-explored, and most sedimentary basins have
the potential for commercial hydrocarbon discoveries. Many untested structural closures
are potentially larger than the giant Maui field in the Taranaki Basin.
As exploration and research programmes advance, understanding of New Zealand’s
sedimentary basins and their petroleum systems continues to evolve. The important
geological elements and the timing of generation, migration and accumulation of
hydrocarbons are now broadly understood, enabling a more methodical approach to
identification and appraisal of prospects. Coupled with increasing levels of exploration
over recent years, this has led to an enviable success rate for wildcat drilling and a
recent commercial discovery success rate of one in three in Taranaki Basin. Ongoing
exploration can be expected to lead to further finds here and in other basins.
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