Vaughan Stagpoole, Andy Nicol, Rob Funnell, Richard Cook
For further information contact Vaughan Stagpoole
This research aims to better understand the geological history of the eastern margin of the Taranaki Basin, and to reduce the risk for petroleum exploration companies working in the area. The 600km-long Taranaki Fault dominates the eastern margin of the sedimentary basin. It trends roughly north-south between the Marlborough Sounds and offshore west of Auckland , but is inactive and not exposed at the surface. Nevertheless the Taranaki Fault is a large structural feature that forms one of New Zealand's major geological boundaries.
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Deformed Miocene strata near exposed basement on the eastern margin - north Taranaki. The fault is about 5 km offshore from this point
Several producing oil fields are associated with the fault. However, the area is geologically complex and hydrocarbon exploration in the area is fraught with difficulties.
Find out more about :
This year our Hydrocarbons Group presented the latest findings from their public-good research on this part of the Taranaki Basin to a well-attended industry workshop. GNS research on the Taranaki fault includes
A CD compilation of presentations, papers and reports is available from GNS
contact GNS hydrocarbons here
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 The Taranaki Fault
The Taranaki Fault is a crustal scale thrust of at least 600 km in length that lies at the eastern margin of the Taranaki Basin . The fault forms the pre-Miocene edge of the basin. Younger sediments form an unbroken cover over the fault so that it is not exposed at the surface. The Taranaki Fault is part of a larger fault system, including the Waimea-Flaxmore and Manaia faults and the Tarata Thrust, which extends from the Alpine Fault in the South Island up through western North Island .
Early petroleum exploration using gravity data indicated that the eastern margin of the Taranaki Basin was bounded by a large fault, although it was not until the acquisition of seisimc reflection data in the 1970s that the immense size and nature of the fault was recognised. Commercial oil reserves have been found in structures associated with the fault mainly under the Taranaki Peninsula.
Interpretation of seismic-reflection data indicates that the dip of the principal fault surface ranges from 25 to 45° and increases southwards. The principal fault is corrugated on length scales of 10's to 100's of kilometres, and is accompanied by multiple slip surfaces which often splay from the main fault within 2-5 km of the upper tip. Splays can be entirely within Tertiary or basement rocks, or may produce inter-fingering of basement and Cretaceous-Tertiary strata. These splays are discontinuous and extend for no more than 10-50 km along strike.
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Fault Movement history
Analysis of displacement data indicates at least two periods of accelerated shortening and displacement on the fault, during the Mid-Late Eocene (ca. 43-35 Ma) and the Early Miocene (ca. 23-18 Ma). These periods of more rapid deformation are interspersed with intervals during which contraction was either slow or absent. Eocene and Miocene displacements vary along the length of the fault with a greater proportion Miocene displacement in the south. This distribution of displacement reflects a southward migration of fault activity which is mirrored by an apparent southward migration of basin depocenters adjacent to the fault.
Cenozoic displacement of the Taranaki Fault with periods of extension are in yellow and periods of shortening are in orange. Crosses indicate the time that thrusting stopped (interpreted from seismic reflection data). Rapid deformation occurred during the Mid-Late Eocene (ca. 43-35 Ma) and, in the north, the Early Miocene (ca. 23-18 Ma). On the Taranaki Peninsula and further south, intense shortening continued into the Pliocene.
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Stratigraphy of the basement overthrust
GNS palaeontologists have been reviewing data from wells that penetrate the basement over-thrust. There findings are in available in a GNS Science report 2004/12 by Ian Raine, Richard Cook and Percy Strong.
Compilation of the age of strata above, below and incorporated between thrusts of the Taranaki Fault.
(Link to PDF of report - 562 kb)
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Taranaki Fault tectonostratigraphic model
Two cross-sections depicting the development the eastern margin of the Taranaki Basin and the history of faulting are shown here. The northern line is near Kawhia and the southern line is south of the Taranaki peninsula.
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Northern Line
In the Cretaceous a basin had formed along what is now the eastern margin of the Taranaki Basin . A reverse fault with about 3000m of throw is depicted here as forming the eastern edge of the basin, although this is uncertain. Regional subsidence occurred during the latest Cretaceous and Paleocene. Uplift along the eastern margin of the Taranaki basin occurred in the Late Eocene.
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Intermittent movement on the fault occurred in the Oligocene and Early Miocene while the basin continued to subside. By Middle Miocene time thrusting had stopped and sediments were deposited over the hanging wall of the fault.
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During the Late Miocene extension and andesitic volcanic activity occurred within the basin. In the last 5 million years uplift centred east of Taranaki resulted in erosion of sediments on the hanging wall and exposure of basement rock.
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In the Cretaceous a basin had formed along what is now the eastern margin of the Taranaki Basin . Regional subsidence occurred during Paleocene, with uplift beginning on the Taranaki Fault in the Late Eocene.
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Intermittent movement on the fault occurred in the Oligocene and Early Miocene while the basin continued to subside. A hiatus in movement occurred in Late Miocene time and sediments were deposited over the hanging wall of the fault.
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During the Late Miocene renewed compression occurred resulting in erosion of sediments on the hanging wall and exposure of basement rock. Subsidence in the last 5 million years has covered the fault with between 1000 and 2000 m of sediment.
Petroleum generation and expulsion close to the fault
Petroleum generation modelling has helped to constrain the charge history for petroleum accumulations associated with the Taranaki Fault. Models, based on seismic mapping of the basin, provide a predictive history of petroleum generation and expulsion in the region of the Taranaki Fault.
Models indicate oil expulsion first commences from deeply buried Cretaceous source rocks in the north of the basin during the Early Eocene and continues through to the present day. Beneath the Taranaki Peninsula , Cretaceous source rocks have now become over-mature for oil generation but continue to expel gas through to the present day. Paleogene source rocks are predicted to have begun expelling oil in the Early Miocene with increasing rates of generation and expulsion through to the present day.
Thus the present day focus for oil generation and expulsion is in the vicinity of the peninsula for Paleogene source rocks and to the north and south for Cretaceous source rocks.
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Predicted total oil expelled in MMbbl/km 2 (left) and gas expelled in BCF/km 2 (right) up to the present day from Cretaceous formations along the eastern margin of the Taranaki Basin (grey areas are on the hanging wall of fault that are not modelled). Plots presented using Zetaware Trinity software.
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Predicted total oil expelled in MMbbl/km 2 (left) and gas expelled in BCF/km 2 (right) up to the present day from Paleogene formations along the eastern margin of the Taranaki Basin (grey areas are on the hanging wall of fault that are not modelled). Plots presented using Zetaware Trinity software.
A fundamental aspect of the petroleum systems associated with the Taranaki Fault is the relative timing of deformation leading to trap formation and petroleum expulsion from source kitchens. Generally fault movement pre-dates the time of maximum oil generation and expulsion, thus the timing of trap formation is favourable for petroleum charge.
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| generation intensity (green) for Cretaceous source rocks showing the timing of oil expulsion intensity (green) and timing of deformation (light orange) along the eastern margin of the Taranaki Basin . The upper plot is a location map for source kitchens modelled (shown in green).
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| Oil generation intensity (green) for Paleogene source rocks showing the timing of oil expulsion intensity (green) and timing of deformation (light orange) along the eastern margin of the Taranaki Basin . The upper plot is a location map for source kitchens modelled (shown in green).
Present day migration directions close to the fault
Present day migration directions, related to the dip of strata close to the fault. Migration directions are generally away from the fault on the peninsula and towards the fault to the north and south of the peninsula. Hence, traps such as culminations associated with the Tarata Thrust on the peninsula and to the west of the fault are on potential migration paths and likely to receive charge. Similarly, those associated with the principal slip surface of the fault to the north and south (including the area around the Rimu-1 well) are also favourably located for charge.
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Schematic diagrams of strata close to the fault showing potential petroleum migration paths (green arrows) towards the fault north and south of the peninsula (left) and away from the fault on the peninsula (right).
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Links to useful papers and reports on the Taranaki Fault
Some recent workshop and conference papers on the Eastern margin of the Taranaki Basin and the Taranaki Fault:
Constraints on the evolution of the Taranaki Fault from thermochronology and basin analysis: Implications for Taranaki Fault play (pdf - 8Mb) by Kamp and others at the 2004 NZ Petroleum conference.
Structure and petroleum potential of the Taranaki Fault play (pdf 5.2Mb) by Nicol and others at the 2004 NZ Petroleum conference.
Structure of the Taranaki Fault from the interpretation of magnetotelluric observations (pdf - 983 kb) by Stagpoole and others at the 2004 NZ Petroleum conference.
Wide angle seismic surveys - the solution for imaging beneath the Taranaki Fault (pdf 2.3Mb) by Pecher and others at the 2004 NZ Petroleum conference.
Improved imaging of the Taranaki Fault reducing drilling risk (pdf - 613kb) by Maslen and others at at the 2002 NZ Petroleum conference.
Eastern Boundary fault paleontology and timing of thrusting (pdf - 562kb) by Raine and others, GNS Science report 2004/12.
Overview of the structure and associated petroleum prospectivity of the Taranaki Fault (pdf - 2.2Mb) by Stagpoole and others at the Eastern Australasian Basins Symposium II. (pdf - 2.2mb)
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