By New Zealand Correspondent Neil Ritchie
SEVERAL speakers at the Advantage NZ: Geotechnical Petroleum Forum 2014, held in Wellington in early April, talked of the yet-to-be-realised potential of the semi-mature Taranaki basin and several frontier regions.
OMV New Zealand new ventures team leader Chris Uruski and New Zealand Oil and Gas geoscience advisor Mac Beggs both said there was still scope for some commercially significant oil and gas discoveries to be made in Taranaki and perhaps even more finds elsewhere around the country.
Keith Myers, geology and inversion manager with WesternGeco in Asia-Pacific, said new seismic technology was revealing fresh insights into the prospectivity of the Taranaki basin.
Mr Beggs said New Zealand plays had tended to “peter out” but sometimes come back to life decades later, citing the Mangahewa Formation and shallower Mount Messenger Formation as examples. New plays had proliferated in the Taranaki basin, but this had yet to be successfully transferred to any new basins. There were also insufficient samples of most plays, and incomplete and inconsistent data.
However, GNS Science research indicates both proven and unproven kitchens exist within the Taranaki basin, with kitchens in the northern and western Taranaki having the fundamentals for some significant undiscovered new plays.
Most Taranaki plays had relatively short runs but there was remaining scope around indicated kitchens. And some southern basins had similar geology to that of Taranaki, Mr Beggs added.
Uruski said analogies drawn from onshore and near-shore Taranaki to help understand other basins might be misleading.
Seismic data from deep water, particularly from offshore Northwest New Zealand, but also from the Great South and Canterbury basins of the east coast of South Island and the East Coast basin of North Island, showed these regions had a very different geological history from that of Taranaki.
Classically, the petroleum geology of New Zealand’s sedimentary basins was considered to start with the deposition of Late Cretaceous coal-bearing successions. Economic basement therefore included all older rocks.
“However, the results of drilling off northern Taranaki, particularly the Waka Nui-1, Pukearuhe-1 and Pluto-1 wells suggest this is not the case everywhere.”
Waka Nui-1 drilled Jurassic coal-bearing units, which on seismic data correlated across approximately 20,000 square kilometres of the offshore Northland Basin. These potential petroleum source rocks were correlated with Mid-Jurassic units of the Murihiku Supergroup, previously considered to be economic basement.
“Subsequent work around New Zealand indicates other units once thought of as economic basement may also be components of petroleum systems.”
In the Deepwater Taranaki basin, Rakopi Formation coal-bearing sediments form the top-set units of a large, essentially Late Cretaceous, delta that overlies thick rift sediments that might include Jurassic and Early Cretaceous units.
Seismic data from the Great South basin showed a well-bedded succession, previously considered to be economic basement, which lies along strike from Permian to Jurassic rocks of the Murihiku Supergroup of the Southland Syncline. Offshore, these rocks might have both source and some reservoir potential.
Canterbury basin rocks also thought previously to be basement had a well-bedded seismic character. Examples of Jurassic coal-bearing successions such as the Clent Hills Formation were present in onshore Canterbury.
In the Pegasus basin, sediments deposited across the top of the Hikurangi Plateau appeared to be contemporaneous with sediments that had been accreted to the margin in the Early Cretaceous indicating one mechanism whereby rocks of the same age and depositional system might have different histories.
“In short, one man’s “basement” may be another’s petroleum system,” Mr Uruski said.
He later told Oil & Gas Australia the thrust of his presentation “was really to say that exploration of our frontier basins should be easier if we have more possible petroleum systems to look for”.
Meanwhile, Mr Myers said initial analysis of the first multi-client survey conducted by WesternGeco in New Zealand waters during early 2013 – a major 3D survey over much of the North Taranaki Graben – was already providing interesting and encouraging results.
The survey, covering an area of about 4350 square kilometres, was the first high-resolution multi-client done in this country using broadband acquisition and post stack time migration and imaging techniques.
Comparison between older 2D data and the latest fast-track 3D broadband data reduced structural uncertainty in the presence of complex geology and identified new leads.
“The data will provide a new level of understanding in the area,” Mr Myers said.
This data is due to be released to clients as part of the 2014 Petroleum Blocks Offer.
The area is highly immature in terms of exploration drilling; however, sub-commercial hydrocarbon discoveries and shows indicate “significant petroleum potential”.
The Mangaa Formation was laterally extensive, with potential in the Miocene-early Pliocene sands, and some Eocene potential. In fact the Miocene was “laterally very extensive throughout much of the graben”, Mr Myers added.
“The Miocene has the highest potential for charge … and the Pohokura-3 structure, this cannot be anything other than gas charged,” Mr Myers added, referring to the appraisal well drilled during the early 2000s by then operator Fletcher Challenger Energy and partners.
He said there were “a number of themes” that required further investigation, including full 3D basin modelling and interpretation.
“There is a lot more work to do and a lot of potential (to be realised), Myers concluded.
GNS Science basin modeler Karsten Kroeger told delegates that there was also a lot potential in the more southern Kapuni (onshore)-to-Kupe (offshore) southeastern part of the Taranaki basin.
Modelling of the Kupe region suggested the Farewell Formation was currently expelling petroleum and that progressively shallower part of the Farewell had only become mature during the past few million years. The up-dip movement of expelling Farewell kitchens had likely impacted migration pathways and charge of traps.
Mr Kroeger said the modelling provided confidence for investigating unexplored leads in the southeastern part of the Taranaki basin.
Geologist Dave Francis talked about the prospectivity of the Whangai and Waipawa Formations that are widespread on the East Coast where several explorers, notable Canadian listed duo of TAG Oil and New Zealand Energy Corporation are actively exploring for unconventional shale oil and gas.
Late Paleocene Waipawa Black Shale and the Late Cretaceous to mid-Paleocene Whangai Formation were known to be the source of numerous seeps and stains throughout the onshore East Coast and in the Marlborough region of the South Island.
Maturity of these formations was demonstrated by the oils themselves, although many surface outcrops and a few well intersections show them to be slightly immature. There was also evidence for thermogenic gas generation occurring throughout the Miocene.
Francis said recent reservoir studies indicated the East Coast formations had porosities comparable to or higher than those of North American shale formations.
Other speakers talked about the potential of New Zealand’s gas hydrates in such offshore frontier regions as the Hikurangi Margin.
Finally, SCS director Jonathan Salo concluded by saying “today’s unconventional resources are tomorrow’s conventional reserves” as is already happening in North America.