Cretaceous - Cenozoic geology and petroleum systems of the Great South Basin, New Zealand (CD)

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Cook, R.A.; Sutherland, R.; Zhu, H. (et al.) 1999 Cretaceous - Cenozoic geology and petroleum systems of the Great South Basin, New Zealand. Lower Hutt: Institute of Geological & Nuclear Sciences. Institute of Geological & Nuclear Sciences monograph 20 190 p.

Abstract: The Great South Basin lies offshore adjacent to the southeast coast of South Island, New Zealand. It covers an area of about 100 000km/sup2/, and has water depth ranging from 100 to 1250 m. The basin was delineated by more then 30 000 km of seismic reflection data acquired between 1968 and 1983, with geological control from eight wells drilled between 1976 and 1984. The adjoining Canterbury Basin and Bounty Through formed contemporaneously during a phase of regional mid-Cretaceous extension that eventually lifted New Zealand away from Gondwanaland. Extensive normal faulting formed a complex horst and graben architecture that divides the Great South Basin into several structural highs and sub-basins fill with up to 8.6 km of Cretaceous and Cenozoic strata. Basement is defined as the base of mid-Cretaceous rift-related sediments. A compilation of magnetic data, combined with a limited number of basement samples, was used to map the distribution of underlying terranes and intrusions, which trend northwest across the basin. Along the northwest margin, Permian-Jurassic Murihiku Supergroup strata are tentatively mapped across the boundaries of the Brook Street Terrane, Median Batholith, and Western Province. This implies that the Murihiku Terrane is better described as the Murihiku Basin, and that basement terrains southwest of the Maitai Terrane were probably amalgamated by Permian-Triassic time. A revised Cretaceous-Cenozoic stratigraphic framework is defined by regional unconformities and correlative conformities, with each group boundary corresponding to a significant change in tectonic or/and sedimentary setting. Formations and members are defined on lithostratigraphic criteria. Eight mapped seismic reflectors, which correspond to significant stratigraphic boundaries, are tied to biostratigraphic and lithostratigraphic data from wells. The Cretaceous syn-rift basin fill as assigned to the Hoiho Group, which is inferred to range in age from about 105 to 83 Ma (Motuan-Piripauan). Local topography, dominated by the elevated northwestern margin of the basin and the Pakaha Horst, was controlled by active normal faults. Slope-fan and slump deposits formed adjacent to active faults, while alluvial process formed broad plains in basinal areas. The deepest and most extended part of the basin, particularly the Central sub-basin, accumulated well stratified lacustrine or restricted marine deposits fringed by deltas and heavily vegetated plains and swamps. Regional subsidence and exclusive marine flooding of the basin from about 83 Ma (Piripauan) resulted in deposition of the Kawau Sandstone, which defines the base of the Wickliffe Formation and Pakaha Group. Quartzose clastic terrestrial sediments of the Taratu Formation were deposited along the northwest margin, while marine siltstones of the Wickliffe Formation were deposited in deeper parts of the basin. Shallow-marine sandstones and organic-rich lower coastal plain sediments were depo sited near the steadily transgressing shoreline. The upper part of the Wickliffe Formation contains late Paleocene organic-rich dark shales of the Tartan Formation. Minor normal faulting continued during deposition of lower Pakaha Group strata, and widespread drape structures formed as Hoiho Group strata were differently compacted. Unconformities developed above basement highs, with progressive onlap by overlaying strata. Drape structures were the hydrocarbon exploration targets drilled by the Pakaha-1, Pukaki-1, Hoiho-1, Rakiura-1, and Kawau-1 wells. The Eocene Rakiura Group is marked by an increase in carbonate contents of sediments. The lower part of the Rakiura Group is characterised by a thick submarine fan sequence. A new extensional plate boundary propagated into Southern New Zealand during Middle Eocene time, causing significant faulting and changes in sediment source area farther west, but there is little evidence for faulting within the basin. The Oligocene-Quaternary was a time of progressively increasing tectonic tempo in New Zealand. The base of the Oligocene to Recent Penrod Group is marked by a significant change to more carbon-rich sediment in southeastern parts of the basin and an unconformity along the northwestern margin. The change to transpressional tectonics in latest Oligocene or Miocene time was associated with regression, increasing clastic sediment supply by strong ocean currents. Penrod Group sediment is generally less then 500 m over most of the Great South Basin, but a combination of shelf processes, current action, and rapidly changing tectonics has produced complex internal stratigraphy. Deformation during Penrod Group deposition was restricted to the northwest margin of the basin. The Takapu-1, Toroa-1 and Tara-1 were drilled on these structures. Intraplate basaltic volcanism has been intermittent since Late Eocene time. Hydrocarbon indications have been found in four of the eight wells drilled in the basin, and oil/condensate seeps are associated with surface fault traces on Stewart Island. In Kawau-1, Kawau sandstone tested predominantly gas up to 6.8 MMCFPD and the overall reserve is calculated at 461 BCF gas. Source rock include Cretaceous and Paleocene coals, which are widespread and exhibit good potential for mixed gas and oil. Biomarker and isotopic analyses of the Kawau-1 condensate, oil shows from Cretaceous rocks, and the Stewart Island oil seep, suggest that the hydrocarbon sources were mid-Cretaceous coaly sediments, but a significant marine influence is also commonly indicated. Late Cretaceous and Paleocene marine mudstones are demonstrated to be good potential source rocks. The late Paleocene Tartan Formation marine mudstone has measured TOC values as high as 9%, and has exceptionally good source potential for oil and gas. Kinetic modelling indicates that widespread generation and expulsion has occurred from Cretaceous coals, with more restricted generation and possible expulsion from Cretaceous-Paleocene marine formations. Present day maturation levels in Tara-1, where coal is common, indicate that significant generation begins at around 2500-2800 m and expulsion at 3500-3800 m depth. Oil seeps on Stewart Island indicate horizontal migration of 50-75 km. Sandstone reservoir rocks were deposited in non-marine lower coastal plain and fluvial environments, and marginal marine to mid-shelf environments. Deep water marine channels and basin-floor fans recognised on seismic profiles are potential reservoirs. Seal rocks are mudstones and calcareous siltstones, with are widespread in the Pakaha and Rakiura groups, and locally present in the Hoiho Group. The basin is highly prospective because the deposition of adequate source, reservoir, and seal rocks, and the formation of primary structures occurred early in the basin's history. All the necessary components of a viable petroleum system were in place before the critical moment, which was within the interval 70-45 Ma. Later deformation of the basin has been slight and may have caused secondary migration of hydrocarbons along the northwest margin, but it is likely that a high proportion of hydrocarbons produced in the basin have been trapped. The basin is under-explored and highly prospective.(auth/JED)

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