Petrographic summaries of Taranaki petroleum reports

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Smale, D. 1996 Petrographic summaries of Taranaki petroleum reports . Lower Hutt: Institute of Geological & Nuclear Sciences Institute of Geological & Nuclear Sciences science report 96/01 88 p.

Abstract: The body of this report consists of summaries of information relating to Taranaki Basin provenance and diagenesis from oil company reports. Brief introductory reviews of provenance and diagenesis are also given. Basement rocks intersected in the Western Platform are largely Separation Point granites. No wells in the Taranaki Graben reached basement. East of the Taranaki Fault Zone are indurated Murihiku, torlesse, and Waipapa sediments. Granites provided most of the sediment during the late Cretaceous and Eocene, but Brook Street Volcanics were detected in eastern wells in the late Cretaceous, and low-grade metamorphic like those of north-west Nelson in sediments adjacent to the South Island. Influence of granites waned in the later Tertiary as the depositional centre moved further east, with growing influence from north-west Nelson, and conspicuous contribution from volcanics from the Miocene on in North Taranaki Graben. In the later Neogene and Pleistocene influence of the Mesozoic sediments east of the Taranaki Fault Zone is dominant in adjacent sediments. Diagenetic features observed in Taranaki are those recorded in most known studies. Kaolinite develops with quartz overgrowths and feldspar dissolution; illite, chlorite, and some mixed-layer clays develop at deeper levels. Carbonates come and go, but Fe carbonates may do so at times different from calcite. Carbonate emplacement, occurring at pH of 8 or 9, may accompany quartz dissolution. The most notable attempts to unravel diagenetic sequences have been for the Maui, Kupe South, and Kapuni Fields, and a generalisation on onshore Taranaki wells. Some contrasting aspects are described; the Kupe South sequence differs from the Maui sequence in having quartz overgrowths and laolinite developing at a stage later than a second calcite-deposition phase, instead of between calcite-deposition phases, and may reflect a final more acidic regime; chlorite-smectite develops between calcite-deposition phases, instead of as a final phase as in Maui. The Maui-type sequence may be more common on the western side of the Taranaki Graben, and the Kupe South sequence on the east, but more detailed investigations are needed of individual areas or wells. (auth)