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Subsurface stratigraphy and petrology of a cool-water carbonate fracture reservoir - the mid-tertiary Tikorangi Formation, Taranaki Basin, New Zealand Hood, Steven D. Kamp, Peter J.J. The totally subsurface Oligocene-Early Miocene Tikorangi Formation in Taranaki Basin is New Zealand’s only commercially-producing fractured-carbonate reservoir, but is poorly understood in a stratigraphic-sedimentological-diagenetic context. Based on correlation between laboratory and geophysical well log data a carbonate facies-based lithostratigraphy has been established for the first time for the entire formation. Petrogenesis of this complex siliciclastic-carbonate-dominated sequence, (re)deposited as a mix of retrogradational, progradational, and aggradational sedimentary sequences across a range of shallow-shelfal, foredeep and basinal settings, has been controlled by proximity to a rapidly evolving convergent Australian-Pacific plate boundary, to changes in relative sea level within an overall transgressive regime, to foredeep trough accommodation, to location within a cool-water temperate latitude setting, and to the changing availability and sources of carbonate and siliciclastic supply. The history and nature of burial diagenesis have been instrumental in the creation and subsequent modification of fracture porosity within the Tikorangi Formation by producing a tight, low porosity/permeability rock; burial replacive dolomitisation with no associated secondary porosity development; intense fracturing, thrusting, and folding in association with compression at the Australian-Pacific plate boundary; and precipitation of a complex suite of ferroan calcite, baroque dolomite, celestite, and quartzine minerals within fracture systems. Petrographic, trace element, stable isotope (δ¹⁸O and δ¹³C), and fluid inclusion data record a complex history of changing pore fluid chemistry and heating during burial, punctuated by relative changes in the input of downward circulating meteoric and upwelling basinal fluids. Precursory hydrocarbon-bearing fluids from Eocene terrestrial source rocks have migrated along with aqueous fluids after about 8 Ma, with major hydrocarbon emplacement within the fracture systems since the Early Pliocene. The Tikorangi Formation’s mixed siliciclastic-carbonate nature, heterozoan biotic carbonate make-up, low-Mg calcite mineralogy of skeletons and cements, and delayed deep burial diagenetic history are key features of current cool-water carbonate models. However, these evolving models of temperate carbonate sedimentation generally remain to incorporate several features exemplified by the Tikorangi Formation deposits, including: the recognition of a full spectrum of shelf-slope-basin facies and not simply shallow platform carbonates; the importance of mass emplacement events in producing complex mixed shelfal and deeper water skeletal assemblages; the textural diversity of deposits in which mudstones, wackestones and packstones are more common than grainstones; the ubiquitous occurrence of late diagenetic dolomite in the deposits; and the fact that cool-water carbonate facies can form economically important fracture reservoir rocks. While this study has provided new insights into the subsurface stratigraphy, petrology and diagenesis of the cool-water Tikorangi Formation carbonate fracture reservoir, an important research focus for the future is to further our understanding of temperate carbonate-dominated depositional and diagenetic systems as a whole, thereby enabling more concise assessment of their potential economic significance. The University of Waikato 2000 Thesis en https://hdl.handle.net/10289/14434 https://researchcommons.waikato.ac.nz/bitstreams/8023cee0-2380-406e-b658-92a23b9891ca/download 2400dae55c7668c4c1ac562f39590765 https://researchcommons.waikato.ac.nz/bitstreams/e1f4d743-e64d-44f1-bdc8-ba38ac78d00d/download 06a47de7eea71f91dcdb8131a5ad5fc1 https://researchcommons.waikato.ac.nz/bitstreams/7812997e-0046-4b02-bd29-5f88e0c731a8/download e14202ab27e47ddb00d33097327ba050 https://researchcommons.waikato.ac.nz/bitstreams/08a0fdff-44f1-4c5a-8b1b-9e344b6f6fb7/download 00d64a747ab6bb6c6743d0129aaa53ca https://researchcommons.waikato.ac.nz/bitstreams/44997d47-dfff-45f8-afc7-bdc660a57b85/download 4bd6696e248b4cc0bc562bb6d725271d All items in Research Commons are provided for private study and research purposes and are protected by copyright with all rights reserved unless otherwise indicated.