The nontropical Oligocene carbonate-rich Tikorangi Formation is an important oil producer in the Taranaki Basin, New Zealand. Hydrocarbons are hosted and produced from mineralized, natural fracture systems. Petrographic, trace-element, stable-isotope (δ18O and δ13C), and fluid-inclusion data have enabled a complex sequence of eight paragenetic events to be determined. The Tikorangi Formation host rock was cemented by low-Mg calcite (event 1) during burial diagenesis, from temperatures of 27°C, corresponding to 0.5 km burial, and continued until 37°C, 1-km burial depth, producing tight, pressure-dissolved fabrics with essentially no porosity and permeability. The host rock was partially dolomitized (5–50%) (event 2) by Ca- and Fe-rich dolomite rhombohedra at burial depths and temperatures of 1.0–1.5 km and 35–50°C without secondary porosity development. Subsequent brittle fracturing formed by Neogene compression (event 3) is constrained to a period following lithification and dolomitization, but before precipitation of first-generation vein calcite (event 4). This initial ferroan low-Mg vein calcite formed after a period of burial from Fe-rich, meteorically modified fluids at temperatures of about 50–60°C and 1.4–1.9 km burial depth. Baroque dolomite formed (event 5), following a period of Mg-enriched basinal fluid input precursory to hydrocarbon emplacement per se. The dolomite formed mainly as a primary cement but also as a calcite replacement at temperatures following further burial to 2–2.5 km and temperatures of 65–80°C. Formation of celestite and quartzine phases (event 6) coincided with or marginally postdated dolomite at similar depths and temperatures to event 6 and formed as both replacements and cements. Second-generation ferroan vein calcite formed (event 7) at cooler temperatures (53–65°C), perhaps resulting from the introduction of cooler meteoric fluids from upsection. The presence of petroleum-fluid inclusions in the second-generation calcite suggests precursory hydrocarbon-bearing fluids have migrated, along with aqueous fluids from about 10 Ma, with hydrocarbon emplacement (event 8) occurring in the last 6 m.y. following a period of rapid late Miocene burial. An improved understanding of the paragenesis of the Tikorangi Formation may assist in hydrocarbon production from its reservoirs.

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