Abstract

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.

You do not currently have access to this article.