ALBERT

Description: 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. Steven Hood is a postdoctoral research fellow in the Department of Earth Sciences at the University of Waikato in Hamilton, New Zealand. While recipient of a University of Waikato doctoral scholarship, he completed a Ph.D. on the subsurface stratigraphy and petrology of the middle Tertiary Tikorangi Formation fracture reservoir in Taranaki Basin in 2000. His research interests are currently focused on the petrology, paragenesis, and petroleum geology of cool-water carbonates, especially in New Zealand.Cam Nelson received B.Sc. and B.Sc. (honors) degrees in geology at Victoria University, Wellington. He lectured in the Department of Geology at the University of Auckland, where he received his Ph.D. before joining the Department of Earth Sciences at the University of Waikato in Hamilton in 1971 as its founding geological staff member. He was department chairperson from 1988 to 1996 and has been a professor since 1991. His research interests are in sedimentary and marine geology and stratigraphy and Cenozoic paleooceanography and paleoclimatology of the southwest Pacific region. He is past president and office holder of the Geological Society of New Zealand and was elected a fellow of the Royal Society of New Zealand in 1994. Peter Kamp is professor of Earth Sciences at the University of Waikato in Hamilton. He received his M.Sc. degree and his Ph.D. from the University of Waikato. His research interests are in the analysis of sedimentary basins, particularly those of Late Cretaceous–Cenozoic age in New Zealand. Another major research interest involves the techniques of fission-track analysis and (U–Th)/He thermochronometry. His research applications involve the thermal history of sedimentary basins and the exhumation history of basement provinces–mountain belts.