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ABSTRACTMethane-derived authigenic carbonates (MDACs) in Miocene bathyal mudstones in North Island, New Zealand are typically expressed as either sub-seafloor conduit concretions or as seafloor seep limestones, but rarely are both types exposed in outcrop at one locality. Consequently, any potential genetic link between them is usually inferred. This also appears to be the case for global occurrences of MDAC. At the Rocky Knob seep complex near Gisborne both seep limestones and conduit concretions co-occur. The petrography and stable carbon (δ13C) and oxygen (δ18O) isotope compositions of their various authigenic carbonate components (automicrite, fibrous aragonite crystals, and granular, blocky and bladed calcite crystals) show that distinctive isotope and petrographic groupings for precipitates within the conduit concretions match or “correlate” with several of those in the seep limestones. This corroborates their genetic tie and derivation from the same fluids, albeit in different parts (i.e. sub-seafloor vs. seafloor) of the seep complex.

The nontropical Oligocene cabonate-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 (δ[sup18]O and δ[sup13]C), 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 Celsius, Corresponding to 0.5 km burial, and continued until 37 Celsius, 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 Celsius 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 Fetich, meteorically modified fluids at temperatures of about 50-60 Celsius 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 Celsius. Formation of celestite and quartzine phases (event 6) coincided with or marginally postdated dolomite... [ABSTRACT FROM AUTHOR]