Lithostratigraphy and depositional episodes of the Oligocene carbonate-rich Tikorangi Formation, Taranaki Basin, New Zealand

Abstract

The subsurface Oligocene Tikorangi Formation is a unique and important oil producer in the onshore Waihapa-Ngaere Field, Taranaki Basin, being the only carbonate and fracture-producing reservoir within the basin. Core sample data from seven onshore wells (foredeep megafacies) and a single offshore well (basinal megafacies) are correlated with a suite of sonic and gamma-ray geophysical well log data to derive interpretative carbonate facies for the Tikorangi Formation. Four mixed siliciclastic-carbonate to carbonate facies have been defined: facies A-calcareous siliciclastite (<25% carbonate); facies B-very calcareous siliciclastite (25-50% carbonate); facies C-muddy limestone (50-75% carbonate); and facies D-coarse limestone (>75% carbonate). Single or interbedded combinations of these facies form the basis for identifying nine major lithostratigraphic units in the Tikorangi Formation that are correlatable between the eight wells in this study.The Tikorangi Formation accumulated across a shelf-slope-basin margin within a tectonically diversified basin setting, notably involving considerable off-shelf redeposition of sediment into a bounding foredeep. Analysis of gamma, sonic, and resistivity well logs identifies five major episodes of sedimentary evolution. Episode I comprises retrogradational siliciclastic-dominated redeposited units associated with foredeep subsidence. Episode II is a continuation of episode I retrogradation, but with increased mass-redeposited carbonate influx during accelerated foredeep subsidence and relative sea-level rise, the top marking the maximum flooding surface. Episode III involves a progradational sequence comprising relatively pure redeposited carbonate units associated with declining subsidence rates and minimal siliciclastic input, with movement of facies belts basinward. Episode IV consists of prograding aggradation involving essentially static facies belts dominated by often thick, periodically mass-emplaced, carbonate-rich units separated by thin background siliciclastic shale-like units. Episode V is a retrogradational sequence marking the reintroduction of siliciclastic material into the basin following uplift of Mesozoic basement associated with accelerated compressional tectonics along the Australia-Pacific plate boundary, initially diluting and ultimately extinguishing carbonate production factories and terminating deposition of the Tikorangi Formation.