New Zealand Journal of Geology and Geophysics abstracts
Lithostratigraphy and depositional episodes of the Oligocene carbonate-rich
Tikorangi Formation, Taranaki Basin, New Zealand
Steven D. Hood
Campbell S. Nelson
Peter J. J. Kamp
Department of Earth Sciences
University of Waikato
Private Bag 3105
Hamilton, 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.
Keywords carbonate; limestone; geophysical well logs;
lithostratigraphy; depositional history; Oligocene; Tikorangi Formation;
hydrocarbons; fractured reservoir; Taranaki Basin; New Zealand
G02014; Received 28 February 2002; accepted 18 March 2003; online publication
date 10 September 2003
New Zealand Journal of Geology & Geophysics, 2003, Vol. 46: 363-386
0028-8306/03/4603-0363 $7.00/0 © The Royal Society of New Zealand
2003
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