RESERVOIR CHARACTERIZATION AND SEDIMENTARY PETROLOGY OF THE WHAKATAKI FORMATION, NEW ZEALAND

The Miocene Whakataki Formation is located within the East Coast Basin of North Island, New Zealand. It is exposed as a wave-cut shore platform and is characterized by laterally extensive fine-grained and thin-bedded repetitions of mudstone and sandstone. These beds are interpreted as being sand-rich turbidites with 73% sandstone and 27% mudstone (Edbrooke and Browne, 1996) that rarely contain the normally graded Ta division of the Bouma sequence. Though fine-grained turbidites are formed primarily on passive margins or during periods of tectonic quiescence on active margins (Bouma, 2000), these beds were deposited in a forearc basin during a period of renewed tectonic activity (Neef, 1992). Based on the thin-bedded and fine-grained nature of these turbidites as well as the primarily Tb-e Bouma divisions, the Whakataki turbidites were likely formed as the midfan levee-overbank deposits of a slope fan or basin floor fan (Basu and Bouma, 2000; Edbrooke and Browne, 1996). Porosity and permeability analysis indicate that the Whakataki has limited potential for being an oil and/or gas reservoir. Tests show a sharp upward decrease in permeability in each Bouma division (ranging from 26.1 mD in the Tb division down to 0.0 mD in the Td division) while porosity displays a gradual upward decrease (ranging from 18.3% in the Tb division down to 9.8% in the Td division). Tests also indicate that the horizontal permeability is significantly greater than vertical permeability in the sandstone portions of the Bouma sequence. Petrographic analyses concluded that the basal parallel laminated sands (Tb division) are arkose sandstones while the convolute/ripple laminated Tc and parallel laminated Td division are feldspathic wackes. The Te division is a mudstone. The presence of authogenic glauconite throughout the sandstone and mudstone beds indicates that there was a low sedimentation rate during the deposition of the Whakataki Formation. The relatively thick mudstones making up the Te Bouma divisions also indicate that there were low rates of clastic deposition in the basin.