Diagenesis Affecting Reservoir Properties of a Single Turbidite Bed: Early Miocene Whakataki Formation, East Coast Basin, New Zealand

Rosalie M. Pollock, Brad D. Field, Greg H. Browne, and Steve W. Edbrooke
Institute of Geological and Nuclear Sciences, Lower Hutt, New Zealand

The Miocene Whakataki Formation in the East Coast Basin was studied intensively in outcrop to assess lateral and vertical variations in reservoir properties to help up-scale reservoir parameters for fluid flow modelling. These thin-bedded, levee-overbank turbidite facies are analogous to gas-bearing thin-beds drilled nearby in Titihaoa-1.

We studied a 19 m lateral transect along a single bed up to 45 cm thick, comprising parallel laminated sandstones (Bouma Tb division) overlain by ripple laminated sandstones (Tc). The bed comprises fine-grained lithic feldsarenites to litharenites, buried to a maximum depth of 3-4 km, with porosities of 12-16% and permeabilities of 0.5-11 mD. The Tb interval generally has better porosity and permeability than the Tc interval, with reservoir quality principally controlled by secondary porosity, resulting from dissolution of feldspars, and crystallisation of intergranular authigenic clays (predominantly chlorite and mixed-layer clays).

Permeabilities are generally low near the base of the bed and locally high near the middle of the Tb interval and near the base of the Tc interval. Greater volumes of authigenic clay diminish reservoir quality near the base of the bed. Hence, highest fluid flow rates are likely through the centre of the turbidite bed (close to the Tb-Tc boundary). However, lateral variations observed in porosity and permeability over a scale of decimetres to meters, mainly due to diagenetic influences, suggest reservoir parameters derived from cores may not be reliable when extrapolated away from a well and indicate uncertainty ranges of at least ± 20% should be allowed for in models.

AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005