ABSTRACT: Petrology, Diagenesis and Reservoir Quality in the Kupe South Field, Taranaki Basin, New Zealand

MARTIN, KENNETH RICHARD, K. R. Martin Pty Ltd, Kenmore, Queensland, Australia

Reservoirs in the Kupe South field are petrologically complex, fluviatile sandstones of the Paleocene Farewell Formation. The sandstones are texturally and mineralogically immature, lithic arkoses in which biotite mica is an important accessory mineral.

The seismically defined subdivision of the main reservoir interval into two subunits (A, B Sands) is verified petrologically on the basis of differences in detrital mineralogy. These differences are subtle, but are recognizable across the field in the form of variations in quartz and feldspar content, potash feldspar/plagioclase ratios, and the heavy mineral suite.

Differences in diagenetic mineralogy broadly parallel the detrital mineral changes. The A Sand is characterized particularly by the occurrence of authigenic kaolinite and iron carbonates (siderite, ankerite) whereas the B Sand contains pervasive chlorite-smectite mixed layer clay. The precursor appears to have been mainly biotite mica in both cases. The two assemblages are generally mutually exclusive, and while they may have j formed concurrently, it is more likely that the kaolinite + carbonate assemblage formed after chlorite-smectite in response to a meteoric influence within the A Sand.

The effects of rapid post-Miocene burial are reflected in the fracturing of brittle grains and plastic deformation of ductile grains in the reservoir sands.

Reservoir quality is influenced by grain size, detrital mineral composition and diagenetic changes, with the formation of authigenic clays being the single most important factor. Porosity is mostly primary, supplemented by smaller quantities of secondary porosity from labile grain and possibly carbonate dissolution.

AAPG Search and Discovery Article #91015©1992 AAPG International Conference, Sydney, N.S.W., Australia, August 2-5, 1992 (2009)