The Kapuni sandstones from Inglewood-1 well, Taranaki—petrology and the effect of diagenesis on reservoir characteristics

Abstract

All the important known hydrocarbon reserves of New Zealand are contained in Upper Eocene sandstones of the Kapuni Formation. Despite its economic importance the Kapuni Formation has received little petrological study, This article presents information from ten samples from a 7⋅5 m cored interval in the Inglewood-1 well, Taranaki. The available sediments are very coarse to medium grained quartzose sandstones with grain supported texture, Dominance of unstrained monocrystalline quartz, limited amounts of microcline and orthoclase, lack of plagioclase, and low mafic content suggest an alkali granitic provenance. Deposition of the sediments as point bars in migrating channels is suggested by the general decrease in grain size upwards and by the sequence of sedimentary structures resembling those described from Recent point bars. Angularity of grains and presence of some fresh feldspar and micas indicate first cycle deposition. Important and widespread diagenetic effects include syntaxial quartz overgrowths, kaolinitisation and calcite cementation, all of which have drastically reduced pore space, impaired permeability, and decreased the reservoir potential of the sandstones, Aggradation of detrital kaolinite and precipitation of quartz overgrowths from pore waters supersaturated with silica probably began soon after deposition. Carbonic and humic acids produced during early coalification probably aided solution of alumina, hydrolysis of feldspars, and precipitation of kaolinite. Later, chemical changes during the increase in rank to bituminous coal would have raised pore water pH, resulting in the deposition of calcite. Other late diagenetic effects were etching of overgrowths and reaction between calcite and silica. The emplacement of methane would have stopped chemical diagenesis.