Sedimentary lithofacies, petrography and diagenesis of the Kapuni group in the Kapuni Field, Taranaki Basin, New Zealand : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science with Honours in Earth Science at Massey University, Palmerston North, New Zealand

Abstract

The reservoir architecture and quality of the Kapuni Group sandstones in seven wells (Kapuni−1, −3, −8, −12, Deep−1, 14 and −15) in the Kapuni Field are characterised using available core and digital geophysical log data. The study focused primarily on the Eocene Mangahewa Formation, but where limited core permits the older Kaimiro and Farewell formations are also examined. Eleven lithofacies in the Kapuni Group, identified and defined in core on the basis of colour, lithology, bedding, texture and sedimentary structures, are interpreted to represent tidal sand bar, tidal-inlet channel, fluvial-tidal channel, spit platform, sand flat, shallow marine, tidal channel, meandering tidal channel, mud flat, swamp and marsh environments. Correlation of core lithofacies with geophysical log motifs enabled lithofacies identification where core data are not available. Log motifs representing each of the lithofacies were then extrapolated to uncored sections of the Mangahewa Formation in the Kapuni Field wells. Interpretation of lithofacies in core and geophysical log motifs indicate that the Mangahewa Formation was deposited in an estuarine setting. During initial deposition of the Mangahewa Formation tide-dominated estuarine lithofacies were deposited. A major coal horizon, the K20 coal, in the field represents a period of maximum infilling. Above this coal core and log data indicate a wave-dominated estuary exhibiting a clearly- defined, "tripartite" (coarse-fine-coarse) distribution of lithofacies. Provenance studies suggest that low-grade metamorphic and granitic rocks are the dominant source for the Kapuni Group sandstones. Minor input from sedimentary and acid volcanic source rocks are also identified. A volcanic source, however, is more important in sandstones from the Farewell Formation, than in the younger Kapuni Group formations. Probable sources include the low-grade metamorphic rocks of Lower Cambrian to Permian age, Permian to Carboniferous Karamea Granite, Triassic and Jurassic greywacke-argillite sediments. Upper Cretaceous Pakawau Group sediments and Pre Cambrian to Upper Cretaceous acid volcanics. Reservoir quality variations in the Kapuni Group sandstones are directly related to environmental and diagenetic processes that have controlled porosity reduction and enhancement. Porosity has been reduced mainly by mechanical and chemical compaction, clay formation (predominantly kaolinite and illite in the Mangahewa and Kaimiro formations and smectite in the Farewell Formation), carbonate precipitation (primarily siderite and calcite), quartz and feldspar overgrowths and pyrite precipitation. While, porosity has been enhanced primarily by carbonate dissolution and subordinately by grain and clay dissolution and minor grain fracturing. The Mangahewa Formation sandstone lithofacies of tidal sand bar and tidal channel environments exhibit the best reservoir characteristics. Future reservoir development in the Kapuni Field and exploration in the Kapuni Field should focus on identifying and exploiting these lithofacies