Paleocene reservoir sandstones in the Kupe South field, Taranaki basin, contain a diagenetic mineral assemblage that records major shifts in pore-water composition during the burial history of the basin. Early calcite formed at shallow burial largely from meteoric depositional pore waters, whereas later chlorite/smectite records the downward passage of marine pore waters into the sandstones from overlying, marine mudrocks prior to significant sandstone compaction during the late Miocene. Late calcite and ferroan carbonates may record the presence of connate meteoric water expelled upward from nonmarine sedimentary rocks of the underlying Cretaceous sequence, whereas later kaolinite and secondary porosity formation are related to localized meteoric influx resulting from late Miocene to early Pliocene uplift and erosion of the reservoir section. Hydrocarbon entrapment occurred during further Pliocene to Holocene sediment accumulation.
Labile-grain alteration has been less severe in the lower part of the hydrocarbon-bearing section (lower sands) than in the upper part (upper sands), with the result that the lower sands contain mainly chlorite/smectite, and the upper sands contain mainly ferroan carbonates and kaolinite formed by extensive alteration of labile grains and earlier-formed chlorite/smectite.
Reservoir quality in the lower sands is controlled mostly by grain size and the presence of chlorite/smectite, but in the upper sands, the presence of kaolinite is the single most important cause of poor reservoir quality.