Abstract

Palaeocene reservoir sandstones in the Kupe South Field, Taranaki basin, contain a diagenetic mineral assemblage that records major shifts in porewater composition during the burial history of the basin. Early calcite formed at shallow burial largely from meteoric depositional porewaters, whereas later chlorite/smectite records the downward passage of marine porewaters into the sandstones from overlying, marine mudrocks prior to significant late Miocene sandstone compaction. Late calcite and ferroan carbonates may record the presence of connate meteoric water expelled upward from non-marine 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 regional 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, whereas the upper sands contain mainly ferroan carbonates and kaolinite tonned by extensive alteration of labile grains and earlier-formed chlorite/smectite. Differences in the extent of alteration most likely reflect fault-controlled permeability variations within the reservoir. Reservoir quality in the lower sands is controlled mostly by grain size and the presence of chlorite/smectite, whereas in the upper sands, the presence of kaolinite is the single most important cause of poor reservoir quality.

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