Diagenetic features in sediments of the McKee Formation, Taranaki

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van der Lingen, G.J.; Watters, W.A.; Smale, D. 1986 Diagenetic features in sediments of the McKee Formation, Taranaki. Lower Hutt: New Zealand Geological Survey. New Zealand Geological Survey report SL 16 2 v.

Abstract: Sandstones from the McKee Formation and adjacent rocks of the upper Kapuni Group (Eocene-Oligocene) in Taranaki were examined to determine the cause of irregular distribution of low-permeability zones in the McKee field. The sandstones are subfeldsarenites; major clastic components are quartz, microcline, albite, and calcic plagioclase. Most samples come from the Eocene-Oligocene McKee Formation in an overthrust slab of the McKee Field (depth range 2131.5 to 2510.5 m). A second group comes from formations stratigraphically equivalent to the McKee Formation, outside the overthrust slab of the McKee Field (depth range 3547 to 4139 m). A third group comes from the stratigraphically underlying Eocene Mangahewa Formation outside the McKee Field (depth range 3261 to 4357 m). Pressure solution, together with fracturing in samples of the main group, has caused greater compaction than would have occurred by normal packing; resorption of calcic plagioclase has occurred at about the same stage, followed by quartz etching and solution, and then by kaolinite neoformation. The deeper samples (second and third groups) show quartz overgrowths and alteration of kaolinite to illite. Carbonate cementation of the sandstones is stratigraphically erratic. Causes of low porosity in the McKee field are (in likely order of increasing adverse effect on permeability): 1. Pressure solution, and minor recrystallization of fine-grained quartz; 2. Clogging of pores and pore throats by secondary kaolinite; 3. ''Rock flour'' produced by grain fracturing resulting from tectonic overthrusting, concentrated in mottles and planar ''silt-trap zones''; and 4. Carbonate cementation of irregular and random zones. The most significant feature causing an erratic relationship between permeability and porosity appears to be the silt-trap zones. As grain fracturing is peculiar to rocks of the overthrust slab, they are rendered particularly susceptible to erratic permeability. (auth)