Petrography of sandstones from the Miocene Tunanui Formation, Hawke's Bay, New Zealand

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Strogen, D.P.; Field, B.D.; Browne, G.H. 2010 Petrography of sandstones from the Miocene Tunanui Formation, Hawke's Bay, New Zealand Lower Hutt, N.Z.: GNS Science. GNS Science report 2010/32 49 p.

Abstract: This report presents the results of a detailed petrographic and porosity-permeability study of the Middle Miocene (Lillburnian) Tunanui Formation from northern Hawke’s Bay. Most of the studied samples were from a single 1 m thick turbidite bed. Two other Tunanui Formation samples from the area were included for comparison. The sandstones are fine grained (fU-fL) and show little or no upwards-fining through the 1 m thick bed, apart from at the very top, where sand (fL) is interbedded with siltstone. One sample from a thin bedded turbidite Tunanui Formation outcrop was slightly finer grained (vfU). The sandstones are lithic-rich, and classify as litharenites or feldspathic-litharenites. The lithic component is composed of a mixture of sedimentary clasts (altered mudstone, siltstone and rare sandstone), metamorphic (mostly low grade pelitic clasts) and rare igneous clasts (altered volcanics and granitic clasts). The fine grain size and altered nature of the clasts makes identifying the lithic component difficult. The compositions of the samples from the single bed are very similar. Samples from other localities were also similar, but overall slightly more feldspathic. The sandstones have undergone very little diagenesis, with minor pyrite and clay redistribution. Only in one sample from the base of the bed did pore-filling ferroan calcite (22%) occlude most of the porosity. The sandstones generally have good porosity (most 25-29%) and moderate permeability (most 20-90 mD). There was no discernable trend in porosity and permeability through the bed, other than a slight drop at the very top. The only sample with very low poro-perm was that with pore-filling carbonate cement. The preservation of good porosity and permeability is due to very limited diagenesis, despite the high proportion of labile clasts, and implies that the sands have undergone only limited burial. MICP data from two samples are consistent with petrographic observations. Tunanui Formation sandstones from previous studies (from two exploration wells and other outcrops) are often finer grained (silt to fL) than those from this study, but their detrital compositions are similar (mostly litharenites, with some feldspathic-litharenites). Within the total spread of QFL data, results from this study sit within the middle of the range. The diagenetic histories of other Tunanui Formation samples are also very similar to those from this study, the main difference being that the samples from the two wells (Tuhara-1a and Waitaria-2) have undergone more compaction. The porosity and permeability of the other outcrop samples overlap with those from this study, but show a larger range. Overall, the porosity and permeability of samples from the two wells are generally much lower than those from this study, apparently due to compaction of the labile lithic clasts with deeper burial. (auth)