Characterizing modern stream sand to better understand reservoir sandstone provenance in the Taranaki Basin, New Zealand

The purpose of this study was to create a model for interpreting the provenance of offshore, petroleum-bearing sandstone reservoirs in the Taranaki Basin of New Zealand using sand samples from modern streams in the region as proxies. Twenty sand samples from streams draining plutonic, metamorphic, and sedimentary terranes exposed on South Island, New Zealand, were collected for petrographic analysis. Sand samples were air-dried, sieved into five sand fractions from very fine to very coarse, made into thin sections, and stained for recognition of potassium- and calcium-bearing feldspar. Using a microscope equipped with an automated point-counting stepper stage, 400 grains per sample were point counted and separated into 102 monomineralic and lithic fragment grain categories. Recalculated parameters were plotted on various ternary diagrams and analyzed for trends in composition with grain size and terrane. Overall, sand derived from erosion of the Median Batholith tends to have less quartz than sand from the Karamea Batholith and a higher monomineralic percentage of plagioclase, while sand from the Karamea Batholith has a higher proportion of monomineralic potassium feldspar. Sand derived from erosion of the Buller Terrane is the most quartz-rich of the non-plutonic terrane samples. Feldspar is rare in all the Eastern province terranes (Caples, Torlesse, and Dun Mountain-Maitai and Brook Street). Sand from the Dun Mountain-Maitai and Brook Street terranes has the lowest quartz content and is recognizable by distinctive serpentine pseudomorphs replacing mafic minerals. Pervasive pumpellyite is characteristic of metamorphic lithic fragments derived from some Eastern Province terranes, particularly the Caples along with the Dun Mountain-Maitai and Brook Street, exposed to prehnite-pumpellyite metamorphism along the boundary between the Australian and Pacific plates. Marlborough Schist-, Caples- and Torlesse Terrane-derived sands are recognizable on the basis of relatively high monomineralic quartz plus epidote and clinozoisite/zoisite together with either pumpellyite or albite and almost no potassium feldspar. Older Taranaki Basin sandstones, such as the Cretaceous North Cape Formation, appear to be derived from plutonic sources such as the Karamea and Median batholiths; a more detailed comparison of specific feldspar components in offshore wells would help distinguish between the two. Sandstones that are more lithic-rich in some wells and more quartz-rich in others, such as the Late Cretaceous Rakopi Formation, may be derived from the same plutonic sources with input from metasedimentary/-metavolcanic sources such as the Buller and Takaka terranes. Based on quartz, feldspar, and lithic fragment proportions, the Miocene Moki and Mount Messenger formations appear to be derived from the Marlborough Schist (Caples Terrane and Rakaia subterrane of the Torlesse Terrane), with some input from plutonic sources. More detailed analysis of feldspar and lithic types is needed to better fingerprint specific sand sources in these and younger Tertiary reservoir sandstones, especially as the latter may have been partly derived from arc magmatic rocks. This project provides an empirical dataset for continued comparison and analysis of offshore petroleum-bearing deposits and their source on the South Island of New Zealand, and a benchmark of modern sand petrography against which the petrography of older sandstones within the basin can be compared.