Murihiku rocks as potential petroleum reservoirs in Zealandia

Detailed petrographic analysis has been undertaken on Jurassic to earliest Cretaceous Murihiku rocks from Southland and South Auckland locations. They comprise lithic (primarily volcanic) and feldspathic (plagioclase-rich) sandstones that generally contain little detrital quartz, and classify as feldsarenites, lithic-feldsarenites, feldspathic litharenites and litharenites. The oldest (Early Jurassic) sandstones are the most quartz-rich. Sandstones become more lithic up-section, with very high volcanic contents occurring within the younger samples (Late Jurassic to earliest Cretaceous). The higher quartz content in the oldest strata is likely to reflect contributions from plutonic or metamorphosed rocks into the forearc during the Early Jurassic, a contribution that decreased with time. However, some quartz enrichment in Early Jurassic sandstones may also have occurred by preferential sorting of quartz grains during transport into the depositional environment. Generally, Murihiku sandstones have poor reservoir properties based on the petrographic and petrophysical results but they may constitute a viable fractured reservoir target.