Reservoir potential of Late Cretaceous terrestrial to shallow marine sandstones, Taranaki Basin, New Zealand

Late Cretaceous coals and coaly source rocks are the main source of hydrocarbons in the Taranaki Basin, yet to date there have not been any hydrocarbon discoveries within Cretaceous strata, and sandstone distribution and reservoir quality for this interval have been poorly understood. The Late Cretaceous sediments were deposited in several sub-basins across Taranaki, with their distribution largely determined by sediment supply, subsidence, and sea level change. In this study, we describe potential reservoir facies in well penetrations of Cretaceous strata in Taranaki, as well as from outcrop in northwest Nelson, on the southern edge of the basin.Study of Cretaceous outcrop has shown that facies distribution is a major control on reservoir quality. Shoreface, intertidal, and estuarine depositional facies contain both excellent, well sorted sandstone reservoir facies as well as low net-to-gross, reservoir-poor facies. These contrast with deposits from generally less well sorted, and more heterolithic coastal plain environments. Much of the facies distribution is attributed to relative sea level fluctuations, which occurred throughout deposition.Late Cretaceous sandstones display a range of mean grain sizes and compositions, with the latter related to geographic location and sediment source. Burial depths are highly variable across the basin, ranging from <1 km to >6 km, and this has had a profound effect on reservoir quality. The main petrographic controls on reservoir quality are sandstone composition, volume of clay minerals, degree of mechanical compaction (related to maximum burial depth), and proximity to a source of acidic pore fluids for the generation of secondary porosity.Results from this study suggest that Cretaceous strata are a viable reservoir play over much of the western part of the Taranaki Basin and into parts of the south-eastern basin. Palaeogeographic maps and textural/compositional data can be used to high-grade regions for further exploration.