Grain‐size characteristics for distinguishing basin floor fan and slope fan depositional settings: Outcrop and subsurface examples from the late Miocene Mount Messenger Formation, New Zealand

Abstract An outcrop section of late Miocene deep‐water sediments of the Mount Messenger Formation in Taranaki, New Zealand, displays distinctive physical sedimentary features that allow differentiation of basin floor and slope fan depositional units. Sandstone grain‐size characteristics have been examined in this study to differentiate these two types of deep‐water deposits. Outcrop data indicate that basin floor fan sandstones are relatively sand rich in comparison to silt‐rich slope fan sandstones. Both basin floor and slope fan sandstones show better sorting with increasing grain size, though cross‐plots show the nature of this relationship differs slightly for basin floor and slope fan samples. These relationships appear to hold for both outcrop and subsurface sandstone samples from the formation. This finding is unexpected given the c. 600 m stratigraphic thickness of the formation, representing several million years of depositional history, and implies a uniform sediment texture was supplied to the basin through time. The differentiation of basin floor fan and slope fan deposits is significant especially in subsurface settings involving petroleum well data. Hydrocarbon exploration strategies will vary markedly for basin floor fan versus slope fan reservoirs, making such differentiation of lithofacies types important to optimise hydrocarbon discovery. With subsurface data, the interpretation of these two reservoir sandstone lithofacies is often difficult to make. The grain‐size changes appear to mimic the contrasting depositional mechanisms operative in these two deep‐water settings.