Outcrop and behind-outcrop characterization of a late Miocene slope fan system, Mt. Messenger Formation, New Zealand

The late Miocene Mt. Messenger Formation of Taranaki, New Zealand, consists mainly of well-sorted, very fine to fine-grained thin-bedded sandstone and siltstone deposited in middle- and upper-bathyal water depths. Lithofacies are dominated by turbidite beds, comprising planar-laminated and climbing-ripple-laminated sandstone and massive siltstone. These strata were studied in a 200 m-thick, 4 km-long coastal outcrop section and in two, behind-outcrop boreholes from which Fullbore Formation Micro Imager (FMI) and Platform Express logs and core were obtained. The boreholes were 47 and 105 m deep and were drilled about 100 m behind the cliff outcrop about 150 m apart. In addition, a high-resolution seismic reflection line was shot adjacent to the coastal cliff and was tied to the updip outcrop exposures. The outcrop section displays abundant erosional cutout of beds by scour or channels that considerably limit lateral bed continuity. At a smaller scale, bed continuity is also interrupted by bioturbation structures. We interpret the outcrop section as a series of vertically stacked or shingled slope fan or channel-levee/overbank complex deposits, comparable to Mutti type III fans. Comparison of behind-outcrop dipmeter, seismic reflection profiles, and FMI data with outcrop data indicates that three main channel-levee/overbank depositional units are present. Siltstone and alternating thin-bedded sandstone and siltstone that display an upward decrease in dip magnitude and variable dip orientation on dipmeter logs are interpreted as channel fill deposits. Vertically stacked, interleaved packages of thin-bedded, typically planar-laminated sandstone (Bouma Tb divisions), climbing-ripple-laminated sandstone (Tc), and siltstone (Te) with complex upward-decreasing dipmeter patterns are interpreted as proximal channel-levee and overbank deposits. Laterally extensive interbedded Tb and Tc sandstone and Te siltstone lithofacies with low-angle dips and little variability on dipmeter logs are interpreted as distal channel-levee and overbank deposits. These three depositional settings are important to distinguish, as each exhibits different fluid-flow behaviors in an analog reservoir. Comparison of the outcrops with typical subsurface data sets as used by petroleum geologists (logs, core, seismic reflection) has provided criteria for distinguishing subtle stratigraphic features that can influence production from a subsurface reservoir analog.