BROWNE, GREG H. and ROGER M. SLATT
Superbly exposed outcrops of Late Miocene slope fan (channel-levee) strata along the Taranaki coast of New Zealand provide a well-to seismic-scale analog of thin-bedded, slope fan reservoirs in the Gulf of Mexico and elsewhere. At the well scale, a 200m thick stratigraphic section is dominated by thinly-interbedded, very well sorted, very fine-to fine-grained sandstone (Bouma Tb/c) and mudstone; climbing ripple-laminated sandstone is particularly abundant. Net:gross averages about 0.85, but varies stratigraphically. Similar characteristics are observed in cores and FMI* logs from two 50-100m deep wellbores obtained 150m behind the outcrops. At the seismic scale, stratal packages of interbedded sandstones and mudstones onlap other packages at discordant angles, as observed in both outcrop and on a high resolution seismic line acquired along the beach in front of the outcrop.
Small- to large-scale erosional scour surfaces, many of which are mud-draped, are common and result in cut-out and lateral discontinuity of sandstone and mudstone beds. Environments in which erosional scours might occur with climbing ripple-laminated sandstone include a channel-lobe transition zone, a relatively steep-gradient slope such as might occur adjacent to salt or a fault, and the shoulder of a levee in proximity to its associated channel; erosion would be enhanced in any of these environments under conditions of very high sediment supply. Within these environments, lateral discontinuity of beds might decrease down the depositional dip as the slope gradient and sediment concentrations, and thus the erosive power of turbidity currents, diminishes. Consequently, the complexity of fluid flow pathways in analog reservoirs might also diminish downdip, giving rise to potentially different drilling and development strategies.
AAPG Search and Discovery Article #90941©1997 GCAGS 47th Annual Meeting, New Orleans, Louisiana