Arnot, M. J.¹, G H Browne¹, P. R. King¹
(1) Institute of Geological and Nuclear Sciences, Lower Hutt, New Zealand

ABSTRACT: Outcrop-based Investigation of Thin-siltstone Beds in Basin Floor Fan Deposits and their Effects on Reservoir Flow

The presence of thin discontinuous siltstone beds within deep-water clastic reservoirs can potentially have a significant impact on reservoir flow. The vertical frequency of such siltstone beds in the subsurface can be obtained from core and well logs. However, these beds are typically not correlatable over inter-well volumes, so that changes in their lateral continuity and nature are difficult to determine. Such data is best inferred from outcrop analogues.
Coastal cliff sections of the Late Miocene Mt. Messenger Formation in northern Taranaki, New Zealand, expose thick-bedded sandstone and inter-bedded thin siltstone deposits. They are inferred to be analogues for intervals in Mt. Messenger Formation and Middle Miocene Moki Formation reservoirs in the nearby subsurface. Lateral discontinuity of the siltstone beds along this section is due to scouring, resulting in local vertical amalgamation of sandstones beds and improved vertical connectivity. The nature of the thin siltstone beds in these deposits ranges from “simple” horizontal beds to inclined beds and locally, compensation style merging of siltstone beds is also observed.
2D flow modelling based on the outcrop sections shows that, although the sandstones are well connected, the presence and variability in the nature and number of discontinuous siltstone beds, imparts heterogeneity in flow properties over relatively short horizontal distances (10’s meters). This variability in flow properties is below typical grid cells sizes in most reservoir models. To incorporate this scale of heterogeneity into a coarser scale reservoir model will involve some element of upscaling.

AAPG Search and Discovery Article #90026©2004 AAPG Annual Meeting, Dallas, Texas, April 18-21, 2004