Analysis of facies proportions as a tool to quantify reservoir heterogeneity

Abstract

The geometry, orientation and stacking patterns of sedimentary geobodies play a key control on fluid migration within subsurface reservoirs, aquifers, and repositories. Outcrops and more recently virtual outcrops are commonly used as analogues to characterise geobody architecture and stacking. Traditionally, both reservoirs and their analogues are described with respect to the bulk proportion of different facies and architectural elements within the system. These are combined with geometric measurements of the different geobodies to describe heterogeneity.

This study presents a new methodology for quantifying the range of facies proportions within architectural element panels taken from outcrop analogues. This method measures the facies proportions within a series of vertical profiles and considers the statistical distribution, rather than the simple mean across the entire panel. The basic premise being that within a layer cake system, the spread in facies proportions will be very narrow whereas in a more “jigsaw” type system the spread will be much wider.

To test this hypothesis, a series of nine virtual outcrops from a range of clastic depositional systems including fluvial, shallow marine and deep marine settings, have been analysed. Depositional facies (architectural elements) were interpreted in LIME and a series of depositional strike-orientated orthorectified panels were extracted. The panels were then analysed, and the extracted facies proportions were translated to “net sand” distribution. Results were then compared with conventional analysis of reservoir heterogeneity.

Different depositional systems show distinct patterns. When arranged in depositional dip order, there is a systematic decrease in sand distribution spread down dip with significant resets at the boundaries between the major gross depositional environments (continental, shallow marine/shelf, deep marine).