Characterisation and modelling of heterogeneity in thinly-bedded, shallow-marine sandstone reservoirs

Abstract

Sandstone reservoirs deposited in wave-dominated, shallow-marine environments are inherently heterogeneous, containing intervals of thinly-interbedded sandstones and mudstones. Permeability contrasts between interbedded sandstones and mudstones are one such heterogeneity. These interbedded intervals contain sandstone-filled erosional scours formed by episodic flows set up by storm waves during periods of relatively high riverine sediment discharge (‘storm floods’). Erosional scours may increase connectivity between sandstone beds that are otherwise isolated by mudstone interbeds, thus creating effective vertical permeability. The geometry, dimensions and spatial distributions of erosional scours are uncertain. Consequently, these aspects of erosional scours are not represented in three-dimensional reservoir models, and their contribution to field performance is thus poorly understood.

The contributions of this thesis are: (1) to present a quantitative analysis of the geometry, dimensions and spatial distributions of erosional scours observed in heterolithic, wave-dominated successions of interbedded sandstone and mudstone deposits exposed at outcrop, (2) to characterise the extent and distributions of mudstone barriers that are remnants of erosional scouring in these deposits, and (3) to determine the resulting controls on sandbody connectivity and effective kv/kh ratio. These aspects of facies architecture and their impact on reservoir performance are addressed by constructing a high resolution digital outcrop model using photogrammetry from nearly continuous exposures of an outcrop analogue (‘G2’ parasequence, Grassy Member), and a suite of numerical reservoir modelling experiments that are calibrated to data derived from this outcrop analogue to quantify the impact of erosional scours on sandbody connectivity, sandstone proportion and effective reservoir properties. The proportion of sandstone is controlled by sandstone-bed and mudstone-interbed thickness and by parameters that describe the geometry, dimensions and lateral-stacking density of sandstone-filled scours. Sandbody connectivity is controlled by the interplay between the thickness of mudstone interbeds and sandstone-filled erosional scours. Effective kv/k ratio is controlled by the proportion of sandstone, which represents the effects of variable distributions and dimensions of mudstones produced by scour erosion, provided that scour thickness is greater than mudstone-interbed thickness. The outcrop dataset and numerical modelling experiments provide the means to estimate the impact of scour geometry, dimensions and distributions on sandbody connectivity and effective kv/k ratio at the scale of typical reservoir model grid cells (200 x 100 x 20 m) from a combination of core data that constrain the thickness of mudstone interbeds and sandstone proportion, and outcrop analogue data that describe the thickness of sandstone-filled erosional scours.