Three-Dimensional Surface-Based Modeling and Flow Simulation of Heterolithic Tidal Sandstone Reservoirs: Examples from the Eocene Dir Abu Lifa Formation Reservoir Analogue, Western Desert, Egypt
Tidal heterolithic sandstone reservoirs are highly heterogeneous across a wide range of length-scales. Consequently, effective flow properties may be poorly predicted using data that do not accurately represent the three-dimensional (3D) distribution of mudstone and sandstone within the reservoir. We present a novel, surface-based modelling approach, which honors the observed geometry of geologic surfaces that control such lithologic variability (e.g. contacts between laminae, beds, and facies units). Quantitative geometrical data to condition the models are obtained from an outcrop analogue, the Eocene Dir Abu Lifa Member (Western Desert, Egypt), which records deposition in a tide-dominated deltaic and estuarine setting.
The workflow uses template surfaces to represent
heterogeneities in 3D depending on their geometry, rather than their
length-scale. The region of interest is subdivided into “elemental
volumes” that stack together, and in which heterogeneities have the same
geometry. Different geometric input parameters are used to characterize the
distribution and 3D orientation of template surfaces in the elemental volumes (e.g.
laminae thicknesses within a cross-bed). Mudstones are modelled as mud drapes
that line the heterogeneity surfaces, with their extent and continuity defined
using a mudstone frequency function derived from the outcrop analogue.
Generic 3D mini-models (volume of 9 m3) of sandy tidal bar deposits comprising stacked cross-beds have been generated with a range of mud drape coverage, which can be linked to a sandstone fraction observed in core. A cornerpoint grid conforming to the surfaces was generated, and the models were flow simulated until steady state was reached. Results show that effective permeability measurements are highly dependent on the volume of rock considered, as heterogeneity surfaces are continuous at centimetre length-scale (e.g. core plugs) but discontinuous at metre length-scale (e.g. cross-beds). At metre length-scale, effective vertical permeability decreases faster than effective horizontal permeability as sandstone fraction decreases, because mud drapes become more laterally extensive. Effective horizontal permeability also decreases faster in the dip direction of the cross-beds than in their strike direction as sandstone fraction decreases. This pattern of 3D anisotropy is related to a higher density of mud drapes in the toesets, relative to the foresets, of cross-beds within sandy tidal bar deposits.
AAPG Search and Discovery Article #90142 © 2012 AAPG Annual Convention and Exhibition, April 22-25, 2012, Long Beach, California