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Bureau Seminar, April 22, 2011

Strategies for Modeling Depositional Heterogeneity of Carbonate Ramps Using Outcrop Analogs and Multiple Point Statistics


Dr. Ted Playton
Chevron Energy Technology Company, San Ramon, CA

Outcrop exposures offer continuity, correlation, and resolution of geological data well beyond that of the subsurface, and serve as ‘idealized’ analogs for reservoirs. In this study, measured sections, photomosaic mapping, DGPS, and LIDAR data were collected along a 38 km dip exposure of Lower Jurassic carbonate ramp strata in the High Atlas of Morocco, and converted into a static model using multiple point statistics (MPS). The goal was to generate new modeling strategies through simulation of ramp depositional heterogeneity during both transgressive (TST) and highstand (HST) conditions, and for each of the facies belts observed (inner, middle, outer, and basinal ramp settings).

The MPS approach uses combinations of hard data constraints (i.e. well data) and soft geologic concepts (i.e. depositional models) to populate 3D grid space. Outcrop LIDAR and GPS data were integral for the model stratigraphic framework and representation of complex stratal patterns. Soft constraints entailed Training Images and a Facies Probability Cube, which together capture juxtaposition relationships and spatial proportions and likelihoods of the facies belts while honoring the hard measured section and traced outcrop surface data. Using this approach, ramp depositional heterogeneity was successfully simulated within the sequence stratigraphic architecture, including stratigraphic partitioning of facies belts, ramp progradation and retrogradation, changes in facies belt width, and various types of strike variability.  

This outcrop-based modeling effort provides strategies that can be incorporated into subsurface modeling workflows. For example, only two systems tract-specific (TST and HST) Training Images were required to capture stratigraphic facies belt partitioning. This enabled simulation of muddier, peloid-dominated settings in the TST and grainier, ooid-dominated settings in the HST for all mapped sequences. High degrees of facies belt interfingering in the Training Images allowed for replication facies belt contraction, expansion, and migration while preserving juxtaposition rules.  Depocenter (trend) maps guided the simulation of the paleogeography and large-scale reentrant-promontory configurations.  This facies modeling study is the first of a larger effort that includes uncertainty analysis, experimental design, flow simulation, and field testing with subsurface data of key assets.  Further experiments aim to address the effects of 1) data configuration (i.e. well spacing), 2) facies delineation schemes (i.e. depositional rock types versus depositional cycle types versus petrophysical rock types), and 3) different modeling targets on the preservation of the critical geological heterogeneity that impacts subsurface flow and reservoir quality distribution.



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