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In carbonate reservoirs, subsurface data are typically sufficient to delineate the stratigraphic framework, identify flow units, characterize lithofacies, and define petrophysical properties close to the borehole. However, these data provide limited information regarding lateral heterogeneity within individual flow units at the sub-seismic scale. This is especially true for dolomite reservoirs where the original rock fabric is strongly overprinted by the dolomitization process. Because of the complexity of dolomite reservoirs, characterization and 3-D geologic modeling often depend on analog outcrop data to quantify spatial relationships (especially lateral variability), augment subsurface data sets, and improve reservoir modeling efforts.
However, few outcrop assessments of lateral petrophysical variability in dolomites exist. To date, this type of spatial variability has only been documented within reservoir analog dolomites of Permian (San Andres Fm.) and Mississippian (Madison Fm.) age. These separate studies documented multiple scales of lateral petrophysical variability based on 1-D sampling, with both analogs exhibiting similar cyclic patterns. Chemical variables also exhibit similar scales of lateral variability in the Mississippian rocks. The observed lateral cyclicity in these properties is believed to represent complex, but subtle, 3-D geometrical patterns. It is hypothesized that these patterns are formed by geochemical self-organization during dolomitization.
The AVID Consortium (Analysis of Variability In Dolomites) represents an expansion of previous work conducted by the principal investigators on reservoir-scale variability in dolomites. This work focuses on two unresolved issues: (1) what are the two-dimensional geometries of the petrophysical structures, and (2) can an understanding of the potential geometries be used to enhance 3-D characterization and modeling of carbonate reservoirs? The AVID consortium is the first phase (Phase I) of ongoing research on variability in carbonate reservoirs.
Field data for the study have been collected along multiple lateral transects (1-ft [30-cm] sample spacing) in the Madison Formation, Lysite Mountain, Wyoming, and the Avon Park Formation in central Florida. The samples are analyzed for porosity and permeability to document and quantify their spatial patterns. The inclusion of the Eocene Avon Park Formation also has the benefit of analyzing lateral variability in a dolomite of a different age.
A key aspect of the research is that by understanding the character and distribution of petrophysical properties in dolomites, predictive models can be developed for use in carbonate reservoir characterization and 3-D reservoir modeling.
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Current supporting companies: ConocoPhillips, ExxonMobil, Shell
Mississippian carbonate sequences evaluated at Lysite Mountain, Wyoming. The porous and permeable dolomitized facies within these sequences were sampled for analysis of lateral petrophysical heterogeneity. |
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