LATERAL PETROPHYSICAL VARIABILITY WITHIN DOLOMITE ROCK FABRICS: IMPLICATIONS FOR PETROPHYSICAL CHARACTERIZATION AND MODELING

Different scales of lateral petrophysical variability are observed within rock fabric facies of dolomitized shallow-water carbonates. To properly characterize and model the spatial variability of petrophysical properties that effect fluid flow and storage within dolomites, it is essential to have an accurate quantitative description of small-scale lateral variability within the different dolomite rock fabrics. Outcrop analogs of subsurface dolomite formations are used to provide the critical information to address the issue of lateral variability within dolomites that is not available from typical subsurface data. Porosity and permeability measurements were acquired for approximately 1700 samples from Mississippian-age dolomite units (Madison Formation) exposed at Sheep Mountain and Lysite Mountain, Wyoming. These dolomite units are among several dolomite outcrops of different ages that are being studied to evaluate lateral petrophysical variability in dolomites and its origin.

Eight lateral transects (45 to 500 ft; 14 to 150 m) and twelve vertical transects (15 to 100 ft; 5 to 30 m) were obtained within dolomitized lower shoreface and upper shoreface mudstones, wackestones, and grainstones. Variography on the lateral traverses show three distinct scales of lateral petrophysical variability, including a significant hole effect. Nested variogram models can be used to account for the different scales of variability. Short-range lateral variability is reflected by short correlation distances of 8 to 12 feet (2 to 4 m) and can be modeled using a spherical model. The nugget effect is generally high and accounts for approximately 50 percent of the total variance. A distinct cyclicity is present on the lateral experimental variograms with periodicities of 40 to 60 feet (10 to 20 m). The periodicities can be modeled using hole-effect variograms. Petrographic analyses, including point-counting pore type, show distinct lateral changes in percentage of intercrystalline to moldic porosity.

Stochastic 2-D cross-sectional models explore the effects of these heterogeneities on fluid flow. Streamline simulations using different petrophysical models demonstrate the relative effect of the different scales of variability on flow behavior.