Permian and Mississippian dolomites contain nested patterns of lateral periodic variation in porosity and permeability. These patterns include a near-random component, short-range variability, and a long-range periodic trend. Phase I of the AVID Consortium (Analysis of Variability In Dolomites) has shown that (1) Eocene dolomites also exhibit the same three scales of lateral variability in porosity and permeability, (2) the short-range variability has the most dramatic impact on fingering, sweep efficiency, break-through time, and bottom-hole pressures , and (3) the longer-range petrophysical cyclicity should be incorporated into dolomite reservoir models depending on the magnitude of the variability.
Phase II of the AVID Consortium will (1) assess the origin of lateral cyclicity in petro- physical properties by sampling across a dolomitization front in the Neogene Seroe Domi Fm., and (2) use reaction-transport models to define the geologic factors that determine the length scale of the short- and long-range trends. Defining the origin(s) of petrophysical variability and the factors that affect the length scales of the variance have obvious impact to reservoir characterization and modeling. The more complete the understanding of the potential distribution of petrophysical properties, the more accurate the resultant characterization.
Two possible origins for the lateral patterns are inheritance from the limestone precursor or formation during dolomitization. An outcrop test of these alternatives requires sampling across a dolomite-to-limestone reaction front in rocks that have experienced only one episode of dolomitization and no subsequent diagenesis. The Seroe Domi fits those criteria. Three, lateral transects will be sampled at 1-foot intervals. Each transect will start in pure dolomite and extend into the undolomitized precursor. All samples will be analyzed for porosity and permeability to quantify their spatial patterns. If short- or long-range patterns are present in both limestones and dolomites, then the patterns in the dolomites are an inheritance from the limestone. If lateral patterns are present in the dolomite but not the limestone, then formation during dolomitization is indicated. Lack of spatial patterns in either lithotype will mean either the patterns are not universal, or that they may only originate during burial diagenesis.
The two-dimensional reaction-transport modeling will use initial porosity and permeability reflective of Holocene sediments and Cenozoic limestones. Dolomitization by evaporitic brine, normal seawater, and hot basinal brine will be considered, as well as varied fluid-flow rates. Multilayered simulations will represent vertical facies variations, and bed thickness will be varied to determine the effect of flow-unit thickness. To test for inheritance of spatial patterns, periodic patterns in the variance of the initial petrophysical properties will be embedded into some simulations. The remaining simulations will have no periodic templates within the initial conditions, thus testing for the creation of petrophysical patterns by dolomitization. The presence, absence, and geometry of spatial patterns in the output data can be related directly to each input parameter, thus defining the geologic scenarios that can create and/or preserve lateral spatial structures in the petrophysical properties. The same sets of results will also assess of the factors that determine the length scale of short- and long-range variability.
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.