The Hidden World of Dolomites: Self-Organization of Porosity and Permeability at the Decameter Scale During Dolomitization
Dolomites are typically characterized by representative but spatially uncorrelated sampling. This strategy can define the general petrophysical character of a dolostone, but is there a hidden world of information that has been missed? We sampled, at a 1-ft spacing, two ~490 ft lateral transects through two dolostone beds of the Mississippian Madison Formation. Porosity, permeability, trace-element concentrations, and abundance of pore types were determined. Each attribute shows periodic oscillations in abundance across both transects. Based on variogram analyses, these features occur at periodicities of ~30 ft for porosity and log permeability, ~45 ft for Fe, ~60 ft for % moldic porosity, and ~100 ft for Sr and Na. The fact that all attributes exhibit these oscillations indicates that the pattern is neither a depositional characteristic nor a function of pore type. Rather, we suggest the oscillatory patterns are the product of self-organization during dolomitization. That is, feedbacks during dissolution, transport, and precipitation produce the periodic structures from an initial non-patterned state without any inherited template. If the reaction front is planar, the product might be a series of wave fronts. More likely is that the reaction front fingers and meanders, which in turn leads to reaction-induced flow diversion, the re-focusing of the flow past the reacted mass, and formation of a 3D spot pattern in attributes. In either case, a single lateral transect will yield the oscillatory structures observed. The patterns in porosity and permeability will affect recovery efficiencies and break-through times of water floods.
The Role of Diagenesis in Determining Reservoir Properties II
2005 AAPG Annual Convention (June 19-22, 2005) Technical Program