Diagenetic Origins of Lateral Periodic Variations in the Properties of Dolomite – Evidence of Self-Organizing Phenomena?
David A. Budd1, Matthew J. Pranter1, Zulfiquar A. Reza1, and Anthony J. Park2
1 University of Colorado, Boulder, CO
2 Sienna Geodynamics & Consulting Inc, Bloomington, IN
Mississippian dolostones from Wyoming contain periodic oscillations in the lateral distribution of trace-elements, porosity, and permeability. Random variations account for 50-70% of the total variability. The balance occurs in periods ranging from 1.5 to >48 m and are resolved only by sampling of ~150-m lateral transects. Similar periodic patterns in porosity and permeability also occur in Eocene and Permian dolostones. Possible origins are: (1) inheritance from the depositional precursor, (2) self-organizing processes during dolomitization, or (3) overprinting by later diagenesis. Of these three, inheritance is most unlikely given that dolomitization completely alters the microfabric of the precursor in geochemically open systems. Although overprinting can not be ruled out in the known examples, generation by self-organizing processes during dolomitization presents an intriguing explanation.
If the periodic patterns are the product of feedbacks between dissolution, precipitation, and reaction transport during dolomitization, then the patterns represent a new approach to the study of dolomites. Geochemical modeling suggests the patterns may provide previously unobtainable information on ancient dolomitizing systems, including fluid-flow vectors at the scale of decameters, migration rates of the dolomitizing front, and size of flow units. Understanding the lateral distribution of petrophysical properties can also improve models of fluid flow in dolomite petroleum reservoirs and contaminant transport between matrix and conduits in dolomite aquifers. Further, if 30-50% of the variability in a geochemical attribute in any bed is due to lateral periodicity, one must ask if a spot sample is a suitable chemostratigraphic proxy for ancient geologic processes and conditions.