Development of Permeable Dolomite by Void Coalescence: Smackover Dolomite of Southern Alabama
Mary L. Barrett, Lawrence A. Hardie
Massive dolomitization of the Smackover Formation (Upper Jurassic) is associated with the paleotopographic high of the Conecuh-Wiggens arches. The replacement front geometry suggests lateral and upward flow of dolomitizing fluids. Paragenetic sequence and fluid inclusions indicate replacement temperatures below 70°C. The dolomite texture is dictated by the original rock fabric. Cemented grainstones preserve primary textures and retain original pore geometry (present permeabilities = 0.1-3+ md). However, muddy peloidal beds are converted to a mosaic with only ghosts of primary textures observed. Pores are enlarged (present permabilities = 1.0-10+ md), but concurrent dissolution is not evident. Thus, the enhanced permeability must result from textural rearrangement dur ng dolomitization.
We suggest that the process of void coalescence may have caused pore enlargement. This process is well known in the experimental sintering of ceramics and metals, and occurs when intragranular and intergranular pores move and coalesce during grain boundary migration (GBM). Smaller voids are destroyed while the larger voids grow in length and width. Thus, the overall number of voids is reduced as the remaining voids are enlarged. Porosity may decrease, but permeability is enhanced by an order of magnitude.
Although GBM is commonly thermally activated, it can be induced by diffusion and may be applicable to low-temperature dolomitization such as in the Smackover. Dolomitization involving GBM may account for the enhanced permeability although replacement could be volume for volume, a problem long recognized in massive sucrosic dolomites.
AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.