LUCIA, F. JERRY, Bureau of Economic Geology, The University of Texas at Austin, Austin, TX

ABSTRACT: Predictive Model for Porosity Distribution in Cyclic Carbonate Rocks Dolomitized by Refluxing Hypersaline Brines

Key constraints in a geologic model that predicts the distribution of dolomite and porosity of dolomite are (1) the chemistry of the dolomitizing fluid, (2) the source of the fluid, (3) the flow-paths of the fluid, and (4) the duration of the dolomitization process. In a hypersaline-reflux system, the source of dolomitizing fluids is an evaporitic tidal flat or pond, and the flow paths are downward and basinward. The extend of the dolomitization process in any specific area is a function of the residence time of the tidal flat or hypersaline pond in that area and the pore-volumes of dolomitizing fluid. Because dolomitizing fluids carry additional carbonate to the site of dolomitization, the longer the duration of the dolomitizing process the lower the porosity of the resulting rock.

Consider hypersaline-reflux dolomitization active in a ramp or platform

in which fluctuating sea level results in cyclic stacking of upward-shoaling carbonate units capped by tidal-flat deposits This situation that can be equated with sea-level highstand and falling sea level. A single, high-frequency sea-level fluctuation cycle will result in deposition of a wedged-shaped dolomite unit that, in its landward part, is thicker, has more tidal-flat residence time, and was flushed with a larger volume of refluxing dolomitizing brine than in its seaward part. A stack of several of these upward-shoaling tidal-flat cycles could form a dolomite unit hundreds of feet thick in which the seaward parts are more porous than the landward parts.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.