Bureau of Economic Geology, The University of Texas at Austin (www.beg.utexas.edu).
RCRL presentation, ExxonMobile, Houston, Texas, January 26, 2003
Evaporative Reflux Dolomitization and Reservoir Prediction
F. Jerry Lucia
Evaporative Reflux is a hydrodynamic process for moving concentrated seawater through carbonate sediment by evaporating seawater at the air-brine interface resulting in an increase in density and creating a hydrodynamic head sufficient to drive seawater downward. Sufficient head to drive evaporated sea water through underlying sediments is know to form by this process in hypersaline ponds and in evaporitic supratidal sediments, and modern dolomite, and occasionally dolostone, is always found in the associated sediments. This model for dolomitization is useful for predicting dolomite patterns in reservoirs because 1) the source of dolomitizing water can be linked to depositional facies, namely tidal flats, 2) the direction of flow can be predicted, namely downward and basinward from the tidal-flat facies, and 3) the outlet is know, namely the ocean floor.
The association of tidal-flat facies with dolostones is a commonly observation in the geologic record and the model predicts that dolomitization should extend from the tidal flat facies downward and seaward. However, within a succession of tidal-flat-capped cycles some tidal flats may be dolostone and others limestone, and the dolostone may extend upward as well as downward into limestone from the tidal-flat facies. The scale of dolomitization attributed to reflux varies between geologists. Some would limit reflux dolostone to the tidal-flat facies and others use the model to explain hundreds of feet of dolostone. The association of dolostones with evaporite deposits is also a common observation. However, there is no correlation between the thickness of evaporite deposits and the thickness of the associated dolostones. The source of the evaporitic water is also controversial. Can a simple shallowing ramp model create sufficient head through evaporation to drive seawater through the underlying sediment, or is a physical restriction required?
These questions can be addressed using modern flow modeling methods constrained by 1) modern sequence stratigraphic studies, 2) reconstruction of porosity and permeability history using modern understandings of how pore size in carbonate rocks changes through time and 3) structural history. Once the source of dolomitizing water is determined, the geometry of the resulting dolostone will be directly tied to the volume of water circulated through the sediment and the permeability distribution between the source and the outlet. Predicting the geometry of dolostone bodies becomes principally a hydrodynamic problem that can be studied using modern flow simulation programs and modern understanding of carbonate reservoirs.