MAJOR, R. P., F. JERRY LUCIA, and STEPHEN C. RUPPEL, University of Texas at Austin, Austin, TX
Mole-per-mole replacement of calcite by dolomite yields an approximately 12% increase in porosity because dolomite is denser than calcite. However, data from the Pliocene-Pleistocene Seroe Domi Formation of Bonaire, Netherlands Antilles, demonstrate the dolomitization of these rocks resulted in porosity reduction. Rocks proximal to the source of dolomitizing fluids exhibit a greater amount of porosity occlusion than more distal rocks, indicating that dolomitization is a porosity-destroying process and that the degree of destruction can be calibrated to the flow path of dolomitizing fluids. Porosity observed in Bonaire dolomite varies over a distance of hundreds of meters along fluid-flow paths.
In three examples of dolomites in which the pathways of dolomitizing fluids can be interpreted from the spatial geometry of dolomite compositions, the distances over which these changes occur are pertinent to interpreting diagenetically controlled reservoir heterogeneity in oil and gas fields.
Bonaire Pliocene-Pleistocene foreslope grainstone exhibits a progressive increase in unit cell dimensions, calcium content, and strontium content from updip low-porosity rocks to downdip high-porosity rocks. Dolomite is in transitional contact with downdip limestone, which is interpreted to have been dolomitized by marine-derived hypersaline brines that moved downdip from a superjacent sea-marginal lagoon or saline lake.
The Lower Ordovician Ranger Peak Formation (west Texas) contains solution breccias thoroughly dolomitized and is bordered by completely and partly dolomitized limestone. The dolomitized limestone adjacent to one thin-solution breccia pipe (approximately 1-m wide) extends parallel to bedding for hundreds of meters outward. Dolomite adjacent to the breccia has more compact unit cell parameters, less calcium, and less strontium than does dolomite several tens of meters away, suggesting that dolomitizing fluids moved downward through the breccia and outward into the surrounding limestone.
Permian (Leonardian) Clear Fork Formation deposits (Monahans field, west Texas) compose a 200-m-thick, third-order, shoaling-upward sequence. The upper 140 m of this sequence, which was deposited during a third-order highstand, is entirely dolomite, having relative compact unit cell dimensions. The lower 60 m of the section is backstepping fourth- and fifth-order cycles deposited during third-order sea level rise. In these interbedded dolomites and limestones, the dolomite has relatively expanded unit cell dimensions and elevated calcium compositions. The dolomite spatial distribution and composition suggest that dolomitizing fluids moved downward from tidal flats deposited during late highstand.
Tracing dolomitizing fluid pathways may predict diagenetically controlled porosity trends in ancient rocks. Because porosity trends are associated with significant changes in petrophysical characteristics and fluid storage capacity at a between-well scale, this interpretation scheme may aid mapping of flow units in hydrocarbon reservoirs.
AAPG Search and Discovery Article #91018©1992 AAPG Southwest Section Meeting, Midland, Texas, April 21-24, 1992 (2009)