--> Abstract: Moldic Porosity beneath Bar Crests, Permian San Andres Formation, Algerita Escarpment, New Mexico, by S. D. Hovorka and C. Kerans; #91004 (1991)

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Moldic Porosity beneath Bar Crests, Permian San Andres Formation, Algerita Escarpment, New Mexico

HOVORKA, SUSAN D., and CHARLES KERANS, Bureau of Economic Geology, University of Texas at Austin, Austin, TX

Detailed geometries of 12 parasequences in the upper San Andres Formation were mapped in outcrop on the Algerita Escarpment, western Guadalupe Mountains, New Mexico, to determine the control of depositional facies on permeability. Parasequences are composed of transgressive mudstone, burrowed and cross-bedded grainstone, and discontinuous fenestral mudstone-packstone caps. The thickest bar has 4 m of depositional topography and a karsted upper surface.

Petrographic study and associated porosity-permeability analysis focused on a parasequence with abundant moldic porosity. Moldic porosity in reservoirs results in lower permeability than the same amount of intergranular porosity. Grainstone bar geometry that is apparent when facies are mapped on the scale of individual parasequences controls the distribution of moldic porosity. The area of moldic porosity corresponds to the thick part of the parasequence, to the area in which fenestral fabric is developed at the top of that parasequence, and to the area of greatest topographic expression of the overlying thick, karsted parasequence. Petrographic relationships demonstrate that grain dissolution occurred after initial cement precipitation, prior to much compaction, and synchronous with cclusion of much intergranular porosity by blocky cement.

Relationships to depositional geometry and precompaction origin of moldic porosity support a model for creation of moldic porosity in a freshwater lens. During the sea level fall defining the top of the parasequence, fresh water introduced beneath bar crests caused dissolution of metastable grains and reprecipitation of carbonate as cement. Emergent bar crests are therefore areas where moldic porosity may cause lower permeability than might be predicted based on increased thickness of grainstone.

 

AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)