Relationships between Cement, Rock Fabric, Porosity and Permeability, Lower Vicksburg Formation, McAllen Ranch Field, South Texas

GRIGSBY, JEFFRY D., and RICHARD P. LANGFORD, University of Texas at Austin, Austin, TX

Detailed petrographic mapping of cement distribution, combined with sedimentologic description and petrophysical data, is a powerful technique for describing distribution of reservoir properties. Two cores from the lower Vicksburg Formation, McAllen Ranch field, south Texas, indicate that porosity and permeability, which vary over several orders of magnitude, are related to a complex interplay between centimeter-scale cement variations, grain size, sedimentary structures, and the development of secondary porosity (dissolution of feldspars and unstable lithic fragments).

Porosity and permeability plugs in combination with thin section analysis, taken every 5 cm through several representative intervals, reveal the complex controls on reservoir properties. For example, within an upward-coarsening interval a strong relationship exists between grain size, the development of secondary porosity, and permeability. The coarsest sands of the interval are cemented by quartz overgrowths and variable amounts of calcite. The preferential development of secondary porosity in this "band" results in an average permeability of 0.169 md. In contrast, as grain size decreases, calcite cementation increases, resulting in a decrease in the development of secondary porosity and an average permeability of 0.008 md. When chlorite is the predominant cement, permeability is dep ndent on the development of secondary porosity and is independent of grain size. Within a chlorite-cemented interval, with a relatively constant measured porosity of 16.6 (plus or minus 1)%, permeability ranged from 0.026 md to 0.372 md. Secondary porosity ranged from 1.5% to 5.5% over this same interval. These variations, which occur in distinct bands in core, result in a complex but predictable distribution in porosity and permeability.

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