Combining Log and Porosimeter Data for Enhanced Recovery

Stephen F. Urschel, Swapan K. Ghosh, Gerald M. Friedman

The vertical sequence in the San Andres Formation, west Texas, has been studied to define reservoir and nonreservoir lithologies.

A combination of core, thin-section, and well-log analyses revealed a shoaling-upward stratigraphic sequence with well-developed reservoir and cap-rock facies. Through the use of gamma-ray, neutron-density, sonic, and resistivity logs, a trend of increasing porosity with increasing depth in the vertical sequence has been found. Log-derived lithology cross-plots indicate that reservoir facies can be predicted by log response. Mercury porosimetry studies defining porosity and pore-throat distribution confirm that the best potential reservoir facies are dolomitized oolitic grainstones and shallow shelf dolomicrites and dolowackestones. Reservoir properties for these facies were enhanced through dolomitization and leaching of both carbonate and sulfate minerals. A trend of improving reser oir characteristics with increasing depth in the vertical sequence parallels the log-derived porosity trend, which suggests that logs may be used to predict reservoir characteristics in the absence of other data.

Mercury capillary pressure curves show that the influx of terrigenous quartz silt has reduced permeability in regions where this silt is abundant. Therefore, occurrence of silty intervals within a vertical sequence may control the flow of reservoir fluids.

Mercury porosimeter data for the upper intertidal and supratidal facies reveal consistently low porosities and permeabilities in these rocks, and lead to the interpretation that these facies form an effective cap rock.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.