C. L. Dinwiddie, C. J. Lorinovich, J. W. Castle, F. J. Molz, III, R. A. Bridges, and S. Lu

Clemson University, Clemson, SC

ABSTRACT: Application of the New Drillhole Minipermeameter Probe to Characterization of Facies- Dependent Permeability Variations in Shallow-Marine Sandstones, Southern Utah

Appreciable heterogeneity in outcrop analogs is evident when permeability measurements are made on small scales. Field measurements have commonly been made using the conventional surface-sealing gas minipermeameter probe; however, it has proven difficult to maintain a constant, uniform force of the optimal magnitude on this probe’s tip seal, unless the measurements are made with a mechanical device in the laboratory. Surface irregularity and weathering are also serious problems impacting field measurements with the conventional probe.

In this presentation, we describe the application of a newly-designed small drillhole minipermeameter probe for performing in situ permeability measurements. Small cylindrical holes are created in an outcrop with a masonry drill, followed by probe insertion, seal expansion and calculation of the intrinsic permeability via measurement of the injection pressure, flow rate, and knowledge of the system geometry. Advantages of this approach are elimination of questionable permeability measurements from weathered outcrop surfaces, provision of a superior sealing mechanism around the air injection zone, and its potential for making measurements at multiple depths below the outcrop surface.

Field-testing of the drillhole probe occurred along a 6x21-m sandstone outcrop of the shallow-marine Upper Cretaceous Straight Cliffs Formation near Escalante, Utah. Data obtained with a sample spacing of 15 cm along near-horizontal (along bed) transects and near-vertical (across-bed) profiles demonstrate facies-dependent variation in permeability values. Permeability ranges from 100-800 millidarcies in a massive-bedded, poorly to well-sorted, very fine to fine-grained, bioturbated sandstone facies, showing relatively little variability (less than plus or minus 25%) over a scale of several meters. The low degree of variation in permeability in the bioturbated facies is attributed to homogenization due to burrowing. In contrast, permeability ranges from 450-5,500 millidarcies in a poor to moderately sorted, fine- to coarse-grained, cross-bedded sandstone facies. Permeability in this facies varies by more than an order of magnitude over a scale of only a few centimeters. The high degree of permeability variation in the cross-bedded facies is attributed to small-scale variations in grain size and structure, related to depositional processes.

Thirty-two representative samples were taken from both facies for thin-section study. Petrographic analysis revealed that the lower permeability samples contain more clay than the higher permeability samples in each facies. The samples from the cross-bedded facies consist of angular, medium to coarse quartz and lithic grains, and contain larger pores than the finer-grained quartz sands of the burrowed facies. Permeability variations and lithologic characteristics in both facies can be correlated between vertical profiles. The contrast in permeability variation between the bioturbated and cross-bedded facies has important implications for subsurface fluid-flow prediction using outcrop analogs. The probe itself proved durable and robust, with a single probe being sufficient to make over 500 measurements in an arid and dusty environment. Data quality supports the conclusion that the drill-hole probe is a viable field instrument.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado