LAWDERMILK, WENDY FAYE, University of Tennessee, Dept. of Geology, Knoxville, TN

Abstract: Rules-Based Quantitative Description of Bed-Normal Joint Patterns in Sedimentary Rocks

In order to develop a more complete characterization of joint network patterns this study will combine geostatistical and geomechanical methods to model bed-normal joints. Joints occur in virtually all rock types and geologic settings, influencing mechanical strength, fluid migration and storage, and geomorphology. Bednormal joints are a particularly prevalent fracture pattern in sedimentary rocks, regardless of bedding dip. However, joint networks are difficult to characterize due to the number of variables that contribute to network geometry and formation.

The Devonian Brallier and the Silurian Mifflintown Formations in central Pennsylvania will be the rock units used as field sites for the investigation. These formations consist of thin beds (10-20 cm) cut by bed-normal joints. Each site will be examined for particular characteristics: (1) bed thickness; (2) stratigraphic sequence; (3) spacing and size of joints; (4) geometric interactions of different joint sets; (5) persistence of joints as a function of lithology, fracture spacing and fracture size; (6) lateral proximity of joint terminations across bedding surfaces; and (7) growth histories from fractographic features.

These characteristics will be used to establish a probabilistic description of the geometry of the joint patterns; to relate fracture patterns to growth histories; and to describe the mechanical controls on joint pattern geometry in terms of lithology, bedding characteristics and fracture mechanics. This rules-based approach provides an opportunity to predict approximate characteristics of joint systems in equivalent rock masses with more limited fracture data. 

AAPG Search and Discovery Article #90925©1999 AAPG Foundation Grants-in-Aid