Appalachian stress study. 1. A detailed description of in situ stress variations in Devonian shales of the Appalachian Plateau

Abstract

We describe an experiment to measure variations in the state of stress within a horizontally bedded Devonian shale/sandstone/limestone sequence in western New York. The stress profiles indicate that a major drop in horizontal stress level occurs in the generally massive shales. This drop occurs principally across the lowermost member of a group of sand beds and corresponds to an offest in Sh and SH of 3.5 and 9MPa, respectively. Above the sands, "thrust' regime conditions prevail, although the amount by which Sh exceeds Sv is undetermined since instantaneous shut-in pressures (ISIPs) were clipped at the level of Sv due to fracture rotation. Below the sands, the regime is strike slip with both horizontal stresses showing lateral uniformity depsite substantial variations in topography. The magnitude of Sh in the sand beds themselves and a lower limestone remains at least as great as Sv despite the decline in shale stress. Hence stress constrasts between these beds and neighboring shales become pronounced with depth. The orientation of SH is ENE with a standard deviation of 20°. Fracture traces were usually splayed, occasionally spanning 30° of well bore. No systematic correlation between mean orientation and lithology is evident. -from Authors