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

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 S_h and S_H of 3.5 and 9MPa, respectively. Above the sands, "thrust' regime conditions prevail, although the amount by which S_h exceeds S_v is undetermined since instantaneous shut-in pressures (ISIPs) were clipped at the level of S_v 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 S_h in the sand beds themselves and a lower limestone remains at least as great as S_v despite the decline in shale stress. Hence stress constrasts between these beds and neighboring shales become pronounced with depth. The orientation of S_H 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