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Repeat formation tester (RFT, mark of Schlumberger) pore pressure data from the lower Tuscaloosa Formation (Upper Cretaceous) are analyzed across a pressure transition zone in two gas fields of the Tuscaloosa trend, Louisiana, documenting a pressure seal within a sandstone-dominated interval that maintains a pressure anomaly of ∼20 MPa. Pressure measurements within the formation span a depth interval of 5500–6060 m and are limited to the crests of rollover structures associated with growth faults; the onset of overpressure occurs at a depth between 5620 m and 5690 m in two fault blocks, while the strata are displaced across the fault by 100 to 120 m, suggesting that the pressure seal may be near-horizontal, crosscutting stratigraphic boundaries. The pressure seal zone is as thin as 38 m and occurs within interbedded sandstones and shales. Overpressures in all wells of the Moore-Sams Field and in two wells of the Morganza Field follow local hydrostatic gradients, indicating that portions of the overpressured zone are in fluid communication though isolated from shallower, normally pressured fluids. In other wells, overpressures increase with depth in a stair-step manner to 117 MPa at depths of 5.9 km. The occurrence of the pressure seal in interbedded sandstones and shales where higher permeability is expected suggests that the sandstones of the seal zone are unusually tight. A companion petrographic study of sandstones near the pressure seal suggests that extreme compaction of the sandstones after dissolution of grain-supporting cements may have contributed to the low permeability of the seal zone.

We studied the correlation between near-surface in situ stress and the preferred orientation of microcracks at two quarries in the Milford granite and one quarry in the Conway granite, New Hampshire. The orientation of in situ stress was determined by overcoring doorstoppers and from the strike of induced borehole fractures produced by a pressurized packer. The preferred orientation of vertical microcracks was determined using ultrasonic measurements to determine P wave velocity Vp on core and in situ and from thin section analyses. In situ Vp anisotropy was determined from interborehole travel time data. At all three sites, directions of maximum compressive stress σ1 and induced borehole fractures are aligned with the preferred orientation of open microcracks determined from core Vp and thin section data. An analysis of the microcracking sequence within each pluton suggests (1) that the quarry grain resulted from cooling-induced thermal stresses and (2) a method of distinguishing the paleostress axes at the time of cooling from contemporary stress.

Strain relaxation stress measurements were made in the Mojave Desert southeast of Palmdale, California, at two sites during the summers of 1979 and 1980, using the U.S. Bureau of Mines technique to depths of about 30 m. The field data and finite element modeling studies demonstrate that thermally induced stress dominates the results obtained in the upper 6 m. At depths greater than 6 m the average orientation for the horizontal maximum compressive stress at these sites is N22°W±9° at 2 km south of the San Andreas fault and N13°W±2° at 20 km north of the fault. These azimuths compare favorably with the average of N21°W determined with nearby hydrofracture stress measurements (Zoback et al, 1980). Savage et al. (1981) also found a NNW orientation for the maximum shortening from a geodetic network with a 15-km aperature in the Palmdale area. The fact that essentially the same orientation is recovered by three different techniques which sample to different depths and over different areal extents argues strongly for a contemporary tectonic origin for the stress. Finite element models of the San Andreas fault in southern California develop a stress field similar to that observed regionally (∼15°E for the maximum compressive stress) away from the fault when displacements corresponding to relative motion between lithospheric plates are applied on the boundaries of the models. Near the fault, however, the model principal stresses are rotated counterclockwise similar to those measured near Palmdale, demonstrating the influence of the faults on the principal stress orientations.

Detailed stress measurements in three boreholes penetrating horizontally bedded Devonian silt-stones, sandstones, and limestones above a prominent salt decollement in the Appalachian Plateau of western New York have revealed variations in horizontal stress magnitudes which correlate with lithologic and stratigraphic units in all wells. High differential stress levels (up to 20 MPa) were found in shales of very high clay content, contrary to the proposition that such materials have negligible long-term strength. Elastic modulus data show that stiffer beds generally host higher stress levels and suggest that sand/shale stress contrasts result in large part from elastic shortening of the section in response to regional ENE compression. No correlation between stress and Poisson's ratio was found. However, a major systematic drop in stress level within the generally massive shales, which occurs across a group of sand beds near the base of the Rhinestreet formation, appears to be of different origin. The stress offset corresponds to the top of a section which we conclude, on the basis of local and regional total strain data derived from chlorite fabric measurements, once hosted abnormally high pore pressures. The total strain data also suggest the entire section above the salt has been uniformly shortened during Alleghanian compression. To explain the stress discontinuity, two kinematic patterns for Alleghanian deformation of the Devonian section are proposed, both involving abnormal pore pressure development in the sub-Rhinestreet section in response to limited drainage of fluid. Drainage of this paleo-overpressure is the best available explanation of the stress offset, although an additional remnant stress component must also be present to satisfy the stress data precisely.

At 11 outcrops within 100 km of Blue Mountain Lake, New York, we measured strain relaxation during overcoring of “surface,” “doorstopper,” and Bureau of Mines borehole deformation gauges. The majority of measurements showed a maximum expansion e1 parallel with the contemporary tectonic stress field. To confirm further the orientation of in situ stress, at two sites, vertical fractures were induced at borehole walls using a packer-fracturing technique. Several cores from each site were then tested for mechanical anisotropy using ultrasonic, compressibility, and thin section analyses. The orientations of mechanical anisotropy had a poor correlation with the preferred orientation of microcracks observed in thin section. The various techniques for measuring in situ stress orientations gave internally consistent results where e1 generally aligned with topographic contours and often the mechanically stiff direction of the core. Furthermore, et aligned with the known contemporary tectonic stress, local p axes of earthquakes, Precambrian structures, and local joints. We interpret the alignment of e1 and other structures to be the result of a feedback between the contemporary tectonic stress (ENE in the northeastern United States) and the process of jointing during the development of local topography. Hence e1 is controlled by local structures and is a reflection of the contemporary tectonic stress but not a direct measure of it.

The direction of maximum expansion during strain relaxation of the Tully Limestone at Ludlowville, New York, is oriented about NO4°E and thus is within a couple of degrees of the strike of both set Ia cross-fold joints and the compression direction indicated by the teeth of tectonic stylolites. These joints and stylolites are members of a suite of structures accommodating approximately 9% layer-parallel shortening during the main phrase of the Alleghanian Orogeny within the Appalachian Mountains. Because of the parallelism of the maximum expansion with the main phase Alleghanian compression and because our measurements were made in joint-bounded blocks, we suggest that the expansion represents the relief of a residual stress locked into the Tully Limestone during the Alleghanian Orogeny. The magnitude of the strain relaxation indicates that the residual differential stress was 14 MPa. Using a flow law for pressure solution, we infer that the Tully Limestone deformed at a strain rate of about 3×10−15s−1, and thus the layer-parallel shortening observed in the Tully Limestone may have required an aggregate deformation interval of only 1 m.y.

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. A total of 75 stress measurements were made in three wells a kilometer or so apart using a wireline-supported hydraulic-fracturing system. 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 offset in Sh and SH of 3.5 and 9 MPa, 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 despite 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 contrasts between these beds and neighboring shales become pronounced with depth. The contrast in Sh and SH between the lowermost sand and the immediately underlying shale is at least 6 and 14.5 MPa, respectively. SH levels in the lower strike-slip regime are about 1.75 times greater than Sh and are less than the value required to initiate slippage on favourably oriented frictional interfaces. For the upper thrust regime and the sand and limestone beds, however, the inferred lower bound on SH is close to the slippage threshold for a Coulomb friction coefficient of 0.6. 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. Significantly different orientations were obtained for adjacent tests in which almost identical ISlPs were observed, suggesting that the fractures quickly reorient themselves to propagate normal to the least principal stress direction. Similarly, vertical traces were observed in those tests where ISIPs apparently reflect Sv, suggesting that rotation to horizontal was rapid.

From plots of in situ stress versus depth it is apparent that horizontal stress is several times the vertical stress at depths of