STRESS AND STRENGTH HETEROGENEITY IN THE NEWARK RIFT BASIN

In situ stresses have long been considered to be relatively uniform over large regions away from faults and zones of active tectonic processes. When examined at a local scale in sedimentary basins, however, the in situ stress field often reveals variations at the scale of individual layers even in tectonically passive environments. We present results of an analytical geomechanics study in the Newark Basin, one of a series of Mesozoic rift basin in Eastern North America, which has been considered for geologic carbon storage. The magnitude and orientation of the principal horizontal stresses are evaluated using a combination of direct borehole measurements and observations of borehole failure. While the average maximum NE stress direction (SHmax) is consistent across different parts of the basin, significant variability of SHmax with depth is observed along a 1.5-km-deep stratigraphic hole, where high-resolution borehole images allow for quantitative analysis and modeling of in situ stress indicators. Some of these variations can be attributed to heterogeneity and anisotropy in geomechanical formation properties, as observed in extensive laboratory and logging data. The analysis of these data indicates that the fluvial and lacustrine formations in the Newark Basin are characterized by high strength and significant variability in their geomechanical properties, which correlate poorly with lithology and mineral content of the rock. The heterogeneity of formation strength, however, cannot fully explain variations in the observed in situ stress indicators. Borehole modeling suggests that local stress variations must exist at the scale of meters to hundreds of meters in these formations. This study highlights the importance of evaluating in situ stresses at local scale, and contributes to quantifying the heterogeneity of formation strength and in situ stress field in Mezosoic rift basins and similar tectonic settings.