LINKING CRUSTAL STRESSES WITH HYDROGEOLOGIC PROPERTIES DETERMINED FROM GEOPHYSICAL WELL LOGS NEAR ALPINE, TEXAS

As part of a regional groundwater investigation, geophysical logs were obtained in several municipal water wells located in the Sunny Glen area near the west Texas City of Alpine. Of these wells, two that intersect extrusive rocks of Tertiary age are of primary interest. The first, 308 m deep, was drilled near the center of an east-west trending valley and the other, 275 m deep, along its northern flank. Analysis and interpretation of the logs indicate that rocks penetrated by the two boreholes have significantly different stress environments and exhibit significantly different hydrogeologic properties. Acoustic televiewer logs reveal stress-induced breakouts having orientations that shift abruptly by 90° with increasing depth, from which a polarity reversal in horizontal principal stresses is inferred. An analytical stress model that considers the effects of preferentially oriented valleys on gravity and tectonically induced subsurface stresses is applied to account for this behavior. Pumping tests and concurrent flowmeter logs indicate that water production is associated with: (1) the upper and lower contacts of a shallow, dense trachyte unit, and (2) deeper zones of highly fractured rocks within the interior of a basalt formation. Transmissivities of the trachyte boundaries in the central valley well are twice those of the same features in the ridge flank well, whereas the transmissivity of the deeper basalts is an order of magnitude greater in the flank well than it is in the same rocks of the central well. This discrepancy is examined from the perspective of fracture opening and flow enhancement by computing magnitudes of normal and deviatoric stress invariants as functions of depth and using these values to quantify a Drucker-Prager stability factor. Results show slightly unstable stress conditions throughout the central well that correlate with intervals of moderate transmissivity and highly unstable conditions in the lower part of the flank well that correlate with intervals of significantly greater transmissivity. Thus, the field data and subsequent stress analysis examined within the context of a valley-ridge setting offer evidence of a coupled tectonic-hydrologic interaction at this site.