Fractured-aquifer hydrogeology from geophysical logs: Brunswick Group and Lockatong Formation, Pennsylvania

The Brunsnick Group and the underlying Lockatong Formation are composed of lithified Mesozoic sediments that constitute part of the Newark Basin in so utheastern Pennsylvania. These fractured rocks form an important regional a quifer that consists of gradational sequences of shale, siltstone, and sand stone, with fluid transport occurring primarily in fractures, An extensive suite of geophysical logs was obtained in seven wells located at the boroug h of Lansdale, Pennsylvania, in order to better characterize the areal hydr ogeologic system and provide guidelines for the refinement of numerical gro und water models. Six of the seven wells are approximately 120 m deep and t he seventh extends to a depth of 335 m. Temperature, fluid conductivity, an d flowmeter logs are used to locate zones of fluid exchange and to quantify transmissivities. Electrical resistivity and natural gamma logs together y ield detailed stratigraphic information, and digital acoustic televiewer da ta provide magnetically oriented images of the borehole wall from which alm ost 900 fractures are identified. Analyses of the geophysical data indicate that the aquifer penetrated by th e deep well can be separated into two distinct structural domains, which ma y, in turn, reflect different mechanical responses to basin extension by di fferent sedimentary units: 1. In the shallow zone (above 125 m), the dominant fracture population cons ists of gently dipping bedding plane partings that strike N46 degrees E and dip to the northwest at about 11 degrees. Fluid flow is concentrated in th e upper 80 m along these subhorizontal fractures, with transmissivities rap idly diminishing in magnitude with depth. 2. The zone below 125 m marks the appearance of numerous high-angle fractur es that are orthogonal to the bedding planes, striking parallel but dipping steeply southeast at 77 degrees. This secondary set of fractures is associated with a fairly thick (approxim ately 60 m) high-resistivity, low-transmissivity sandstone unit that is abr uptly terminated by a thin shale bed at a depth of 190 m. This lower contac t effectively delineates the aquifer's vertical extent at this location bec ause no detectable evidence of ground water movement is found below it. Thu s, fluid flow is controlled by fractures, but fracture type and orientation are related to lithology. Finally, a transient thermal-conduction model is successfully applied to simulate observed temperature logs, thereby confir ming the effects of ground-surface warming that occurred in the area as a r esult of urbanization at the turn of the century. The systematic warming of the upper 120 m has increased the transmissivity of this aquifer by almost 10%, simply due to changes in fluid viscosity and density.