Hydrogeologic structure of the Newark Basin, Pennsylvania, from borehole geophysical logs

Abstract

The Brunswick and the underlying Lockatong Formations are comprised of littrifled Mesozoic sediments that constitute part of the Newark Basin in southeastern Pennsylvania (USA). These fractured rocks form an important regional aquifer that consists of gmdational sequences of shale, siltstone, and sandstone, with fluid transport occurring primarily in fractures. An extensive suite of geophysical logs was obtained in seven wells located at the borough of Iamsdale, PA in order to characterize the areal hydrogeologic system. Analyses of these data indicate that the aquifer can be separated into two distinct structural domains which may, in turn, reflect different mechanical responses to basin bxtension: (1) In the shallow zone above 125 m, the dominant fmcucre population consists of gertfly dipping bedding-plane partings that strike N46°E and dip to the NW at about 11°. Fluid flow is concentrated within the upper 80 m and transmissivities rapidly diminish in magnitude with depth. (2) The zone below 125 m marks the appearance of numerous steeply dipping fractures that are orthogonal to the bedding plane features, striking subparallel at N227°E but dipping sharply to the SE at 77°. This secondary set of fractures is associated with a reasonably thick (= 60 m) high-resistivity, low- transmissivity sandstoned siltstone unit that is abruptly terminated by a thin shale bed at a depth of 190 m. This lower contact effectively delineates the aquifer's vertical extent and the observed lack of hydraulic productivity with increasing depth may be associated with the gradational transition into the Lockatong Formation.