Conceptual Model of Ground Water Flow in Fractured Siltstone Based on Analysis of Rock Cores, Borehole Geophysics, and Thin Sections

Abstract

The flow of ground water in flat‐lying sedimentary rocks of the Appalachian Plateau is significantly influenced by the vertical and horizontal distribution of vertical joints and horizontal bedding plane fractures. Although considerable published data document the geographic distribution of joints in outcrops in this region, few data have been published on the vertical distribution of joints and on the vertical and horizontal distribution of bedding plane fractures. The vertical and horizontal distribution of bedding plane fractures at a site located in central New York was investigated by examination of borehole geophysical logs, continuous rock cores from boreholes drilled to a maximum depth of 175 feet, and rock outcrops. This investigation reveals that three zones of bedding plane fractures can be delineated based on the vertical distribution of horizontal fractures, and that the spacing between these horizontal fractures increases with depth. The occurrence of vertical joints is not noticeably influenced by depth. Because bedrock at this site consists of several hundred feet of relatively homogeneous, thin‐bedded, fractured siltstone (the Genesee Formation) having essentially no visible microscopic intergranular porosity, ground water flows exclusively through fractures. Caliper, density, temperature, and neutron logs indicate that submillimeter‐wide fractures serve as conduits that transmit ground water. Slug test data reveal that shallower wells exhibit hydraulic conductivities 10 to 100 times greater than those of deeper wells, confirming that increased (wider) fracture spacing at depth results in a lower permeability. Because the spacing of horizontal fractures increases with depth, ground water flow in shallow bedrock is expected to be very different than that in deeper bedrock. Because both vertical joints and horizontal bedding plane fractures are conduits in shallow bedrock, flow might approximate equivalent porous media conditions. However, flow in deeper bedrock might approach fracture flow because bedding plane fractures are sparse and the principal conduits are only vertical joints. These differences can have important implications for modeling ground water flow.

Estimating the hydraulic properties of the fracture network in a sandstone aquifer

“…As a result, modifications (described in Section 5.2.3) were made to previous CCTV interpretations. Comparison of CCTV and calliper logs showed poor agreement in locating fractures: in addition, repeatability for calliper logs was poor, suggesting that many of the fractures give rise to borehole wall irregularities which are close to the detection limit of the device (as also concluded by Merin, 1992). Accordingly, calliper data were disregarded.…”