9706482 Goodwin Faults can act as either conduits or barriers to groundwater flow, and their permeability can change over time as a result of diagenesis and deformation in the fault zone. We propose to investigate the influence of faults on fluid flow by conducting an integrated geologic/hydrologic investigation of the spatial distribution of fault-zone permeability in poorly consolidated sediments. all previous studies of fault-zone permeability have focused on rocks, yet many important aquifers in the United States consist of poorly consolidated sediments. Our characterization of hydrogeologic units in fault zones in sedimentary aquifers will be based on detailed study of an exhumed normal fault cutting poorly consolidated Cenozoic sediments. The Sand Hill fault is exceptionally well exposed, providing extensive vertical and lateral sections for study of the geometry and continuity of structures and diagenetic features within the fault zone. The fault zone is distinguished by preferential cementation with respect to adjacent sediments, and includes elongate oriented zones of cement that record the orientation of groundwater flow at the time of precipitation. Preliminary studies of fault-zone structures and cement distribution indicate that fault-zone permeability characteristics vary in a predictable manner. We have constructed a conceptual model in which the width and permeability structure of the fault zone vary both laterally and vertically depending on the sediments juxtaposed by the fault. This variation is represented by two end members: (1) Where fine-grained, low permeability units such as silts and clays are juxtaposed by the fault, the zone of deformation is narrow and weakly cemented. (2) Where relatively coarse-grained, initially moderate to high permeability units such as coarse sands are juxtaposed by the fault, the zone of deformation is wide and moderately to strongly cemented. Detailed geologic mapping and petrographic studies of deformation features within the fault zone, cementation patterns both within and outside the fault zone, and stratigraphic variability of sediments will be used to refine this preliminary conceptual model of fault-zone hydrogeology. As a beginning point for understanding the influence of cementation on fluid flow, we will consider both uncemented and cemented states. The permeability variations of uncemented fault-zone hydrogeologic units and lithostratigraphic units cut by the fault will be determined using a combination of gas minipermeameters and falling head permeability measurements. The pre-cementation permeability will be estimated by dissolving cements and determining the permeability of repacked sediments as well as applying empirical formulae correlating grain size with permeability. One fundamental challenge in this study is to use the centimeter-scale permeability values to describe larger scale permeability characteristic of the sediments. Our approach to this problem will be to used the permeability data to characterize meter-scale hydrogelogic units. The larger-scale (i.e., deca to hectometer scale) hydrogeologic units can be considered as discrete structures and characterized by geological mapping and distribution of meter-scale measurements within the units. The conceptural model and quantitative measurements will then be used with classical geostatistical continuum methods based on variograms and cross-variograms as well as discrete statistical approaches to characterize the spatial distribution of hydrogeologic units and petrophysical properties within the Sand Hill fault zone, and to relate these to the spatial distribution and petrophysical properties of lithostratigraphic units juxtaposed by the fault. The comprehensive nature of this project makes it imperative that collaboration occur at all stages. Statistical design will be incorporated with data collection and initial analyses will be used to guide measurement strategies at the later stages.
