ILLUMINATING THE INTERMEDIATE ZONE: EXAMINING SEPARATION BETWEEN OIL AND GAS PRODUCTION AND GROUNDWATER RESOURCES

For decades, the oil and gas industry has been drilling wells both vertical and horizontal in areas that contain important fresh and slightly saline water resources and wells. From these wells, we have a plethora of data about the geology and hydrogeology of aquifers and oil and gas plays. The knowledge gap lies in the areas between oil and gas wells and water wells known by some as the “intermediate zone.” The intermediate zone can be over a 1 km thick, can have non-economic oil and gas lenses, and often has complex geology with highly variable lithology, structures, faults, and fractures. Water in the intermediate zone is commonly brackish to saline and frequently contains methane and other hydrocarbons that can contaminate groundwater resources. Here we examine the intermediate zone in the Western Canadian Sedimentary Basin and several sedimentary basins in the United States. Geologic and hydrogeologic attributes such as thickness, porosity, lithology and water chemistry of the intermediate zone were compiled for each basin. The proximity of oil and gas wells to water wells was also examined. This is of particular concern in areas such as the Michigan and Illinois basins where oil and gas wells and water wells are vertically within a few hundred meters of each other and high salinity groundwater is present at shallow depths. In other cases where oil and gas wells and water wells are in close proximity, such as the Big Horn Basin, waters in the intermediate zone have low salinity water but may contain hydrocarbons. Solute transport models were created to assess transport times across the intermediate zone. Preliminary results suggest that a wide range of transport times are possible depending on intermediate zone thickness and permeability as well as the presence or absence of conduits such as faults or leaky wells.