The pristine and highly valued ecosystem of the Tallgrass Prairie Preserve, located in Osage County Oklahoma, was selected as the project study site. The risk associated with oil production at the site was estimated by combining the probability of an accidental produced fluid release with the severity of the consequences of the released fluids. Event tree analysis, reliability theory and Monte Carlo simulation are used to quantify the probability of occurrence of an event. The consequence estimate is based on the amount of material released, spreading of the released material, exposure assessment of vulnerable receptors, and cleanup costs. The financial consequences of accidental release events were estimated through (a) physical fate and transport model to calculate the extent and penetration depth of a spill; (b) the potential receptors and a risk factor were determined based on quantity of spill, distance from ground surface to groundwater table, distance to nearest potable water well from edge of impacted soil, and background groundwater quality, then; (c) the risk factor provides guidance for determining cleanup limits which coupled with the spill extent determines the cleanup costs. The results of the risk estimation is presented through ArcGIS maps of the study site.