FREEMAN, S.D. (AP) — TransCanada estimates that about 16,800 gallons of oil leaked from the Keystone pipeline into a field in South Dakota. The Canadian energy company says it reported the estimate today (THUR) to the National Response Centre and the Pipeline and Hazardous Materials Administration. TransCanada says the estimate is based on the excavation of soil to expose more than 100 feet of pipe and takes into account a number of factors including oil observed in the soil and the potential area impacted. The company says the leak was reported Saturday but haven't said what caused the leak. The pipeline is shut down while officials investigate. Some 100 workers are at the site, about four miles from the Freeman pump station in Hutchinson County. The pipeline runs from Alberta, Canada, through Kansas and other states, on its way to refineries in Illinois and Oklahoma.