Shell Oil has agreed to spend at least $US115 million ($A126.04 million) to cut harmful pollution at a Houston-area refinery. The oil giant will also pay a $US2.6 million civil penalty under the settlement announced on Wednesday. Shell settled with the US Environmental Protection Agency and the Department of Justice after it was accused of violating the federal Clean Air Act. Shell will install a $US1 million system to monitor cancer-causing benzene levels along the fence line of its Deer Park facility. The data will be publicly available on a website.