Northern Michigan has significant natural gas reserves along the Antrim shale formation in northern Michigan. By some estimates it is the 15th largest gas field in the nation. Drilling activity peaked in the late 1980s and early 1990s, In 2014, Encana, the Canadian company who had been drilling in Northern Michigan, sold their mineral rights to Marathon Oil order to focus on more profitable operations elsewhere. For oil interest, Encana amassed rights for the Collingwood-Utica Shale (Michigan) between 2008 and 2010, mostly in Cheboygan, Kalkaska, Michigan and Missaukee counties. The Collingwood layer is two miles below the surface and would require horizontal drilling.