Petroleum (and all of its byproducts) have shaped our world into what it is today. Discover where oil comes from, how it’s refined and how much of it is left in ground on this week's Please Explain. Dr. John B. Curtis is Professor in the Department of Geology and Geological Engineering at the Colorado School of Mines. Dr. William Fisher is a Professor in the Department of Geological Sciences and Bureau of Economic Geology at the University of Texas.