Total U.S. crude oil production grew by 1.24 million barrels per day (b/d) (11%) in 2019. This increase in production was led by relatively light, less dense crude oil and was largely the result of the growth in crude oil production from shale and tight rock formations. Shale and tight rock formations are now more accessible because of the continued advances in horizontal drilling and hydraulic fracturing. U.S. refinery inputs have also become lighter over the years as refineries use less of the heavier, imported crude oil and more of the lighter, domestically produced crude oil to process into petroleum products.