oil for what is included in the model, and "unconventional" oil for classes excluded. In 1956, Hubbert confined his peak oil prediction to that crude oil "producible by methods now in use." By 1962, however, his analyses included future improvements in exploration and production. All of Hubbert's analyses of peak oil specifically excluded oil manufactured from oil shale or mined from oil sands. A 2013 study predicting an early peak excluded deepwater oil, tight oil, oil with API gravity less than 17.5, and oil close to the poles, such as that on the North Slope of Alaska, all of which it defined as non-conventional. Some commonly used definitions for conventional and unconventional oil are detailed below. Conventional sources Conventional oil is extracted on land and offshore using "standard" (i.e., in common use before 2000) techniques, and can be categorized as light, medium, heavy, or extra heavy in grade. The exact definitions of these grades vary depending on the region from which the oil came. Light oil flows naturally to the surface or can be extracted by simply pumping it out of the ground. Heavy refers to oil that has higher density and therefore lower API gravity. It does not flow easily, and its consistency is similar to that of molasses. While some of it can be produced using conventional techniques, recovery rates are better using unconventional methods. According to the International Energy Agency, production of conventional crude oil (as then defined) peaked in 2006, with an all-time maximum of 70 million barrels per day. Tight oil was typically classified as "unconventional" prior to about 2006, but more recent analyses began to consider it to be "conventional" as its extraction became more common. It is extracted from deposits of low-permeability rock, sometimes shale deposits but often other rock types, using hydraulic fracturing, or "fracking." It is often confused with shale oil, which is oil manufactured from the kerogen contained in an oil shale (see below), Production of tight oil has led to a resurgence of US production in recent years. U.S. tight oil production peaked in March 2015, and fell a total of 12 percent over the next 18 months. But then U.S. tight oil production rose again, and by September 2017 had exceeded the old peak, and as of October 2017, U.S. tight oil production was still rising. Tight oil as a whole peaked in late 2019, although it is