unclear whether this was due to the COVID-19 pandemic; however, the Eagle Ford formation appeared to have peaked in 2015 and the Bakken has reached well saturation in top-producing regions. Unconventional sources As of 2019, oil considered unconventional is derived from multiple sources. Oil shale is a common term for sedimentary rock such as shale or marl, containing kerogen, a waxy oil precursor that has not yet been transformed into crude oil by the high pressures and temperatures caused by deep burial. The term "oil shale" is somewhat confusing, because what is referred to in the U.S. as "oil shale" is not really oil and the rock it is found in is generally not shale. Since it is close to the surface rather than buried deep in the earth, the shale or marl is typically mined, crushed, and retorted, producing synthetic oil from the kerogen. Its net energy yield is much lower than conventional oil, so much so that estimates of the net energy yield of shale discoveries are considered extremely unreliable. Oil sands are unconsolidated sandstone deposits containing large amounts of very viscous crude bitumen or extra-heavy crude oil that can be recovered by surface mining or by in-situ oil wells using steam injection or other techniques. It can be liquefied by upgrading, blending with diluent, or by heating; and then processed by a conventional oil refinery. The recovery process requires advanced technology but is more efficient than that of oil shale. The reason is that, unlike U.S. "oil shale", Canadian oil sands actually contain oil, and the sandstones they are found in are much easier to produce oil from than shale or marl. In the U.S. dialect of English, these formations are often called "tar sands", but the material found in them is not tar but an extra-heavy and viscous form of oil technically known as bitumen. Venezuela has oil sands deposits similar in size to those of Canada, and approximately equal to the world's reserves of conventional oil. Venezuela's Orinoco Belt tar sands are less viscous than Canada's Athabasca oil sands – meaning they can be produced by more conventional means – but they are buried too deep to be extracted by surface mining. Estimates of the recoverable reserves of the Orinoco Belt range from to . In 2009, USGS updated this value to . Coal liquefaction or gas to liquids product are liquid hydrocarbons that are