be present, also destroy the catalysts used in refining. Olefins form insoluble sediments and cause instability. The oxygen within the oil, present at higher levels than in crude oil, lends itself to the formation of destructive free radicals. Hydrodesulfurization and hydrodenitrogenation can address these problems and result in a product comparable to benchmark crude oil. Phenols can be first be removed by water extraction. Upgrading shale oil into transport fuels requires adjusting hydrogen–carbon ratios by adding hydrogen (hydrocracking) or removing carbon (coking). Shale oil produced by some technologies, such as the Kiviter process, can be used without further upgrading as an oil constituent and as a source of phenolic compounds. Distillate oils from the Kiviter process can also be used as diluents for petroleum-originated heavy oils and as an adhesive-enhancing additive in bituminous materials such as asphalt. Uses Before World War II, most shale oil was upgraded for use as transport fuels. Afterwards, it was used as a raw material for chemical intermediates, pure chemicals and industrial resins, and as a railroad wood preservative. As of 2008, it is primarily used as a heating oil and marine fuel, and to a lesser extent in the production of various chemicals. Shale oil's concentration of high-boiling point compounds is suited for the production of middle distillates such as kerosene, jet fuel and diesel fuel. Additional cracking can create the lighter hydrocarbons used in gasoline. "Pale sulfonated shale oil" (PSSO), a sulfonated and ammonia-neutralized variant named "Ichthammol" (chemical name: Ammonium bituminosulfonate) is still in application today. Reserves and production Global technically recoverable oil shale reserves have recently been estimated at about of shale oil, with the largest reserves in the United States, which is thought to have . Worldwide production of shale oil was estimated at in 2008. The leading producers were China (), Estonia (), and Brazil (). The production of shale oil has been hindered because of technical difficulties and costs. In March 2011, the United States Bureau of Land Management called into question proposals for commercial operations in Colorado, Utah and Wyoming, stating that "(t)here are no economically viable ways yet known to extract and process oil shale for commercial purposes". The US Energy Information Administration sometimes uses the phrase "shale (tight) oil" to refer to tight oil, "crude oil ... produced directly from tight oil resources". In 2021, the US produced 7.23 million barrels of such tight oil each