Hard oil refers to a type of heavy viscous oil that is difficult to refine or extract. It is often found in shale deposits and tar sands which require specialized methods to extract. Synonyms for this type of oil include heavy crude, extra-heavy oil, and bitumen. These terms refer to the same type of oil that is dense and has a high viscosity, making it flow less easily than conventional oil. Heavy crude is typically more expensive to refine due to its complex chemical makeup and high sulfur content. Despite these challenges, hard oil represents a significant potential source of energy for the future as traditional sources of oil become more scarce.