Tar sandstones & tar sands are oil-impregnated reservoir rocks that have been subjected to long periods of weathering and petroleum degradation.
The largest deposit in the world is the Athabasca Tar Sand of Alberta, Canada. Although oil prices noticeably skyrocketed starting in 2004, Albertan tar sands have been economically exploited for many years, starting in the 1970s. It is expected & very likely that Canada will be expanding the mining & processing of Athabasca material into crude oil in the future.
Tar sandstone from the 10-12 million year Monterey Formation (Miocene) of southern California, USA.
Many areas in southern California have significant accumulations of petroleum. Natural petroleum seeps can be seen in places (for example, at La Brea Tar Pits in the Los Angeles-Hollywood).