Six stratigraphic intervals, representing one-third of Phanerozoic time, contain petroleum source rocks that have provided more than 90% of the world's discovered original reserves of oil and gas (in barrels of oil equivalent). The six intervals are (1) Silurian (generated 9% of the world's reserves), (2) Upper Devonian-Tournaisian (8% of reserves), (3) Pennsylvanian-Lower Permian (8% of reserves), (4) Upper Jurassic (25% of reserves), (5) middle Cretaceous (29% of reserves), and (6) Oligocene-Miocene (12.5% of reserves)

See Klemme 1994 for a detailed explanation on the origins of the Jurassic source rocks (i. e. 25% of the world's reserve at the time): most of them are marine, but some are fluvial or from lagoonal inland seas (table 3.2).

_{References:
Klemme, 1994. Petroleum systems of the world involving Upper Jurassic source rocks. In Maggon & Dow (Eds), The petroleum system - from source to trap, AAPG Memoir, 60: 51-72.
Klemme & Ulmishek, 1991. Effective petroleum source rocks of the world; stratigraphic distribution and controlling depositional factors AAPG Bulletin, 75: 1809-1851.
Thomas, 2013. Coal Geology, 2nd Edition. Wiley-Blackwell, 444pp.}