Cores from the Deep Sea Drilling Project in the Atlantic Ocean show widespread organic-rich black shales in the middle Cretaceous. However, geochemical studies indicate that the origin and petroleum potential of the organic matter are highly variable. Three main types of organic material can be recognized in these sediments from kerogen studies: (a) marine planktonic material, deposited in reducing environments; (b) terrestrial higher plants, moderately degraded; (c) residual organic matter, either oxidized in subaerial environments and/or sediment transit, or recycled from older sediments.

Vertical and horizontal variations of these three types of organic matter determined from geochemical logs in each main basin of deposition indicate the paleogeography and environment of deposition of organic-rich shales. The petroleum potentials of the sediments are therefore consequences of their paleogeographic settings. Thus, the zones favorable for oil and gas (given adequate maturation), or those devoid of any potential, can be suggested. Complementary studies of wells on the continental shelf of the North American continent indicate that the organic facies in the deep basins may extend to nearshore locations.