 These kinds of rocks have high porosity (around 35%) but low permeability, allowing considerable oil to accumulate in the formation, while a combination of structural and stratigraphic traps keep the oil from migrating further upward.Throughout the history of the field, these upper units have been the most productive, and they were also the first to be discovered, in 1909.Other productive units include the Etchegoin, of Pliocene age (this same unit is productive in many oil fields in the southwestern San Joaquin Valley); the Reef Ridge Formation, of Miocene age; the McDonald/Devilwater pool in the Monterey Formation, of Miocene age; the Agua (Santos) pool in the Temblor formation, of Oligocene age; and the Kreyenhagen Formation, of Eocene age.