The Elk Hills Oil Field has a complex stratigraphy compared to other nearby fields, many of which are single large pools in simple structural traps. Thirteen separate oil pools have been identified so far in the Elk Hills Field, in rock units ranging in age from Oligocene to Pleistocene. The shallowest formation, the Tulare, was the first in which oil was found, at below ground surface, and the deepest, the Oligocene portion of the Temblor containing the Agua Pool at a depth of , was not found until 1977.