Timing and Velocity of Petroleum Migration in the Los Angeles Basin

Petroleum in the Los Angeles Basin took a relatively short time to migrate through carrier beds from the deep basin to present-day reservoirs. We use a numerical model to reconstruct, beginning in Miocene time, subsidence, sedimentation, compaction, thermal evolution, and fluid migration in the basin's central block. The modeling indicates that the heat flow is currently near the continental average (1.5 HFU) but was higher (~2 HFU) during the early evolution of the basin. The thermal history predicted by the model suggests deeply subsided source rocks of late Miocene age began to generate oil about 2.2 Ma. In the simulations, compaction slowly drives groundwater from mature source beds toward reservoir rocks. These slow rates combined with the low solubility of petroleum in water preclude the possibility that oil migrated by a miscible process; instead, the oil must have moved as a phase separate from the groundwater. The buoyant force acting on the oil phase along the steeply dipping carrier beds was as much as an order of magnitude greater than the hydrodynamic force acting on both oil and water. This difference, along with the assumption that capillary forces segregated oil into the most porous and permeable laminae of the carrier beds, leads us to estimate conservatively that the oil migrated through the carrier beds at velocities 6 to > 100 times faster than water. According to our calculations, oil traversed the approximately 13 km from source beds to the West Coyote oil field in about 60,000 to 120,000 years, whereas groundwater required between 1.4 and 1.9 m.y. We calculated that oil had to saturate only a small portion of the carrier bed (~0.1%) to account for the amount of petroleum reaching the field.