TIMING AND VELOCITY OF PETROLEUM MIGRATION IN THE LOS-ANGELES BASIN

Petroleum in the Los Angeles Basin took a relatively short time to mig rate through carrier beds from the deep basin to present-day reservoir s. We use a numerical model to reconstruct, beginning in Miocene time, subsidence, sedimentation, compaction, thermal evolution, and fluid m igration 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 (similar to 2 HFU) during the early evolution of the basin. Th e thermal history predicted by the model suggests deeply subsided sour ce rocks of late Miocene age began to generate oil about 2.2 Ma. In th e simulations, compaction slowly drives groundwater from mature source beds toward reservoir rocks. These slow rates combined with the low s olubility of petroleum in water preclude the possibility that oil migr ated by a miscible process; instead, the oil must have moved as a phas e separate from the groundwater. The buoyant force acting on the oil p hase 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 force s segregated oil into the most porous and permeable laminae of the car rier beds, leads us to estimate conservatively that the oil migrated t hrough the carrier beds at velocities 6 to >100 times faster than wate r. According to our calculations, oil traversed the approximately 13 k m 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 c alculated that oil had to saturate only a small portion of the carrier bed (similar to 0.1%) to account for the amount of petroleum reaching the field.