The geodynamics of faults and petroleum migration in the Los Angeles basin, California

Abstract

A two-dimensional multiphase fluid flow model, coupled with heat flow, using a hybrid finite element and finite volume method is developed and applied to fundamental issues of long-distance petroleum migration and accumulation in the Los Angeles basin, which is intensely faulted and deformed by transpressional tectonic stresses, yet host to the world's richest oil accumulation.

These simulations show that a combination of continuous hydrocarbon generation and primary migration from upper Miocene (∼10 – 5 Ma) source rocks in the central Los Angeles basin synclinal region, coupled with subsiding basin fluid dynamics, favored the massive accumulation and alignment of hydrocarbon pools along the Newport-Inglewood fault zone (NIFZ). According to our multiphase flow calculations, the maximum formation water velocity within fault zones likely ranged between 0.5 to 1.0 m/yr during the middle Miocene to Pliocene epochs (13–2.6 Ma). The estimated time for long-distance (∼25 km) petroleum migration from source beds in the central basin to oil fields along the NIFZ is approximately 90,000 to 220,000 years, depending on the effective permeability assigned to the faults (5 ∼ 50 md) and adjacent inter-bedded sandstone and siltstone “petroleum aquifers.” With an average long-distance flow rate of 0.2 m/yr and fault permeability of 20 md (2.0 × 10⁻¹³ m²), the total petroleum volume of the Inglewood oil field (450 million barrels ≈ 1.6 × 10⁵ m³) would have accumulated over a period of 180,000 years or less. The results also suggest that besides the thermal and structural history of the basin, the fault permeability, capillary pressure, and the configuration of aquifer and aquitard layers played an important role in controlling petroleum migration rates, patterns of flow and saturation, and the overall fluid mechanics of petroleum accumulation.