Effects of episodic fluid flow on hydrocarbon migration in the Newport-Inglewood Fault Zone, Southern California

摘要

Fault permeability may vary through time due to tectonic deformations, transients in pore pressure and effective stress, and mineralization associated with water-rock reactions. Time-varying permeability will affect subsurface fluid migration rates and patterns of petroleum accumulation in densely faulted sedimentary basins such as those associated with the borderland basins of Southern California. This study explores the petroleum fluid dynamics of this migration. As a multiphase flow and petroleum migration case study on the role of faults, computational models for both episodic and continuous hydrocarbon migration are constructed to investigate large-scale fluid flow and petroleum accumulation along a northern section of the Newport-Inglewood fault zone in the Los Angeles basin, Southern California. The numerical code solves the governing equations for oil, water, and heat transport in heterogeneous and anisotropic geologic cross sections but neglects flow in the third dimension for practical applications. Our numerical results suggest that fault permeability and fluid pressure fluctuations are crucial factors for distributing hydrocarbon accumulations associated with fault zones, and they also play important roles in controlling the geologic timing for reservoir filling. Episodic flow appears to enhance hydrocarbon accumulation more strongly by enabling stepwise build-up in oil saturation in adjacent sedimentary formations due to temporally high pore pressure and high permeability caused by periodic fault rupture. Under assumptions that fault permeability fluctuate within the range of 1-1000 millidarcys (10(-15)-10(-12)m(2)) and fault pressures fluctuate within 10-80% of overpressure ratio, the estimated oil volume in the Inglewood oil field (approximately 450 million barrels oil equivalent) can be accumulated in about 24000years, assuming a seismically induced fluid flow event occurs every 2000years. This episodic petroleum migration model could be more geologically important than a continuous-flow model, when considering the observed patterns of hydrocarbons and seismically active tectonic setting of the Los Angeles basin.