James R. Boles

Mantle helium is a significant component of the helium gas from deep oil wells along the Newport‐Inglewood fault zone (NIFZ) in the Los Angeles (LA) basin. Helium isotope ratios are as high as 5.3 Ra (Ra = ³He/⁴He ratio of air) indicating 66% mantle contribution (assuming R/Ra = 8 for mantle), and most values are higher than 1.0 Ra. Other samples from basin margin faults and from within the basin have much lower values (R/Ra < 1.0). The ³He enrichment inversely correlates with CO₂, a potential magmatic carrier gas. The δ¹³C of the CO₂ in the ³He rich samples is between 0 and −10‰, suggesting a mantle influence. The strong mantle helium signal along the NIFZ is surprising considering that the fault is currently in a transpressional rather than extensional stress regime, lacks either recent magma emplacement or high geothermal gradients, and is modeled as truncated by a proposed major, potentially seismically active, décollement beneath the LA basin. Our results demonstrate that the NIFZ is a deep‐seated fault directly or indirectly connected with the mantle. Based on a 1‐D model, we calculate a maximum Darcy flow rate q ∼ 2.2 cm/yr and a fault permeability k ∼ 6 × 10⁻¹⁷ m² (60 microdarcys), but the flow rates are too low to create a geothermal anomaly. The mantle leakage may be a result of the NIFZ being a former Mesozoic subduction zone in spite of being located 70 km west of the current plate boundary at the San Andreas fault.

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.