Influence of faults on late-stage migration of methane-rich gas in a major unconventional hydrocarbon accumulation: Montney Formation, western Canada

Abstract

Faults are thought to act as barriers or conduits for intraformational hydrocarbon migration within unconventional low-permeability hydrocarbon accumulations. However, well-documented studies substantiating this phenomenon at the regional-scale are scarce. Here, we use an extensive molecular and isotopic dataset for produced-gas samples (n = 562) from the Montney unconventional hydrocarbon accumulation of northeast British Columbia to show that regional-scale secondary migration of methane-rich gas was significantly influenced by the distribution and orientation of faults. Cross-plots using iC4/nC4, δ13C ethane and δ13C propane exhibit single coherent distributions, suggesting that the C2+ components of most of these gases share interrelated histories of generation, maturation and primary migration. Conversely, cross-plots using gas dryness and δ13C methane against other selected gas parameters demonstrate geographic differentiation, with samples from the southeastern part of the study area generally exhibiting elevated molecular concentration and isotopic enrichment of methane compared to the samples from the northwestern part of the study area. The compositional and isotopic enrichment of methane in the southeastern samples signify significant late-stage secondary migration of highly mature methane-rich gas from deeper zones to shallower zones, and mixing with in-situ hydrocarbon fluids of lower thermal maturity. Previous structural mapping documents the presence of numerous subsurface thrust-faults related to the Cordilleran orogeny in the northwestern area, while these features are entirely absent in the southeastern area. Consequently, we conclude that thrust-faults, with accompanying clay-rich gouges, formed effective en-echelon barriers to the late-stage secondary migration of methane-rich gas in the northwestern area and prevented substantial mixing of hydrocarbon fluids.