The Early Triassic Montney Formation in western Canada hosts a world-class unconventional petroleum accumulation with a complex history of hydrocarbon charging from both external and internal source rocks. This study focuses on self-sourced hydrocarbons and their intraformational migration within the siltstone-dominated Montney Formation. A review of recent geochemical studies highlights evidence of three main widespread episodes of intraformational hydrocarbon migration. The first episode was characterized by the migration of early-generated oil from internal Montney organic-rich source rocks during rapid burial. The second episode consisted of gas-condensate migration during deep burial and over-pressuring. The final episode involved methane-rich gas migration, mainly during basin uplift and depressurization. Spatial and temporal relationships of these three migration episodes fit a dynamic model of hydrocarbon generation, hydrocarbon migration and pressure evolution tied to basin subsidence and uplift history. Intraformational migration of gas and condensate has direct economic impacts on Montney well performance, such as higher gas-oil ratios and lower hydrocarbon liquid contents than expected from routine thermal maturity proxies. The Montney Formation has abundant publicly available subsurface data and thus provides a well-documented analogue for evaluating intraformational hydrocarbon migration in other unconventional petroleum accumulations.
Intraformational migration of hydrocarbons driven by the changing pressure, volume and temperature (PVT) conditions that accompany basin subsidence and uplift is gaining increased recognition as a common phenomenon in unconventional low-permeability petroleum accumulations (Han et al., 2015, 2019; Wood and Sanei, 2016; Zumberge et al., 2016; Ducros et al., 2017; Euzen et al., 2018, 2019, 2020, 2021; Wood et al., 2021a, 2022). Recognizing significant hydrocarbon migration episodes is important for assessing unconventional oil and gas plays because it provides a basis for understanding intricate geographic distributions of gas-oil ratio (GOR) or condensate-gas ratio (CGR) in terms of first-order thermal maturity trends and second-order migration trends (Wood and Sanei, 2016; Wood and Sanei, 2017; Wood et al., 2021a). Sound delineation of intraformational migration and consequent mixing of hydrocarbon fluids directly impacts play economics by enhancing the ability to target liquid-rich versus drier gas zones, depending on changing commodity prices or corporate resource-development strategy.
