The Early Triassic Montney Formation is a world-class tight gas play in the Western Canadian Sedimentary Basin, mainly composed of siltstone. The majority of the organic matter in the Montney siltstone consists of solid migrabitumen. This represents a former liquid oil phase which migrated into the larger paleo-intergranular pore space. Physicochemical changes in the oil led to precipitation of asphalt aggregates. These asphalt aggregates were then consolidated into solid migrabitumen while being subjected to thermal cracking (or pyrobitumen at higher thermal maturity). Petrophysical measurements of drill-core samples across the basin in conjunction with organic geochemistry and petrographic observations show that reservoir quality in the Montney tight gas is strongly influenced by the pervasive presence of pore-occluding solid migrabitumen. Solid migrabitumen obstructs porosity and hinders fluid flow, and thus shows a strong negative correlation with reservoir qualities such as porosity and pore throat size. Bitumen saturation is the proportion of solid migrabitumen filling the intergranular paleopore network. This is the dominant control on pore throat size and absolute permeability. In the economic portions of the Montney tight gas fairway, siltstones are found to have porosities in the range of 3 to 7%. The conventional determinants of porosity and permeability, such as grain size, sorting, clay content and cementation, have less of an influence on the reservoir quality in this economically key porosity range than the bitumen saturation.