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.
Keywords: Solid migrabitumen, Montney Formation, Reservoir quality,
Wettability, Rock-Eval, Extended slow heating (ESH), Fluid-like hydrocarbon
residue (FHR).
