Solid bitumen in the Montney Formation: Diagnostic petrographic characteristics and significance for hydrocarbon migration

The petrographic study of organic matter (OM) in fine-grained, shale and tight rocks can be complicated by ambiguities in distinguishing primary kerogen (deposited OM) from secondary solid bitumen (migrated and solidified petroleum). In this study, complementary thin-section, field emission scanning electron microscopy (FESEM) and organic petrology methods are used to examine core samples acquired from petroleum wells drilled into the Triassic Montney Formation in western Canada. The results show that OM in the medium- to coarse-grained siltstones of the Montney tight-gas and hydrocarbon liquids fairway is composed almost entirely of pore-filling solid bitumen/pyrobitumen. Here, we document a range of petrographic characteristics that are diagnostic of the solid bitumen and its origin as migrated liquid petroleum. These solid bitumen characteristics include (1) void-filling habit, (2) enclosure of earlier authigenic minerals, (3) margins with smoothly curved meniscate form, (4) internal flow structure, (5) entrained and aligned clay minerals, (6) isopachous rims along inorganic pore walls, (7) globular/granular aggregates and (8) fracture-filling habit. The petrographic results also show that the migration and flow of original oil through the thick Montney Formation (up to 340 m) was controlled at the micro-scale by sedimentary, ichnological, diagenetic and structural features. These features combined to form highly complex pathways and baffles for hydrocarbon migration. Petrographic recognition of OM as secondary migrated solid bitumen rather than primary deposited kerogen has vital significance for assessing unconventional shale and tight hydrocarbon plays including OM typing from Rock-Eval pyrolysis, interpreting depositional facies and environments, and evaluating reservoir quality.