Petrographic and Molecular Characterization of Organic-Rich Mudstones with Petroleum Generative Potential from The...

The Triassic Montney Formation hosts major oil and gas resources in Western Canada. Despite significant historical development of these resources, the origin of its hydrocarbons remains unclear, partly due to limited evidence of primary organic matter within the formation. Most of the hydrocarbons in the Triassic Montney Formation are trapped in low-permeability siltstone facies. For the purposes of this study, however, we investigated the lesser-known and understudied organic- and clay-rich mudstone beds, typically ranging from 2 to 8 cm in thickness, which are interbedded with the siltstone reservoir facies. Petrographic and organic geochemical analyses were conducted on core samples from a well located in west-central Alberta with thermal maturity of 0.95-1.16%Ro. Petrographic analysis, including kerogen maceral composition and fluorescence of petroleum fluid inclusions, reveals that the organic matter in the Montney Formation is closely linked to its host lithology. Organic-rich mudstone facies, with TOC values up to 3.5 wt %, contain primary kerogen (32-74 vol %) and solid bitumen (29-68 vol %). In contrast, the more organic-lean siltstone facies (TOC < 0.8 wt %) contain primarily pore-filling solid bitumen and lack structured kerogen. Kerogen within the organic-rich mudstone facies is mainly composed of liptinite macerals, including amorphinite, liptodetrinite, and alginite, subordinate amounts of acritarch and sporinite, and traces of inertinite and vitrinite macerals. The primary organic matter in the mudstone facies is presumed to be derived from marine sources (i.e., phytoplanktonic algae and bacterial biomass). Its preservation may result from the balance between periodic high productivity (potentially driven by river floods), rapid sedimentation to bury and preserve organic matter, and low dilution by siliciclastic material. Organic petrology analysis suggests an in situ generation of hydrocarbons in the mudstone beds and a potential subsequent short-distance migration to adjacent siltstone beds. Biomarker analysis supports the genetic linkage between rock extracts from these beds. Additionally, three families of petroleum fluid inclusions were systematically observed (using fluorescence microscopy) in several sets of contiguous siltstone and mudstone beds. These findings suggest that the organic-rich mudstone facies partially contributed to the present-day hydrocarbons of the Middle Montney Member in the study area.