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TitleSolid bitumen as a determinant of reservoir quality in an unconventional tight gas siltstone play
AuthorWood, J M; Sanei, H; Curtis, M E; Clarkson, C R
SourceInternational Journal of Coal Geology vol. 150-151, 2015 p. 287-295,
Alt SeriesEarth Sciences Sector, Contribution Series 20140434
Mediapaper; on-line; digital
File formatpdf
ProvinceAlberta; British Columbia
AreaWestern Canadian Sedimentary Basin
Subjectscore samples; pore structure; pore size; pyrobitumen; bitumen; hydrocarbons; thermal maturation; siltstones; Montney Formation; Rock-Eval; organic petrography; Triassic
Illustrationsphotomicrographs; schematic diagrams; graphs; histograms
ProgramShale Reservoir Characterization, Geoscience for New Energy Supply (GNES)
AbstractIn this study of the Triassic Montney tight gas siltstone play in the Western Canadian Sedimentary Basin petrophysical measurements of drill-core samples (porosity, pore throat size and water saturation) are integrated with pyrolysis data, organic petrography observations and SEM imaging to show that reservoir quality in the gas window is strongly influenced by the pervasive presence of solid bitumen (and pyrobitumen at higher thermal maturity). The solid bitumen formed as a pore-filling liquid oil phase that was diagenetically and thermally degraded with further burial and increase in temperature. The proportion of solid bitumen filling the intergranular paleopore network can be expressed as paleo-bitumen saturation (PBS) and this attribute is found to be the dominant control on pore throat size distribution. Samples with low PBS and large pore throat radius (>0.01 µm) have water saturations that generally increase as pore throat size diminishes, a relationship consistent with capillary theory for conventional water wet conditions. Samples with high PBS and small pore throat radius (<0.01 µm) have abnormally low water saturation, a condition inconsistent with capillary theory for conventional water wet rocks. The coincidence of small pore throat size, low water saturation and high PBS is attributed to the presence of well-connected nanopores within the devolatilized, solid bitumen and the hydrophobic nature of the bitumen. Siltstones in economic portions of the Montney tight gas fairway have porosity mostly in the range of 3 to 7% range. The results of this study show that reservoir quality in this economically key porosity range is influenced more strongly by PBS than by conventional determinants of porosity and permeability such as grain size, sorting, clay content and cementation. The concept of solid bitumen as an important control of reservoir quality elucidated here for Montney siltstones likely has application to the technical and economic evaluation of other tight gas plays particularly those in indirect basin-centered gas accumulations.
Summary(Plain Language Summary, not published)
This paper provides comprehensive data on roll of organic matter (solid bitumen) in reservoir quality of Montney tight gas.