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TitleDetermination of hydrocarbon generation and expulsion temperature of organic-rich Upper Ordovician shales from Hudson Bay and Foxe basins using modified hydrous pyrolysis, organic petrography, Rock-Eval analysis and organic solvent extraction
AuthorReyes, J; Jiang, C; Lavoie, D; Milovic, M; Robinson, R; Zhang, S; Armstrong, D; Mort, A
SourceGeological Survey of Canada, Open File 8049, 2016, 60 pages, (Open Access)
PublisherNatural Resources Canada
Documentopen file
Mediaon-line; digital
File formatpdf
ProvinceNorthern offshore region; Ontario; Nunavut
NTS43; 44; 45; 54; 55; 25; 35; 36; 46
AreaHudson Bay; Hudson Strait; Foxe Channel; Southampton Island; Baffin Island
Lat/Long WENS -94.0000 -68.0000 66.0000 50.0000
Subjectsgeochemistry; fossil fuels; geochemical analyses; petrographic analyses; mudstones; shales; source rocks; vitrinite reflectance; bitumen; hydrocarbons; hydrocarbon generation; rock-eval analyses; Ordovician
ProgramHudson/Ungava Hydrocarbon source rocks, GEM2: Geo-mapping for Energy and Minerals
Released2016 11 02
Immature organic-rich Ordovician mudstone and limy shale source rocks were artificially matured using modified hydrous pyrolysis in an attempt to characterize the thermal maturation history and hydrocarbon generation scenario. The experimental hydrous pyrolysis temperature range of 310-350 oC for 72 hours corresponds to hydrocarbon generation (HCG) at laboratory timescales. The Rock-Eval (RE) results show that the Tmax of the pyrolyzed samples increased from 413 oC to 450 oC after the last stage (350 oC) of hydrous pyrolysis. Solvent extraction and RE analysis of pyrolyzed rock chips indicates that Tmax suppression is associated with the high amount of free hydrocarbons and soluble solid bitumen or extractable organic matter in the samples.
The estimated vitrinite reflectance equivalent values calculated from the measured Tmax (0.67 to 1.03 %Roeqv-ext), chitinozoan (0.68 to 1.1 %Roeqv) and bitumen (0.75 to 1.08 %Roeqv) reflectance are in excellent agreement and indicate that all artificially matured samples reached HCG at 310°C. Organic petrographic analysis of the pyrolyzed rock chips from hydrous pyrolysis shows that bituminite and other amorphous kerogen are the first macerals to thermally degrade, followed by prasinophyte alginite. Some of the pore spaces created by the thermally degraded amorphous kerogen and bituminite are partially filled by soluble heavy bitumen. The solubility of the solid bitumen and the amount of free hydrocarbons decreases, and the fluorescence intensity of liptinite macerals shifts from bright yellow to reddish orange with increasing pyrolysis temperature. Free hydrocarbons in fracture and intergranular pore spaces were also observed in the rock matrix after hydrous pyrolysis.
Peak hydrocarbon generation occurs between pyrolysis temperatures of 330 and 350 oC with measured Tmax values of 439 to 455 oC after solvent extraction. The quantity of generated hydrocarbons ranges from 22.75 to 166.22 mg HC/g TOC. Comparisons of the calculated TRs of HI and S2 from unextracted and extracted pyrolyzed rock chips clearly indicate that the unextracted samples underestimate the residual hydrocarbon potential of the pyrolyzed source rocks. The underestimation appears to be higher during the formation of heavy oil and soluble bitumen, which declines exponentially as the pyrolysis temperature increases.
Comparative analysis of the geochemical data from the offshore wells from the Hudson Bay Basin and the results of this study suggest that the observed large Tmax variation among the samples from the offshore wells cannot be attributed to Tmax suppression associated with a high amount of soluble organic matter and labile hydrocarbons. Almost all of the RE parameters show no indication of in-situ oil generation and migration. Petrographically, none of these previously analyzed offshore Upper Ordovician formations showed any direct evidence of in-situ hydrocarbon generation and migration unlike those observed in the artificially matured samples. If indeed the measured and estimated Tmax values (>435 oC) are the effective maximum and current burial temperatures of these organic rich formations, migrated solid bitumen and free hydrocarbon will be visible within the intergranular pore spaces of the rock matrix at this level of thermal maturity.
Summary(Plain Language Summary, not published)
Four immature organic rich mudstone and limestone shales collected from Upper Ordovician hydrocarbon source rocks from the Hudson Bay and Foxe basins in northern Canada were artificially matured using modified hydrous pyrolysis. The study aimed at determining their thermal maturation, phases and kinetics of petroleum generation. The 72 hour pyrolysis temperatures were 310, 320, 330, 340 and 350oC; these temperatures range is within the hydrocarbon generation window. Organic geochemistry and petrographic analyses before and after hydrous pyrolysis stages show that all samples reached hydrocarbon generation based on the measured Tmax and estimated vitrinite reflectance equivalent. Hydrocarbons were expelled after 310 and 320oC; peak expulsion rate was reached after the 330 and 350oC pyrolysis temperatures. Expulsions rate varies between formations, ranging from 22.75 to 166.22 mg HC/g TOC. The estimated transformation ratio (TR) of hydrogen index is between 22 to 82% and 47 to 92% for S2.