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TitleOrganic geochemistry of Silurian graptolitic shale and its petroleum source rock potential, Canadian Arctic Archipelago
AuthorObermajer, M; Dewing, K; Fowler, M G
Source27th International Meeting on Organic Geochemistry, IMOG 2015 Prague, book of abstracts; by IMOG Organizing Committee; 2015, 275-276 pages
LinksOnline - En ligne (PDF, 100 MB)
Alt SeriesEarth Sciences Sector, Contribution Series 20140514
PublisherEuropean Association of Organic Geochemists
MeetingInternational Meeting on Organic Geochemistry; Prague; CZ; September 13-18, 2015
Mediapaper; on-line; digital
File formatpdf
ProvinceNorthwest Territories; Nunavut
AreaCanadian Arctic Archipelago
Subjectsfossil fuels; organic geochemistry; source rocks; shales; hydrocarbons; thermal maturation; hydrogen index; graptolites; Cape Phillips Formation; Sverdrup Basin; Silurian
ProgramWestern Arctic Sverdrup Basin, GEM2: Geo-mapping for Energy and Minerals
AbstractThe widespread series of open marine graptolite-rich rocks that formed during the Ordovician-Silurian time in many areas of the world are amongst some of the most prolific petroleum source rocks. In the Canadian Arctic Archipelago this series is represented by basinal deep-water facies of the Cape Phillips Formation deposited on an enclosed embayment, beginning in the latest Ordovician and continuing for most of the Silurian. These strata, typically a few hundred meters thick and occurring over an area of approximately 500 km x 200 km, are known to contain intervals with elevated contents of organic matter and have been previously considered as one of the best potential hydrocarbon source rocks of the Franklinian successions. Despite that, this stratigraphic interval has received little attention in studies of petroleum systems of the Canadian Arctic which concentrated mostly on the Mesozoic sequences of the Sverdrup Basin where major hydrocarbon discoveries were made.
A Rock-Eval/TOC reconnaissance of over 500, mostly subsurface samples from the Cape Phillips Formation revealed TOC contents greater than 1% in the 80% of the analysed samples, with a maximum of 13.6% and an average of 2.3%. There are at least two main cycles of higher TOC within the unit that can be correlated throughout most of the formation occurrence area, a 50 to 75 m thick interval at the base and a second interval near the top of the formation. Tmax and PI values are commonly greater than 445oC and 0.40 respectively, indicating that most of the samples are thermally mature with respect to oil generation. Although the Hydrogen Index (HI) values approach 600 in some less mature samples, there is an apparent decrease in the HI values to less than 200 in samples with Tmax greater than 440oC. This change occurs over a 430oC to 440oC Tmax range (Fig. 1), suggesting that most petroleum might have been generated relatively early at what could otherwise be interpreted as the onset of the "oil window".
Extract yields, although quite variable, are commonly below 100 mg HC/g TOC, most likely reflecting hydrocarbon cracking, expulsion or destruction. Such interpretation is supported by the proportion of hydrocarbons in extracts which is greater than 40% in almost all of the extracts and greater than 60% in more than half of the samples, perhaps indicating hydrocarbon staining. Saturate fraction gas chromatograms are characterized by a smooth distribution of n-alkanes, often superimposed over a broad but low baseline hump. Pristane/phytane ratios are greater than 1.0 with an average of 1.4. Terpane and sterane biomarkers are fairly consistent amongst extracted samples. Terpane signatures are typically dominated by a large C30 hopane peak, smooth homohopane profile with the relative C31-C35 abundances decreasing with increasing carbon number and minor predominace of Ts over Tm. In several samples, however, these signatures appear quite mature as they are dominated by high abundances of tricyclic terpanes. Some of those samples also show low amounts of diasteranes relative to regular steranes. This characteristic might be source-related, as the presence of diastaranes in the Cape Phillips samples is otherwise more prominent. The regular steranes typically show C29>C27>C28 regular steranes pattern, likely reflecting contribution from several types of algae.
The overall biomarker characteristics are similar to those of the crude oil produced from Devonian reservoirs at Bent Horn. As the Cape Phillips Formation appears mature in close proximity to Bent Horn field, it has been considered as a main source of this oil. However, the pyrolysis data indicating possibility of an early generation of hydrocarbons within these strata could suggest potential for some occurrences of Cape Phillips sourced oil also in the areas of relatively lower thermal maturity of this formation, such as the south-eastern region of the Franklinian Basin.
Summary(Plain Language Summary, not published)
The study presents results of organic geochemical analyses of rock samples and organic extracts from the Cape Phillips Formation in the Canadian Arctic Archipelago. This unit occurs over an area of approximately 500 km x 200 km and is typically a few hundred meters thick. As it contains several intervals with elevated contents of organic matter it is considered as one of the best potential hydrocarbon source rocks and has been correlated to crude oil produced from Devonian reservoirs at Bent Horn field. Detailed organic geochemical data reported here include Rock-Eval/TOC pyrolysis, solvent extraction and gas chromatography, gas chromatography - mass spectrometry, and could be quite useful for studies and assessments of petroleum systems of the Canadian Arctic.