Acritarch Fluorescence as a New Thermal Maturity Indicator
OBERMAJER, MARK; STASIUK, LAVERN D.; FOWLER, MARTIN G. and OSADETZ, KIRK G., Geological Survey of Canada, Calgary, Alberta, Canada
The relative fluorescence of acritarchs (400-700nm range) was investigated
as an alternative technique of determining the level of thermal maturity
of Ordovician to Devonian source rocks in Canada. The sedimentary strata
examined include the Blue Mountain, Lindsay (Collingwood Mbr), Guelph (Eramosa
Mbr) and Marcellus formations from southern Ontario, Yeoman, Winnipegosis
and Manitoba strata from Saskatchewan, as well as the Elk Point Group from
Alberta. These strata contain oil-prone, predominantly marine organic matter
(Type II and I) with varying proportions of bituminite and alginite as
dominant maceral components. Acritarchs, which occur as persistent maceral
inclusions within such organic facies, show excellent potential not only
for paleodepositional interpretations but also for thermal maturity assessment.
In general, the fluorescence properties of acritarchs parallel those
of Leiosphaeridia alginite. However, at the same maturity level the maximum
wavelength of the fluorescence intensity (lmax) and the red/green quotient
(Q) values are usually lower for acritarchs. Correlations with geochemical
maturity indicators, such as Rock-Eval Tmax and the isomerization ratio
of regular steranes (S/S+R, bb/aa+bb), indicate that for a Type II kerogen
both lmax and Q values of acritarchs vary little until the onset of oil
generation. At the maturity level corresponding to Tmax<435°C, lmax
values are commonly below 460nm whereas Q is below 0.5. As the zone of
initial oil generation is approached the lmax shows a sudden shift to 500nm,
and then progressively increases throughout the oil window (500-600nm).
However, these changes are accompanied by only a subtle increase in Q values.
For a Type I kerogen no significant variations in lmax and Q have been
observed up to maturity level corresponding to Tmax=450°C.
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