Abstract: Acritarch Fluorescence as a New Thermal Maturity Indicator

OBERMAJER, MARK, LAVERN D. STASIUK, MARTIN G. FOWLER, and KIRK G. OSADETZ

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 (Imax) 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 Imax and Q values of acritarchs vary little until the onset of oil generation. At the maturity level corresponding to Tmax <435oC, Imax values are commonly below 460nm whereas Q is below 0.5. As the zone of initial oil generation is approached the Imax 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 Imax and Q have been observed up to maturity level corresponding to Tmax=450oC.

AAPG Search and Discovery Article #90939©1997 AAPG Eastern Section and TSOP, Lexington, Kentucky