Dolomite Fluorescence and Its Exploration Significance in the Swan Hills Formation, Upper Devonian, Western Canada Sedimentary Basin

Paul E. Fejer

In the Upper Devonian Swans Hills Formation two distinct fluorescence colours are observed under blue-violet incident light (i.e. 405nm excitation): yellow and green. Thick (55m average) massively dolomitized areas (e.g. Kaybob South field) tend to fluoresce yellow, while areas with thin (3m average), facies controlled stringers of dolomite (e.g. Ante Creek field) tend to fluoresce green. Green fluorescing dolomites also form a halo around thick massively dolomitized areas (e.g. Caroline field). Where both yellow and green fluorescing dolomites occur together, the green fluorescing dolomites cross-cut and thus post-date the yellow fluorescing dolomites. The yellow and green fluorescing dolomites are geochemically distinct with the yellow fluorescing dolomites having higher ¹⁸O/¹⁶O and Mn/Fe ratios and higher Sr content.

Using a UMSP50 (univeral microscope spectro-photometer) and blue-violet incident light the emission spectrum of the yellow fluorescencing dolomites is 620 nm and is probably caused by Mn²⁺ replacing Ca²⁺ in the dolomite structure. The intensity of yellow fluorescence varies from weak to strong and is probably a function of the Fe/Mn ratio. The emission spectrum of the green fluorescing dolomites is in the 500-560nm range. The intensity of green fluorescence generally weak and is thought to be due to a rare earth or other trace element concentrations too small to quantify using inductively coupled plasma/mass spectrometry analytical techniques.

The yellow and green fluorescing dolomites are interpreted to have formed from either a single fluid with the green fluorescing dolomites representing mature end member of that fluid; or from two distinct fluids. With a single fluid model a reduction in the hydraulic head may explain the change from pervasive yellow fluorescing dolomites to the more facies specific green fluorescing dolomites. Since yellow fluorescing dolomites in the Swan Hills are associated with more significant reservoir development than green fluorescing dolomites, evidence of yellow fluorescing dolomites in sparsely explored areas may be a useful indicator of significant reservoir development in the area.

AAPG Search and Discover Article #91019©1996 AAPG Convention and Exhibition 19-22 May 1996, San Diego, California