SULFIDE FORMATION IN RESERVOIR CARBONATES OF THE DEVONIAN NISKU FORMATION, ALBERTA, CANADA - AN ION MICROPROBE STUDY

The processes affecting sulfur during diagenesis in carbonates have be en investigated by ion microprobe analysis of delta(34)S values of pyr ite, marcasite, and anhydrite from the Devonian Nisku Formation in the Western Canada Sedimentary Basin. Pyrite and marcasite from three Nis ku wells have delta(34)S ranging between -35 and +20 parts per thousan d CDT; most lie between -35 and -5 parts per thousand CDT. The delta(3 4)S values of sulfides are heterogeneous within individual thin sectio n-sized samples, varying by as much as 25 parts per thousand. The delt a(34)S values increase by up to 50 parts per thousand with increasing well depth in two different Nisku wells, and delta(34)S also increases by up to 15 parts per thousand as grain size increases from 40 to ove r 100 mu m in individual samples. The appearance of native sulfur is a ccompanied by an increase of up to 30 parts per thousand in pyrite del ta(34)S values. Bulk and ion probe analyses of anhydrite are relativel y uniform (delta(34)S = +22 to +30 parts per thousand CDT). The predom inately low delta(34)S values of Nisku sulfides indicate bacterial sul fate reduction. Textural relations indicate that Fe-sulfide formation in Nisku carbonates may have occurred by two different mechanisms. Muc h of the sulfide has delta(34)S values that suggest that it was associ ated with bacterial sulfate reduction, although most Fe-sulfides did n ot form until after pervasive matrix dolomitization (depths of 300-100 0 m). Other sulfide may have formed later, during deep (similar to 4 k m) burial via thermochemical sulfate reduction. The range in delta(34) S values in a single thin section and correlations between pyrite morp hology and isotopic values suggest that sulfate reduction was a very l ocalized process, and that the sulfate reduction environment varied co nsiderably on a small scale.