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

摘要

The processes affecting sulfur during diagenesis in carbonates have been investigated by ion microprobe analysis of delta(34)S values of pyrite, marcasite, and anhydrite from the Devonian Nisku Formation in the Western Canada Sedimentary Basin. Pyrite and marcasite from three Nisku wells have delta(34)S ranging between -35 and +20 parts per thousand CDT; most lie between -35 and -5 parts per thousand CDT. The delta(34)S values of sulfides are heterogeneous within individual thin section-sized samples, varying by as much as 25 parts per thousand. The delta(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 over 100 mu m in individual samples. The appearance of native sulfur is accompanied by an increase of up to 30 parts per thousand in pyrite delta(34)S values. Bulk and ion probe analyses of anhydrite are relatively uniform (delta(34)S = +22 to +30 parts per thousand CDT). The predominately low delta(34)S values of Nisku sulfides indicate bacterial sulfate reduction. Textural relations indicate that Fe-sulfide formation in Nisku carbonates may have occurred by two different mechanisms. Much of the sulfide has delta(34)S values that suggest that it was associated with bacterial sulfate reduction, although most Fe-sulfides did not form until after pervasive matrix dolomitization (depths of 300-1000 m). Other sulfide may have formed later, during deep (similar to 4 km) burial via thermochemical sulfate reduction. The range in delta(34)S values in a single thin section and correlations between pyrite morphology and isotopic values suggest that sulfate reduction was a very localized process, and that the sulfate reduction environment varied considerably on a small scale. [References: 63]