Petrographic and Geochemical Evidence for Burial Diagenetic Processes: Example from Devonian Swan Hills Formation (Rosevear Field), Alberta, Canada

Jonathan Kaufman

The Rosevear field of the west-central Alberta subsurface is comprised of Middle to Upper Devonian platform and reef-complex deposits of the Swan Hills Formation. The reservoir is formed by massive replacement dolostones, which occur adjacent to a marine channel within the reef complex. Dolomitization resulted in biomoldic and vuggy porosity that was partially filled by, in order of decreasing age, saddle dolomite, anhydrite, bitumen, coarse-crystalline calcite, and pyrite.

The anhydrite has a low ⁸⁷Sr/⁸⁶Sr ratio (0.7083) compared to the significantly more radiogenic values (> 0.7100) measured in saddle dolomite and coarse-crystalline calcite. Furthermore, the ^dgr³⁴S composition of the anhydrite ranges from 15 to 23^pmil, values which are lower than the ^dgr³⁴S estimated for Upper Devonian seawater sulfate (25^pmil). A potential source for the strontium and sulfur in the anhydrite is the underlying Middle Devonian evaporites of the Elk Point Group. Middle Devonian seawater strontium and sulfur isotopic values are compatible with the isotopic chemistry of anhydrite. A smaller component of formation water with radiogenic strontium and heavy sulfur was also involved in anhydrite precipitation.

Thermochemical sulfate reduction (TSR) has resulted in calcitization of anhydrite as indicated by the occurrence of partially digested anhydrite crystals (in optical continuity) floating within coarse-crystalline calcite. These calcites have variably depleted carbon isotopic compositions ranging from -5 to -13^pmil but show a narrow oxygen isotope spread (-7.3 to -8.7^pmil). The association of calcite with bitumen suggests that isotopically light carbon was released during oxidation of hydrocarbons. Pyrite enriched in ³⁴S(^dgr³⁴S = 26.5^pmil) and the presence of 8% hydrogen sulfide in reservoir gas are also indicative of TSR.

Minimum temperature estimates for the onset of TSR range from 80° to 140°C. In the absence of abnormally high geothermal gradients at the Rosevear field, these temperatures would have been reached upon burial to approximately 2,000 m--depths achieved in the Late Cretaceous-early Tertiary.

AAPG Search and Discovery Article #91022©1989 AAPG Annual Convention, April 23-26, 1989, San Antonio, Texas.