Abstract

Petrologic and geochemical investigations on several Devonian Crossfield reservoirs west of the Crossfield trend, Alberta, provided new evidence for the role of reflux brines on the formation of early-formed pervasive fine-crystalline matrix dolomite and modification of porosity in these reservoirs. Other associated diagenetic processes during early and late burial history include compaction, recrystallization of early matrix dolomite, precipitation of secondary anhydrite, blocky calcite, saddle dolomite and thermal sulphate reduction (TSR)-related diagenetic minerals.

The evidence for density-driven reflux from the Stettler brines for the formation of the pervasive fine-crystalline matrix dolomite include:

1. dolomitization occurs in sediments that were relatively uncompacted, and the degree of preservation of the original fabrics, such as fenestrae and peloidal grains, is quite significant in the dolomicrite and fine- to medium-crystalline matrix samples;

2. matrix dolomite is relatively fine- to medium-crystalline and ranging from (<10 μm–200 μm) with the mean size range of <100 μm;

3. the fabric of the matrix dolomite ranges from planar-S to nonplanar and is typically non-luminescent to very dully luminescent. Replacement of precursor limestone is generally mimetic, with most original depositional structures preserved;

4. the development of sub-parallel, anastomosing stylolites that were sub-sequently healed by matrix dolomitization processes suggests that the sediments underwent shallow burial prior to dolomitization. However, subsequent fabric-destructive episodes of stylolitization and the development of dissolution seams cross-cutting dolomite matrix fabrics indicate that most major chemical compaction events occurred with increasing depth and after the pervasive dolomitization of the precursor matrix;

5. oxygen and carbon isotopic composition of the dolomites ranges from δ¹⁸O of −6.17 to −4.79‰ (VPDB) and δ¹³C values of 0.22 to 1.47‰ (VPDB) for all of the dolomicrite and fine-crystalline dolomite matrix samples. These values overlap and/or are slightly lower than the postulated Famennian seawater isotopic signatures, probably representing precipitation from evaporated Famennian seawater. The amount of negative shift in these isotopes is likely the result of recrystallization processes that have affected the matrix dolomite during subsequent burial diagenesis; and

6. ⁸⁷Sr/⁸⁶Sr ratios of the matrix dolomites (0.70805 to 0.70830) indicate that the dolomicrite and fine-crystalline dolomite precipitated from Famennian (or slightly evaporated) seawater. The ⁸⁷Sr/⁸⁶Sr ratios of primary evaporite are also close to Famennian seawater values, which further suggest that the dolomicrite and fine-crystalline matrix dolomite precipitated from Famennian seawater. Medium-crystalline and coarse-crystalline and matrix dolomites are more radiogenic (0.70949 to 0.71053) and, in conjunction with oxygen/carbon values, may further confirm that recrystallization processes have diagenetically altered matrix dolomites during the burial process.