Abstract

The Mississippian carbonates of the Upper Debolt Formation, Sikanni Field, British Columbia, are gas-producing, fractured and brecciated, dolomitized rocks. These rocks have undergone complex diagenetic changes, represented mainly by various generations of calcite cementation and dolomitization. Reservoir porosity is represented by secondary, fracture, vuggy and intercrystalline porosity. Five types of dolomite have been identified: early dolomite, matrix dolomite, pseudomorphic dolomite, coarse crystalline dolomite, and saddle dolomite. Early dolomite (50-200 mu m) replaced both micrite and calcite cement of grainstones prior to appreciable compaction. Low delta 18 O values (-7.45 to -8.65 per mil PDB) relative to the postulated Mississippian marine carbonate values suggest that early dolomite may have been recrystallized by later fluids. Matrix dolomite (10-200 mu m) occurs along dissolution seams and replaces fossils of wackestone and packstone facies. Matrix dolomite formed during shallow to intermediate burial as suggested by its association with the dissolution seams and high Fe+2 and Mn+2 contents. However, its wide range of delta 18 O values (-3.8 to -10.03 per mil PDB), and radiogenic 87 Sr/ 86 Sr ratios (0.7085 to 0.7091) indicate that it may have been recrystallized by a later fluid. Pseudomorphic dolomite mimetically and nonmimetically replaces crinoids and it postdates the matrix dolomite. Coarse crystalline dolomite (500-2000 mu m) and saddle dolomite (500-1600 mu m) are co-genetically related to fractures and breccia horizons. Coarse crystalline dolomite pervasively replaced packstone and grainstone facies producing secondary intercrystalline porosity that was later partly occluded by pyrobitumen. Saddle dolomite partially to completely occludes fractures, brecciated areas, and vugs. The similarity of delta 18 O values for coarse crystalline dolomite (-8.14 to -9.85 per mil PDB), and for saddle dolomite (-7.82 to -10.8 per mil PDB) suggests that they were precipitated from comparable fluids. Fluid inclusion data show homogenization temperatures ranging from 87 to 214 degrees C (average 135 degrees C) for both dolomite types and salinity of ca. 8 wt. per cent equivalent NaCl. These data imply hot, slightly saline fluids were responsible for their formation. The radiogenic 87 Sr/ 86 Sr ratios for coarse crystalline dolomite (0.7086 to 0.7091) and for saddle dolomite (0.7087 to 0.7099) are atypical for Mississippian seawater suggesting an extraformational fluid. The geochemical and petrographic data, combined with the proximity of both dolomite types in relation to the thrust faults suggest that basinal fluids, somehow affected by meteoric waters, enriched in Mg (super 2+) , radiogenic 87 Sr/ 86 Sr, and depleted delta 18 O values were funneled upwards along faults and fractures which developed during Late Cretaceous to Early Tertiary time due to the Laramide Orogeny.

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