HYDROTHERMAL DOLOMITIZATION OF THE MISSISSIPPIAN UPPER DEBOLT FORMATION, SIKANNI GAS-FIELD, NORTHEASTERN BRITISH-COLUMBIA, CANADA

The Mississippian carbonates of the Upper Debolt Formation, Sikanni Fi eld, British Columbia, are gas-producing, fractured and brecciated, do lomitized rocks. These rocks have undergone complex diagenetic changes , represented mainly by various generations of calcite cementation and dolomitization. Reservoir porosity is represented by secondary, fract ure, vuggy and intercrystalline porosity. Five types of dolomite have been identified: early dolomite, matrix dolomite, pseudomorphic dolomi te, coarse crystalline dolomite, and saddle dolomite. Early dolomite ( 50-200 mu m) replaced both micrite and calcite cement of grainstones p rior to appreciable compaction. Low delta(18)O values (-7.45 to -8.65 parts per thousand PDB) relative to the postulated Mississippian marin e carbonate values suggest that early dolomite may have been recrystal lized by later fluids. Matrix dolomite (10-200 mu m) occurs along diss olution seams and replaces fossils of wackestone and packstone facies. Matrix dolomite formed during shallow to intermediate burial as sugge sted by its association with the dissolution seams and high Fe+2 and M n+2 contents. However, its wide range of delta(18)O values (-3.8 to -1 0.03 parts per thousand PDB), and radiogenic Sr-87/Sr-86 ratios (0.708 5 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 dolo mite (500-2000 mu m) and saddle dolomite (500-1600 mu m) are co-geneti cally related to fractures and breccia horizons. Coarse crystalline do lomite pervasively replaced packstone and grainstone facies producing secondary intercrystalline porosity that was later partly occluded by pyrobitumen. Saddle dolomite partially to completely occludes fracture s, brecciated areas, and vugs. The similarity of delta(18)O values for coarse crystalline dolomite (-8.14 to -9.85 parts per thousand PDB), and for saddle dolomite (-7.82 to -10.8 parts per thousand PDB) sugges ts 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 Sr-87/Sr-8 6 ratios for coarse crystalline dolomite (0.7086 to 0.7091) and for sa ddle dolomite (0.7087 to 0.7099) are atypical for Mississippian seawat er suggesting an extraformational fluid. The geochemical and petrograp hic data, combined with the proximity of both dolomite types in relati on to the thrust faults suggest that basinal fluids, somehow affected by meteoric waters, enriched in Mg2+, radiogenic Sr-87/Sr-86, and depl eted delta(18)O values were funneled upwards along faults and fracture s which developed during Late Cretaceous to Early Tertiary time due to the Laramide Orogeny.