ABSTRACT

Recent data from the subsurface Tangent, Eaglesham and Normandville fields, located east of the Peace River Arch in northern Alberta, indicate that these Wabamun Group subtidal carbonates have undergone a complex diagenetic history related to multiple stages of fracturing and faulting. Burial dolomitization, which generated mainly matrix dolomites, was responsible for the development of porous reservoir rocks which are spatially highly variable.

Primary porosity was reduced by marine and early burial diagenesis, which formed micrite, syntaxial overgrowths, granular, blocky and coarse fracture-filling calcite cements. Minor amounts of microdolomite and floating dolomite rhombs formed during shallow burial. Further porosity reduction by mechanical compaction and cementation, except for minor dissolution, and initial fracturing, resulted in predominantly tight limestones.

Matrix dolomitization is associated with the second, most extensive phase of fracturing, which greatly increased both the porosity and permeability of the tightly cemented Wabamun limestones. Dolostones, which comprise 85% matrix dolomite, are the principal reservoir rock but have highly variable porosity. Large dissolution cavities developed locally after formation of the matrix dolomites.

The third phase of fracturing crosscuts the matrix dolomites. In the Tangent field, these fractures locally form irregular cavity systems which are partly filled with finely crystalline and saddle dolomites. These fine-crystalline dolomites probably represent dolomite silt and sand derived from brecciation of the earlier matrix dolomite during faulting. Renewed faulting was responsible for rotation of breccia clasts. Saddle dolomite was precipitated during at least two phases, on the sides and undersides of some clasts, and in fractures. Late-stage bladed anhydrites and fracture-filling coarse calcites precipitated during hydrocarbon generation, presumably in the Late Cretaceous, except locally where source rocks may have been heated by ascending hot brines.

The timing of dolomitization and fracturing is difficult to determine. Early fractures most likely occurred in the Late Devonian to Early Carboniferous during early burial and mechanical compaction. The second phase of fracturing probably occurred during the Mississippian to Pennsylvanian when the Normandville and Dunvegan faults underwent movement during the foundering of the Peace River Arch. The third phase of fracturing is linked to the more rapid subsidence and burial by the Cretaceous and Early Tertiary clastic wedge, which presumably reactivated some of the earlier faults.

The combined stratigraphic, petrophysical and geochemical data support the following interpretations. 1) Early dolomite rhombs and patches resulted from the movement of seawater through semi-lithified sediment near the sediment-water interface or during early compaction. 2) Matrix dolomites formed at temperatures of between 40 and 65°C and depths of about 500 to 1500 m. Probable fluid sources for matrix dolomites were Devonian brines buried with these and underlying sediments and expelled by mechanical compaction, possibly aided by seismic pumping, along stage II fracture-fault conduits. 3) Sucrosic, saddle and reworked dolomites postdate stage III fractures and occurred during deep burial. Most saddle dolomites formed at temperatures around 100°C, but a few formed near 200&de ;C. These dolomites formed from warm, highly saline, basin brines and/or from hot brines that ascended along fractures and faults and other stratigraphic conduits from deeper parts of the basin and/or the crystalline basement.