ABSTRACT

Significant accumulations of hydrocarbons have been trapped in Halfway-aged deltaic sediments in the Progress Area, Township 77 to 79, Range 8 to 10 W6M. Integration of sequence stratigraphic concepts with reservoir engineering data explains hydrocarbon distribution, reservoir quality and production performance of these pools.

Reservoir quality inner fringe sands of the Lower Halfway Formation grade upward from prodelta silts and shales of the Doig Formation and are overlain by progradational beach and lower shoreface facies of the Upper Halfway. Laterally, to the north and east, these clastics grade into a tight evaporitic sequence. Deeply incised channels, which mark the culmination of the highstand systems tract, and a major sea level fall, abruptly terminate the sequence to the south and west. Mud dominated fluvial and estuarine fill sediments in these channels comprise the lowstand and transgressive systems tracts of the next sequence. Conformable sedimentation continues into overlying and areally widespread evaporitic seals of the Charlie Lake Formation.

Lower Halfway sands, composed primarily of very fine quartz, contain mainly intergranular porosity which is partly occluded by a paragenetic sequence of silica overgrowths, dolomite and anhydrite cements. Reservoir quality, which ranges from 8 to 12 per cent and 1.5 to 14 mD decreases to the west as dolomitic mud increases. In contrast, Upper Halfway sands are coarser, contain more lithic rock fragments and chert, and have both intergranular and vuggy porosity. Reservoir development, which ranges from 3 to 12 per cent, increases in an orderly manner to the west. A petrographically distinct, well sorted, medium grained sandstone with 15 to 20 per cent porosity and up to 300 mD permeability occasionally caps the Upper Halfway. Occurrence of this sand as elongate lenses over estuarine fill sediments precludes affinity to the highstand systems tract of the first sequence.

Gas wells currently producing from only deltaic sands have an average deliverability of 150 E³m³ (after stimulation) while those containing medium grained sand attain deliverabilities of 250 E³m³. Although some downdip oil producers are in pressure communication with the gas cap, many appear from static gradient data to be separate, suggesting the presence of an estuarine filled channel. Small individual pools with variable hydrocarbon charge have accumulated where medium grained sand lenses occur over mud-filled channels. Material balance and pressure matching of these pools suggest sizes ranging from 64 to 390 hectares.

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