ABSTRACT

The Middle Triassic (Ladinian) Halfway Formation of the Peejay Field, located in northeastern British Columbia, is an oil and gas play. It has been interpreted as a prograding barrier island shoreline sequence. A subsurface study involving the description of 120 cores and interpretation of 320 geophysical well logs has enabled the interpretation of reservoir facies, distribution, geometry, and quality. The construction of facies cross-sections and maps, extensive use of petrophysical information, and thin section samples has aided the location and interpretation of lateral and vertical heterogeneities in reservoir quality.

The tidal-inlet coquina sediments are of good reservoir quality, and their distribution and orientation form two laterally adjacent fairways striking northwest-southwest. Surrounding these sediments are crossbedded, porous, shoreface sands.

It has been observed that 82 per cent of the cores studied contain either bioclastic or coquina sediments. The average coquina and bioclastic sandstone thicknesses are 5.76 ft. and 3.42 ft., respectively, thus the pools are very thin. Only 38 per cent of these cores contain a stacked sequence of inlet-fill sediments. Present observations have shown that the reservoir quality of shoreface sands may be as good as that of the inlet-fill sediments. Reservoir lateral continuity may be better in the former, although diagenetic effects must also be considered and mapped.

Secondary diagenetic processes create biomouldic and interparticulate porosity by the leaching of the molluscan coquina inlet sediment, or may reduce reservoir quality by the precipitation of cements that plug up pore throats, reduce pore throat diameter, and increase the tortuosity.

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