The Middle Triassic Halfway Formation is a significant producer of oil and gas in the eastern British Columbia and western Alberta portions of the Western Canada Basin. This formation produces hydrocarbons primarily from stratigraphic traps.
The Halfway Formation was deposited on the northeastern shoreline of an epicontinental seaway that occupied much of western Canada during Middle Triassic time. The primary locus of sediment influx to the basin was located in northeastern British Columbia. From that area, sediment was transported eastward and southeastward under the influence of longshore currents set up by waves approaching from the west. During periods of shoreline progradation, wave-dominated deltas formed in eastern British Columbia, and barrier/strand-plain deposits accumulated downdrift in the western portion of Alberta. These barrier/strand-plain deposits were backed by arid, hypersaline lagoons, tidal flats, and sabkhas.
Two general stratigraphic sequences are recognized within the Halfway shoreline sandstones: (1) an upward-coarsening sequence consisting of offshore marine shales and siltstones, overlain by cross-bedded shoreface sandstones and capped by foreshore flat-bedded sandstones; and (2) a coarse-grained, tidal inlet-fill sequence consisting of flat-bedded to cross-bedded sandstone and shell-hash conglomerates.
In barrier island areas, the upward-coarsening sequence predominates. Because waves reworked the sediments deposited above the fair-weather wave base, the upper shoreface/foreshore sandstones originally had good primary intergranular porosity and permeability. However, these upper shoreface/foreshore deposits are generally thin (less than 5 m), and later diagenetic events have greatly reduced the primary intergranular porosity. These sandstone bodies are most continuous along depositional strike (northwest-southeast).
In some areas, inlets have scoured through the shoreface deposits, and the inlet-fill sequences, which are up to 20 m thick, commonly rest upon offshore marine siltstones and shales. Tidal currents moving through the inlets transported and deposited sand-size sediment as well as large amounts of shell material. As a result, these inlet-fill deposits had excellent primary intragranular porosity and permeability that was subsequently enhanced by the leaching of the calcareous shell material. Leaching resulted in the development of additional secondary moldic porosity and permeability. Among the Halfway deposits, the inlet-fill sequences have the greatest reservoir potential. They are laterally discontinuous along depositional strike (northwest-southeast) and more continuous along depositional dip (northeast-southwest).
