ABSTRACT
The Spirit River Formation is a prolific gas producer in the Deep Basin of Alberta. The lower Wilrich Member consists of coarsening-upward clastic cycles from 50 to 100 m (165 to 330 ft) thick. In the southern part of the study area, the Falher Member (the major producer) comprises nonmarine clastics and coals. Around the Elmworth gas field, this member comprises five transgressive and regressive cycles, with alternating marine and nonmarine deposits. Each cycle consists of low-angle, laminated, fine-grained sandstone with varying amounts of coarser sandstone and conglomerate, locally capped by a thin coal. The fine sandstones were deposited by waves in shallow-marine and shoreface environments. Granule conglomerates, which are well sorted, matrix-free, and present immediately under the coal, are interpreted as beach deposits. Pebbly conglomerates with poorer sorting, sandy matrix, some cross-bedding, and sharp bases cut into shoreface sandstone are interpreted as distributary-channel deposits, or as shoreface deposits reworked a short distance from a channel.
Coarser sandstones and conglomerates of the shore zone form conventional reservoirs with porosities up to 15% and permeabilities to 1 darcy. The fine-grained shoreface and shallow-marine sandstones are “tight sands” with permeabilities of 0.001 to 0.5 md. Petrography has shown that they have lost porosity because of (1) formation of quartz overgrowths, (2) crushing of sedimentary rock fragments, and (3) cementation by carbonates and clays. These processes have also operated in the conglomerates but much primary porosity has been preserved and some secondary porosity has been formed. Deep gas generation from coal has filled the well-connected pores in the conglomerate and the poorly connected pores left in the tight sands.
The tight sands have lower permeabilities than some shales and form the seals that trap gas in the basin by retarding updip flow. The Spirit River gas deposits can be considered the result of combined stratigraphic-diagenetic trapping.
