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Conglomerates in the Falher Member of the Lower Cretaceous Spirit River Formation of Alberta are the reservoir rocks for the giant (2-3 Tcf or 5-8 × 1010 m3) Elmworth gas field. Three types of conglomerates are present: (a) unimodal--granules or pebbles lacking any matrix, (b) bimodal grain supported--a framework of pebbles with fine to medium sand in the interstices, and (c) bimodal sand supported--pebbles floating in fine to medium sandstone. The distribution of these three types is controlled by the original depositional environment.
The granules and pebbles are composed of chert and silicified sedimentary and volcanic rock fragments. The sand grains are dominantly quartz. During diagenesis, very small (.01-0.1 mm) drusy quartz crystals form on the cherts and rock fragments, whereas quartz grains undergo heavy quartz overgrowth formation because of differences in nucleation and growth of quartz crystals. As a consequence, most unimodal conglomerates (low in quartz) have not experienced major reduction of porosity and permeability. Bimodal conglomerates, both sand and pebble supported, have lost significant amounts because of cementation of the quartz-rich matrix. Streaks and patches of kaolinite and calcite cements have also reduced porosity and permeability locally. Gas production from Falher wells, therefore, de ends not only on the amount of conglomerate, but also on the proportion of unimodal and bimodal types.
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