ABSTRACT

A subsurface investigation of the Triassic Doig Formation of northeastern British Columbia documented two reservoir facies. Both are a product of mass movement of sediment gravity flow processes on a tectonically active continental margin. A basinward thickening of the entire Triassic section indicates basin subsidence during deposition. Wave set-up, perhaps in conjunction with seismic activity, initiated sediment movement. Sedimentary facies and reservoir parameters were determined from analysis of approximately 150 cores and 900 well logs.

Laterally discontinuous Doig sandstones are up to 60 m thick and trend northeasterly within the project area. The main reservoir facies are incised density flow deposits and laterally extensive slump deposits. Reservoir quality within these sands is extremely variable, with porosity ranging from less than 5 to 15 per cent. In core, these deposits consist of moderately well sorted, very fine grained sandstones with no vertical grain size variation. The sandstone is generally massive but contains intervals of oversteepened bedding, disturbed clay laminae, fragmented shell material, and rip-up clasts of fine grained shelf sediments. These deposits have a sharp basal contact and a blocky gamma-ray signature, very similar to fluvial channel-fill sequences. The channelized density flow deposits form thick but laterally restricted reservoirs. Both reservoir types are enclosed in fine grained shelf deposits that constitute good seals to hydrocarbon migration. The best production to date is in the Buick Creek Field, with initial flows of 346 BOPD. The slump deposits are thinner and tend to be more elongate parallel to paleoshoreline. These sands were subject to some wave reworking. Modern analogues where similar processes and products of deposition are known to occur include the Gulf of Alaska continental shelf and the Fraser River Delta slope.

Successful development of Doig reservoirs must incorporate geological models that assist in understanding the complex and highly variable reservoir quality of sandstone units. Modern-day analogues provide a basis for developing applicable geological models.

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