W. CANADA PLAINS TRIASSIC-1 TRIASSIC GAS POTENTIAL SEEN HIGH IN WESTERN CANADA PLAINS REGION

Tim D. Bird, Jim E. Barclay, Ross I. Campbell, Pei J. Lee
Geological Survey of Canada
Calgary

Geological analysis and statistical assessment of Triassic gas resources in the plains portion of the Western Canada sedimentary basin suggest that 51% of the total gas resource remains to be discovered.

Of the undiscovered Triassic gas potential, 89% is considered to be present in mature established plays. As many as 154 pools with a volume greater than 280 million cu m (10 bcf) and 11 pools greater than 2,800 million cu m (100 bcf) remain to be discovered.

The most attractive mature plays with the greatest potential are:

The Halfway/Doig shelf (Peace River structure)Monias play;
The Baldonnel subcrop-Laprise play; and
The Halfway/Doig shore zone (Peace River structure)-Sinclair play.

These plays make up almost 60% (182,580 million cu m -6.5 tcf) of the total expected (undiscovered) resource.

Eleven percent of the estimated expected volume in this study occurs in conceptual and immature plays. Of this amount, the two immature plays combine to yield over 80% of the expected volume, with the remainder (up to 10,000 million cu m) expected in conceptual plays.

W. CANADA GAS ASSESSMENT

This article summarizes results of a recently completed study - Geological Survey of Canada Bulletin 483 1 - of conventional gas resources contained in Triassic strata of the plains portion of the Western Canada basin.

The report includes a detailed geological play analysis and numerical assessment of undiscovered gas potential in Part 1, by Bird et al.1 of the GSC, and an economic analysis, Part 11, by Waghmare et al.2 of the Energy Sector of Natural Resources Canada in Ottawa. This bulletin on the appraisal of the natural gas potential is part of a series of reports resulting from a comprehensive assessment by GSC of Western Canada gas resources.

The Western Canada gas assessment project was divided into seven major play groups on the basis of geological criteria, following major stratigraphic units or structural/tectonic provinces. Each group has a distinct set of geological factors which control size, distribution, and type of hydrocarbon play or reservoirs The major play groups are: the Devonian, Carboniferous-Permian, Triassic, Jurassic to Lower Cretaceous (Mannville), Middle Cretaceous Colorado Group, Upper Cretaceous-Tertiary, and Rocky Mountain foreland belt. A comparison of discovered and undiscovered resources is given in Fig. 1.

About 5% of the discovered in-place gas reserves in the interior plains of Western Canada basin is contained in Triassic rocks (Fig. 1). Two-thirds of the discovered in-place gas reserves of the Triassic are contained in the Interior Plains, with the remaining third in Rocky Mountain foreland belt of the Cordilleran Orogen (Foothills-Deformed Belt). The study area is largely confined to the Peace River arch/embayment-where much of the Triassic succession occurs in the subsurface (Fig. 2).

REGIONAL GEOLOGY, PLAY DEFINITION

Triassic strata belong to a platformal succession of mixed siliciclastic, carbonate, and evaporate sediments deposited along the western portions of the Western Canada basin. The thickest deposits, and most of the oil and gas, occur in the westward thickening and deepening depocenter called the Peace River embayment. Triassic rocks can be divided into three main assemblages, related to large scale transgressive-regressive marine cycles, allowing characterization of play groups (Fig.3).Exploration plays consist of stratigraphic and stratigraphic-structural combination traps with reservoirs in aeolian, shoreline, tidal channel, shallow marine sandstones, coquinas, and tidal flat and marine shelf carbonates.

Geological analysis by subsurface mapping, using Alberta and British Columbia government pool data, literature studies, and discussions with government and industry geoscientists, enabled the grouping of Triassic pools into 10 mature and two immature plays in four main formations (table 1). Plays are classed as mature or immature depending on the number of pools that have been discovered in each. In-place gas reserves for mature and immature plays, total 288,400 million cu m (10.2 tcf) discovered in 622 pools. In each established play, pools both discovered and undiscovered, are assumed to form a natural geological population that is governed by geological controls such as depositional style, structure, or trap geometry. These geological factors control the play boundary and the resulting distribution of pools within that play.

NUMERICAL ASSESSMENT

Once exploration plays are defined, quantitative analyses based on exploration discovery histories and pool size distributions are used to assess play potential. We estimate undiscovered potential resources contained in mature, immature, and conceptual plays using a numerical assessment technique, termed the discovery process model, which uses the size (volume) and the discovery sequence of individual pools or plays within a natural population of pools or plays. 3a Conceptual plays have no discoveries or reserves but are inferred to exist by geological and/or statistical analysis. Mature plays require geological analysis to delineate the type and extent of the pool population for each play, prior to statistical analysis. In contrast, the number and magnitude of immature and conceptual plan,s is primarily inferred from the statistical analysis of mature plays. Geological analysis of immature plays provdes subjective comparisons matching discovered resources in a model of the total play population (discovered plus undiscovered resource). Fig. 4 shows a play size-by-rank plot of all Triassic plays.

MATURE PLAYS-RESULTS

Table 2 shows all Triassic mature plays ordered by largest expected volume of gas in-place. Results of the mature play analysis indicate that three mature plays have significant potential to hold additional amounts of natural gas. These are:

Halfway and Doig formations-shelf sandstone reservoirs, influenced by Peace River arch/embayment structures (e.g., Monias field) with an expected potential of 88,934 million cu m (3.2 tcf);
Subcropping Baldonnel formation carbonate reservoirs involved in Laramide-aged gentle folds (e.g., Laprise field) with an expected potential of 66,610 million cu m (2.4 tcf); and 3) Halfway/Doig formation shoreline related sandstone reservoirs, influenced by Peace River arch/embayment structures (e.g., Sinclair field) with an expected potential of 27,036 million cu m (0.96 tcf).

IMMATURE, CONCEPTUAL PLAYS-RESULTS

Estimates of the potential and size of immature and conceptual plays were derived from the discovery process model using the lb mature plays as the 'pool' database, in effect modeling the total play population (Fig. 4). The expected potential for conceptual and immature plays is 33,777 million cu m (i.2 tcf). Compared with mature plays, immature and conceptual plays have less potential. This result is consistent with the long history of exploration for Triassic plays and the idea that mature play definitions are sufficiently broad to include most play concepts.

The expected potential from all play types; mature, immature, and conceptual is 305,901 million cu m (10.8 tcf) distributed in about 3,300 pools-remember these figures refer to gas in-place and do not incorporate any economic constraints. A more speculative, probable potential value of 767,300 million cu m (27 tcf) provides a more optimistic estimate of gas remaining to be discovered in all play types.

REFERENCES

Bird, T.D., Barclay, J.E., Campbell, R.I., and Lee, P.J., Triassic gas resources of the Western Canada Sedimentary Basin, Interior Plains, Part 1: Geological Play Analysis and Resource Assessment, Geological Survey of Canada Bull. 483, 1994.
Waghmare, R.R., Dallaire, S.M., and Conn. R.F., Triassic gas resources of the Western Canada Sedimentary Basin, Interior Plains, Part 11: Economic Analysis, Geological Survey of Canada Bull. 483,1994.
Reinson, G.E., Lee, P.J., Barclay, J.E., Bird, T.D., and Osadetz, K.G., Western Canada basin conventional gas resource estimated at 232 tcf, OGJ, Oct. 25, 1993, pp. 92-95.
Reinson, G.E., Lee, P.J., Warters, W., Osadetz, K.G., Bell, L.L., Price, P.R., Trollope, F., Campbell, R.I., and Barclay, J.E., Devonian Gas Resources of the Western Canada Sedimentary Basin, Part 1: Geological Play Analysis and Resource Assessment, Geological Survey of Canada Bull. 452, 1993, pp. 1-1 28.
Barclay, J.E., with contributions by Gibson, D., Triassic Petroleum Geology, Western Canada Basin and neighbouring Yukon Territory and American Regions: bibliography and summary, Bull. of Canadian Petroleum Geology, Vol. 41, No. 4,1993, pp. 437-452.
Edwards, D.E., Barclay, J.E., Gibson, D.W., Kvill, G.E., and Halton, E., Triassic strata of the Western Canada Sedimentary Basin, in Geological Atlas of the Western Canada Sedimentary Basin, G.D. Mossop (ed.), Canadian Society of Petroleum Geologists, Calgary, Alta., 1994.
Gibson, D.W., and Barclay, J.E., Middle Absaroka Sequence: The Triassic stable craton, in The Western Canada Sedimentary Basin - A case history, B.D. Ricketts (ed.); Canadian Society of Petroleum Geologists Special Publication No. 30, Calgary, 1989, pp. 219-232.