STUDY FINDS DEVONIAN GAS RESOURCES OF WESTERN CANADA ATTRACTIVE TARGET

Gerald E. Reinson, Pei J. Lee
Geological Survey of Canada
Calgary

This report summarizes results of a recently completed study on the conventional natural gas resources estimated to be contained in Devonian strata of the Western Canada sedimentary basin. 1

This study is the first in a series dealing with conventional gas resources of the basin south of 62 N. Lat. Estimates of regional resource potential have been prepared periodically by the Geological Survey of Canada, 2 3 4 5 using systematic geological basin analysis and statistical resource evaluation methods.

The initial computer based statistical evaluation methods were developed within the GSC, 6 7 and subsequently refined into the Petrimes system 8 9 which is employed here for estimating resource potential of established plays.

The assessment of natural gas resource potential of western Canada is the most difficult project of its kind undertaken by GSC because of the enormity of the pool data base, the number of plays, and the geological complexities of those plays.

Hence the assessment was divided into major play groups using geological criteria, primarily major stratigraphic time-rock units or structural/tectonic provinces. That is, each stratigraphic or structural unit will have distinct geological factors controlling size, distribution, and type of hydrocarbon play or reservoir.

The major play groups in the western Canada gas project are Devonian, Permo-Carboniferous, Triassic, Deformed Belt, Lower Cretaceous Mannville group, Middle Cretaceous Colorado group, and Upper Cretaceous-Tertiary.

The Devonian assessment was undertaken first because of the existing comprehensive geological data base and because there is an upside potential for finding significant reserves in relatively large economic pools.

The Devonian ranks as the most prolific hydrocarbon producing interval in the Western Canada basin. Devonian oil accounts for more than 60% of recoverable conventional crude in Alberta." Further, more than one quarter of the total in place gas reserves in the Western Canada basin are contained in Devonian rocks (Fig. 1).

The objectives of the comprehensive Devonian study' were to estimate the total gas potential in the Devonian system of Western Canada (exclusive of the Foothills belt) and to outline the principal Devonian gas plays with respect to geological controls and resource potential.

Categorization of each play and its potential is extremely useful to industry for evaluating specific prospects or overall exploration programs.

ASSESSMENT PROCEDURE

The terms reserve, resource, and potential as defined by GSC 3 are retained in this report.

Resource is defined as all hydrocarbon accumulations that are known or are inferred to exist. Reserves are that portion of the resource that has been discovered, and the term potential describes that part of the resource that is inferred to exist but is not vet discovered.

The terms potential and undiscovered resources are synonyms and may be used interchangeably. It should be noted that the term reserve also has been used elsewhere to refer to initial marketable gas volume, so discovered in place volume is used rather than reserve to avoid confusion.

The terms pool, prospect, play, and field have the following designated meanings: A prospect is defined as an untested exploration target within a single stratigraphic interval that may or may not contain hydrocarbons. A play is a family of pools and/or prospects that share a common history of hydrocarbon generation, migration, reservoir development, and trap configuration.11

The term gas field designates an area that produces gas without stratigraphic interval restrictions. Any number of discrete pools at varying stratigraphic levels may exist within a field. A gas pool is defined as a discovered gas accumulation, typically within a single stratigraphic interval, that is hydrodynamically separate from another gas accumulation.

Individual pools may include nonassociated gas or various combinations of nonassociated, associated, and solution gas. The potential estimates given here reflect total natural gas resources. However, the predominant type of gas occurrence in each mature play is indicated in Table 1.

Plays are groups into two categories: established plays (those that are demonstrated to exist by virtue of discovered pools with established reserves) and conceptual plays (those without discoveries or reserves but which geological analysis indicates may exist).

Established plays are grouped further into mature and immature plays. Mature plays are those in which the profile of the discovery sequence is sufficient to allow analysis using the discovery process model in the Petrimes assessment procedure. 8 9

Immature plays are those in which the number of pools (and therefore the discovery sequence) is inadequate for application in this model.

The Petrimes discovery process model is based on the assumption that the group of discovered pools in any given play represents a biased sample of the underlying Pool population. 11 The exploration process is biased in the sense that the largest pools tend to be found first. The discovery process model uses the two most reliable data sets, pool size and discovery date, to estimate play potential and undiscovered pool sizes. Individual pool volumes are then summed to give an estimate of the total gas resource in that play. The pool data used here are based on data sets from provincial agencies in Alberta" and British Columbia. 12

The discovery process model is also used to estimate undiscovered resources in conceptual plays by assuming that mature plays belong to a single population.1 The 25 mature plays can be treated as a 'pool' data set from which the model generates an underlying population of ranked play sizes. From this, the number and individual size of conceptual plays can be estimated.

MATURE PLAY ANALYSIS

The analysis of Devonian gas potential involved delineation and systematic evaluation of 28 established plays.

Twenty-five are classified as mature, while three are considered to be immature (Table 1). Each play is designated by geological formation/member, depositional or trap type, and characteristic gas pool. The detailed report1 contains a description of all established mature plays, including definition, geology, exploration history, and estimated resource potential, with supporting figures.

The Western Canada basin occupies an area of 1.4 million sq km, encompassing southwestern Manitoba, southern Saskatchewan, Alberta, northeastern British Columbia, and the southwestern comer of the Northwest Territories (Fig. 2).

The basin is bounded on the north by the Tathlina arch, on the east by the Precambrian shield, and on the west by the Cordilleran disturbed belt. The Peace River and Sweetgrass arches divide the basin into discrete sub-basins.

The established plays conform to three broad exploration regions that are governed somewhat by these tectonic figures (Fig. 2). The exploration regions are rather arbitrary but still provide a framework for comparative analysis of similar plays.

Natural gas reserves in the Northern District and Peace River arch region occur mainly in Keg River and Slave Point carbonate reefoid complexes of Northeast British Columbia and the north flank of the Peace River arch. In the Central District and Deep Basin region, gas reserves are associated with major oil plays in central Alberta and with the Upper Devonian erosional sub-crop edges of eastern Alberta.

In the Deep Basin area of west central Alberta, sour (and sweet) gas reserves occur along extensive shelf margins and in large reef complexes of the Beaverhill Lake, Woodbend, and Winterburn groups. Gas reserves in the Southern exploration region are confined mainly to platform carbonates in the uppermost Devonian cycle (Wabamun group).

Discovered and potential gas reserves for the 25 mature plays are listed in Table 2. The total figures for the 25 mature plays are 1,567,200 million cu in (56 tcf) of discovered in-place gas, with an additional 564,500 million cu m (20 tcf) of estimated undiscovered potential.

The total resource for each play is the sum of potential and discovered volume. Mature plays are ranked according to discovered and potential in-place volume in Tables 3 and 4. When comparing the results, interesting trends useful for planning exploration strategies are evident.

As expected, the very mature gas plays, Leduc isolated reef (Westerose) in central Alberta and Swan Hills shelf margin (Kaybob South) in the Deep Basin area (Table 3), have the largest discovered gas resources. The large volumes in the Westerose play reflect its early discovery and subsequent intensive drilling, primarily for oil. Large discovered in-place gas volumes in the Kaybob South play reflect the recent Caroline discovery, which added substantial reserves to this still expanding exploration play. When the mature plan,s are ranked according to potential, a much different order emerges (Table 4). The relationship between discovered and potential in-place volumes also illustrates some interesting comparative trends among plays (Fig. 3A). The Cranberry, Adsett, and Parkland plays have relatively low discovered in-place volumes but significant play potential. Moreover, the Windfall and Kaybob South plays, although containing large discovered in-place resources, also have significant potential. Estimated largest undiscovered pool size for each mature play, when compared to overall play potential (Fig. 3B) also shows the upside potential of the Cranberry, Adsett, Windfall, Kaybob South, and (unexpectedly) the Westerose play.

The largest discovered in-place volumes occur close to more populated areas where drilling activity historically has been the highest (Central District and Deep Basin region). The potential for discovering significant additional resources is highest in the Northern District and Peace River arch region. This reflects the relatively underexplored state of the Slave Point reef complexes (Cranberry), Slave Point interior platform (Adsett), and Wabamun structural/stratigraphic (Parkland) plays.

Significant potential still exists in mature plays of the Central District and Deep Basin areas. The Swan Hills shelf margin (Kaybob South) and Leduc/Nisku reef complexes (Windfall) plays in the Deep Basin have very favorable potential, as does the Leduc isolated reef (Westerose) play.

The relatively low discovered and potential volumes in southern Alberta are due to the fact that only one mature gas play has been identified there. However, the conceptual play analysis indicates that significantly more gas resources could be present in the Western Canada basin; perhaps much of this occurs in conceptual plays in the Southern Alberta exploration region.

CONCEPTUAL PLAY ANALYSIS

As mentioned previously, estimates of the potential and size of conceptual plays were achieved using the 25 mature plays as the 'pool' data base and generating a play size-by-rank plot (Fig. 4) in a similar manner as the pool size-by-rank plots for the mature plays.

The rectangular bars in Fig. 4 represent plays that remain to be discovered. The mean volume of these undiscovered plays is summed to give an estimate of the conceptual play potential.

The conceptual play analysis suggests that there are about 100 Devonian gas plays in the Western Canada basin. Not all of these plays are significantly large enough to be explored far. Thus it is appropriate to truncate the generated play size-by-rank curve, since the very small plays will never be identified.

A realistic estimated of play numbers and conceptual play potential results when the play-size distribution is truncated at 30,000 million cu m - this cutoff is approximately 1 tcf (Fig. 4).

This corresponds to the largest 40 plays. Of these, 17 are established mature plays and three may be the immature plays that were geologically defined but not statistically evaluated. That leaves 20 plays, greater than 1 tcf in size, remaining to be discovered. Three of these conceptual plays are extremely large, in the order of 90,000 to 225,000 million cu m (3 to 8 tcf).

The sum of the mean volume of the 23 undiscovered immature and conceptual plays along with the volume for the entire non-truncated play population is given in Table 5.

The credibility of the conceptual potential estimate; depends to some degree on whether it can be demonstrated geologically that enough new plays actually exist to contain such additional gas volumes. During the past decade, exploration strategies have not focused on wildcat exploration for new plays.

The last new Devonian plan, to be discovered was the Nisku isolated reef (Brazeau River) play in 1979. The Caroline gas pool, discovered in 1986, is not a new play but an extension of the Swan Hills shelf margin (Kaybob South) play.

In our detailed report,1 we demonstrate that based on stratigraphic models and comparison with established mature plays there are several possible settings for new plays containing large undiscovered resources. These conceptual play possibilities are grouped into three types: 1) carbonate facies belts based on the cyclical pattern of Devonian sedimentation and distribution of the established mature plays; 2) deeply buried lowstand clastic deposits; and 3) structurally-controlled drape or subcrop plays.

RESULTS, DISCUSSION

The total Devonian gas resource in the Western Canada basin is estimated at 126 tcf. This total is derived from the discovered in-place volume of 56 tcf, mature play potential of 20 tcf, and conceptual play potential of 50 tcf (Tables 2, 5).

Fifty-six percent of the resource remains to be discovered (Fig. 5); 40% is contained in conceptual plays and only 16% in mature plays. Compared with conceptual plays, mature plays have a much lower overall potential.

The 20 tcf of mature play potential suggests that only 26% of the total gas resource in mature plays remains to be discovered. Further, no undiscovered pools 1 tcf in size or larger are predicted to be in any of the 25 mature plays.

Conversely, 17 undiscovered pools with in-place gas volumes of 100 bcf or larger are predicted to be present in the 25 mature plays. This indicates significant upside potential in several of the mature plays to make continued exploration attractive.

The plays with the highest potential are:

Slave Point reef complexes situated north of the Peace River arch and characterized by Cranberry gas field.
Leduc/Nisku reef complexes such as Windfall in the Deep Basin region of west-central Alberta.
The Swan Hills shelf margin play of western Alberta, which is typified by the Kaybob South and Caroline fields.
The Slave Point plat-form play of Northeast British Columbia in which Adsett field is the primary example.

Seventy-two percent of the total Devonian gas potential is predicted to occur in conceptual plays. Although we cannot predict the size of individual pools in conceptual plays, the large volumes of several of these plays suggest that attractive pool sizes should be present.

To obtain an appreciation of the significance of this Devonian gas potential study, it is interesting to compare the results with the 1983 gas potential estimates for the entire Western Canada basin.13

In the 1983 report the recoverable discovered resources for the entire basin were listed as 75 tcf, with a predicted potential (average expectation) of 88 tcf. By yearend 1989, an additional 60 tcf of recoverable gas was proven, leaving only 28 tcf more of recoverable potential for the entire basin on the 1983 prediction.

This study suggests that there are 76 tcf of gas remaining (approximately 35 tcf recoverable) to be discovered in the Devonian alone.

Thus if the entire sedimentary succession of the Western Canada basin is taken into account, conventional gas potential could be considered to be substantial, possibly in the order of three to four times that of the estimated Devonian potential reported here.

ACKNOWLEDGMENTS

This article summarizes the numerical results of the detailed Devonian study published in spring 1993. The authors thank the contributors to the comprehensive study since this summary would not have been possible without their original input. Kirk Osadetz and Grant Mossop critically reviewed the manuscript and provided useful comments and suggestions.

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