ABSTRACT

An important gas trend within the southern Rocky Mountain Foothills of northeast British Columbia in under current exploration and development. The presence of gas reserves has been established in Lower Cretaceous/Jurassic sandstones and Upper Triassic carbonates, with locally important reserves in Middle Triassic sandstones.

This paper deals with Upper Triassic gas reserves and potential present in complexly folded and faulted low-grade carbonate reservoirs of the Pardonet and Baldonnel Formations. The geological, geophysical and reservoir parameters pertaining to these accumulations are best illustrated in the Sukunka/Bullmoose area where five discoveries have been made, three of which are partly defined by development drilling.

These Upper Triassic sour gas accumulations are believed to have formed in the following manner: Organic matter was deposited contemporaneously with the Pardonet-Baldonnel carbonates and with shales of the immediately overlying Jurassic Fernie Formation. The organic matter was transformed into hydrocarbons, which migrated a short distance into locally permeable and porous carbonates as the Triassic became buried by a thick Jurassic and Cretaceous clastic sequence. As a result of deep burial and the Laramide Orogeny, the mature stage of organic diagenesis was reached, leaving only gaseous hydrocarbons and bitumen. The Laramide tectonic event created a series of compressional-type structures and an extensive fracture system, allowing the gas to re-migrate into anticlinal traps. This fracture network is also responsible for significantly enhanced permeability and thus excellent productive capability. AOF tests as high as 6 300 10³m² (223 MMcf) per day have been recorded. Proven and probable reserves of sour gas in the general Sukunka/Bullmoose area defined to date are estimated at 28 328 10⁶m³ (1.1 Tcf). Because of the high H₂S and CO₂ content, sales gas is estimated to be in the order of 16 544 10⁶m³ (584 Bcf).

The area's exploration history has been long and difficult, hampered by a combination of high-cost operations and complex geology. Although exploration was undertaken in the late 1950s and a discovery made at Sukunka in 1965, it was not until the mid-1970s that progress was made in defining the extent of the original Sukunka discovery and in discovering other fields. The increase in commodity price for natural gas, improved technology, and persistence in exploration have been factors in the emergence of this promising gas trend in British Columbia.

1 This paper was originally presented at the C.S.P.G. 50th Anniversary International Conference, "Facts and Principles of World Oil Occurrence", Calgary, Alberta, June 1978.

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INTRODUCTION

The area of interest lies in the Rocky Mountain Foothills of northeast British Columbia, between the Pine River and the Alberta border (). A number of gas fields or single-well gas discoveries are present in the region. The area is still relatively unexplored, as most of the discoveries have been made in the last seven years. Gas-producing zones have been identified in the Upper Triassic Pardonet and Baldonnel carbonates, Middle Triassic Halfway and Cretaceous/Jurassic Nikanassin sandstones. This paper describes the exploration history leading up to the discoveries, the stratigraphy, local and regional structures, and reservoir data relevant to the Upper Triassic gas accumulations. As most of the technical data are from the Sukunka/Bullmoose field area, the paper will focus on this locality.

Four discoveries have been made in generally tightly folded and fractured Upper Triassic Pardonet and Baldonnel carbonate reservoirs in the Sukunka/Bullmoose field area. Another Upper Triassic gas discovery has been made at Murray River, d-48-I/93-I-14, 42 km southeast of the area. Proven and probable reserves of approximately 28 328 x10⁶m³ (1.0 Tcf) of sour gas or 16 544 10⁶m³ (584 Bcf) of sales gas have been established, mainly in the Sukunka and Bullmoose pools where the producing structures have been partially delineated by drilling.

Initial well deliverability is usually very good, ranging from 340 10³m³/d (12 MMcf/d) at East Sukunka d-37-A to 1 444 10³m³/d (51 MMcf/d) at c-45-J on the West Sukunka structure; these flow rates are against 8 000 kPa line pressure. The estimated combined deliverability potential from the seven wells completed in the Sukunka, East Sukunka, West Sukunka and Bullmoose pools is 6 034 10³m³/d (213.4 MMcf/d).

The region is extremely difficult to explore, as the terrain is rugged with local relief exceeding 1,200 m. The physiographic form is related to complexly faulted and folded Lower Cretaceous clastic sediments. The combination of complex structural geology and rugged topography has proved both difficult and costly to explore, and explains why exploration has taken so long to delineate significant reserves.

B.P.'s (formerly Triad Oil Co. Ltd.) exploration and development activity spans twenty-five years. Approximately $120,000,000 has been spent by B.P. and partners to the end of 1979 before production finally started in 1980. It is recognized that performance of low-porosity; low-permeability reservoirs is difficult to predict. More specific estimates on reservoir conformance to prediction will not be available until the individual pools have been produced for some time.

GEOLOGICAL SETTING

The Sukunka/Bullmoose field area is located within the Rocky Mountain Inner Foothills belt of British Columbia (), about 100 km southwest of Dawson Creek. A number of regional features are of interest: the area is located where the Laramide fold belt forms a re-entrant between two broad arcs of the Canadian Rocky Mountains; it lies on the intersection of the Peace River Arch and West Alberta Ridge (pre - Middle Devonian positive features); it lies on a north-south Upper Devonian - Carboniferous facies change from mainly carbonates in the east to mainly shales in the west, and it is the site of significant erosion of Upper Carboniferous sediments and remained high through Pennsylvanian and Permian time. An early Paleozoic, north-south structural hinge line runs through the area; to the east, Devonian sediments rest on Precambrian 'basement' while to the west, a thick wedge of Precambrian and pre-Devonian sediments has been identified in outcrops.

GENERAL STRATIGRAPHY

The Laramide orogenic events have exposed increasingly older stratigraphic units, from Upper Cretaceous in the plains area to the east to Paleozoic in the mountains to the west.

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Within the Inner Foothills, Where the Triassic gas pools are found, only the Lower Cretaceous and earlier sediments have been preserved from recent erosion. The major stratigraphic units and their generalized lithology and thickness are shown in .

Lower Cretaceous

The Lower Cretaceous Bullhead Group forms most of the surface exposures along the Sukunka/Bullmoose structural trend. This group includes the Gething and Cadomin Formations.

General physiographic setting and loction of major discoveries, Rocky Mountain Foothills, British Columbia.

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Stratigraphic nomenclature and thickness of formations/members, general Sukunka River area.

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Gething Formation: The Gething Formation, 380 to 450 m thick, includes a succession of a fine- to coarse-grained sandstone, varicoloured shale, often carbonaceous and silty, and conglomerate. It contains rich, metallurgical-grade coal deposits that companies are now examining for mining prospects.

Cadomin Formation: The Cadomin Formation is traceable over a wide area of western Canada. It consists of conglomerate and well-cemented sandstone. Because of its resistant nature, the Cadomin provides an excellent mapping unit. It varies from 10 to 100 m in thickness.

Lower Cretaceous - Jurassic

Nikanassin Formation: The Nikanassin spans the Jurassic-Cretaceous boundary (Stott, 1967). It consists of a relatively incompetent sequence of lithic arenites, quartzitic sandstone and dark carbonaceous mudstone. Thin coal beds are present in the upper part of the formation. In the Grizzly Valley Field area, to the southeast of Sukunka-Bullmoose, massive sandstone units with low-grade porosity are present. These sandstones, when located in favourable structural position, are fractured and provide reservoirs for sweet gas accumulations. In the Sukunka/Bullmoose area, massive sandstones generally are not present, and at surface, the Nikanassin displays characteristic chevron-type folding, associated with numerous reverse faults (). Sandstone content decreases while shale and siltstone content increases toward the base. As a result, the contact with the underlying Fernie shales is gradational which, combined with structural complexity, makes it difficult to establish the true thickness of the Nikanassin Formation. It is estimated to range from 900 to 2300 m.

In outcrop sections, Stott (1967) preferred using 'the better defined and dated Minnes Formation' rather than extending the term 'Nikanassin' into the area. He raised the Minnes Formation to Group rank to include the Monteith, Beattie Peaks and Monach Formations and the overlying 'unnamed beds'. He reports thickness in excess of 1800 m for the complete sequence in the 'Foothills South of Peace River.' Because of structural complexities and lack of recognizable units in the subsurface of Sukunka, the present authors prefer to continue using the Nikanassin Formation nomenclature.

Jurassic

Fernie Formation: This formation varies from 260 to 470 m in the Sukunka/Bullmoose field area. In descending order the formation consists of the following units: fine-grained sandstone, siltstone, and dark gray silty shale; black carbonaceous thin-bedded shale and black carbonaceous, argillaceous limestone and very fine crystalline limestone. The lowermost limestone unit, 10 to 35 m thick, is termed the Nordegg Member. In the Sukunka area, a thin, glauconitic sandstone marker is present at the top of the Nordegg. The shales of the Fernie Formation behave as an incompetent structural unit and exhibit plastic deformation.

Triassic

With the exception of the Halfway, all other formations of the Triassic can be readily correlated with their equivalents outcropping in the Rocky Mountains to the west of the Sukunka/Bullmoose field area. The Triassic sequence in this area is about 1000 m thick. In descending order, the Triassic consists of the following formations:

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a) Chevron folding, Nikanassin Formation, west Sukunka area. b) Overturned anticline of Mississippian Rundle at leading edge of thrust plate, west Sukunka area. (cf. ).

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Pardonet and Baldonnel Formations: These formations comprise siltstone, limestone and dolomite, 75 to 150 m thick. They provide the main hydrocarbon reservoir in the sedimentary section explored to date in the Sukunka area.

The Pardonet Formation consists of carbonaceous-argillaceous limestone, siltstone and dolomite. The limestones are quartzose and bioclastic and in places form coquina banks of pelecypod shells. This formation thins eastward to zero because of pre-Jurassic erosion. The contact with the underlying Baldonnel Formation is sharp and usually defined at the base of a shaly siltstone with high gamma radiation ().

The Baldonnel Formation in subsurface consists primarily of dolomite. In outcrop sections studied in the Front Range approximately 25 km west of Sukunka, Gibson (1972) recognized two distinctive units within the Baldonnel Formation: an upper unit consisting of a dolomitic, quartzose, aphanitic to oolitic and bioclastic limestone and dolostone; and a lower unit, the Ducette Member, consisting of highly quartzose siltstone, very fine grained sandstone and limestone. The Ducette Member has been identified only in outcrop in the Sukunka River - Peace River region.

Charlie Lake Formation: This formation, 300 to 380 m thick, is an evaporitic sequence comprising anhydrite, dolomite, sandstone, and siltstone. Bedded anhydrites are more prominent in the upper part of the formation while very fine grained sandstone, red siltstone, and dolomite are characteristic of the lower part. Similar sediments, with the exception of anhydrites, have been described in outcrop sections west of Sukunka (Gibson, 1972).

Halfway Formation: The Halfway Formation in subsurface varies from 5 to 100 m in thickness. It is a marine clastic sequence consisting mainly of fine-grained calcareous or dolomitic sandstone. Lithic fragments, chert, clay and feldspar are accessories. In outcrop sections to the west of the field area, the Halfway sandstone facies is not developed; its equivalents are included in the Llama Member (Gibson, 1972).

Doig and Montney Formations (and equivalents): These formations, although not fully penetrated by wells in the Sukunka/Bullmoose locality, are reasonably well known from well and outcrop sections in surrounding areas. Their combined thickness is in the order of 450 m. In wells drilled east of Sukunka/Bullmoose, an increase in siltstone and very fine grained sandstone occurs in the upper part of the Doig Formation. The contact between the Doig and overlying Halfway is placed at the base of massive, fine-grained sandstone. The Doig sandstone is very fine grained. Black shale, silty shale and phosphatic sediments are present in the Black Shale Member (Manko, 1960) of the basal Doig Formation. The contact between the Doig and Montney Formations is placed at the base of this member. The Montney Formation consists of a monotonous siltstone sequence.

In outcrop sections, sediments lying between the Charlie Lake Formation and the Permian are included in the Sulphur Mountain Formation. Gibson (1972) divides the Sulphur Mountain Formation into the Llama, Whistler and Vega-Phroso Siltstone Members. The uppermost Llama Member consists of resistant siltstone, limestone, and minor sandstone and dolostone, about 400 m thick. The Llama member includes equivalents of the Halfway and Doig Formations of

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Plains terminology. The Whistler Member, 20 to 90 m thick, is a distinctive dark grey and black shale and siltstone unit containing phosphatic sediments; it is readily correlatable with the Black Shale Member in the Plains. It consists of argillaceous, calcareous or dolomitic quartzose siltstone and silty shale. The Vega-Phroso Siltstone Member thickens from about 90 m in the Sukunka area to 300 m near the Alberta border.

Permian

In the Rocky Mountains to the west of the Sukunka/Bullmoose area, extensive outcrop data of the lithologically and tectonically complex Permian and Upper Carboniferous sedimentary sequence exist from the work of McGugan and Rapson (1976) and Bamber and Macqueen (1971, 1979). In the outcrop sections west of the Field area, the Permian and Upper Carboniferous units are thin or absent. Also of interest is the comment by Bamber and Macqueen (1979) that "Truncation of Lower Carboniferous rocks is greater in the Monkman Pass / Pine Pass area than in any other part of the Rocky Mountains."

Four shelf carbonate and clastic formations are included in the Permian Ishbel Group (Bamber and Macqueen, 1979). These are, in descending order: the Mowitch, Ranger Canyon, Fantasque and Belcourt Formations. The Mowitch Formation consists of a thin sandstone unit with chert and carbonate lenses, 0 to 22 m thick; near the big bend of the Sukunka River, we have observed an upper unit consisting of glauconitic sandstone. The Ranger Canyon Formation is a thin (2 to 6 m) complex deposit of phosphatic chert and siltstones with some sandstone and silicified carbonates. The Fantasque Formation, 0 to 33 m thick, a facies variation of the Ranger Canyon, consists of siltstone and some chert. The Belcourt Formation consists of marine shelf carbonates varying in thickness from 0 to 13 m. Unconformities separate all units (except the Ranger Canyon - Fantasque facies variants) and indicate pulses of transgression, uplift and erosion. Thickness of the combined units ranges from 0 to 74 m.

In the Plains area, Permian sediments are included in the Belloy Formation. These sediments, about 10 to 40 m thick, consist of chert, dolomite, dolomitic and glauconitic sandstone and sandy dolomite. The correlation of the Belloy Formation with formations of the Ishbel Group is uncertain.

Upper Carboniferous

Hanington Formation: Bamber and Macqueen (1979) have identified erosional carbonate remnants of Later Carboniferous age at two localities: west Sukunka River and Mount Hanington, near Kakwa River south of Wapiti Lake. The Hanington Formation consists of skeletal (mollusc, echinoderm, brachiopod, bryozoan, foraminiferal, ostracod and other indeterminate debris) and pelletoidal limestone with minor argillaceous limestone. Its thickness in the west Sukunka River area is 67 m.

Lower Carboniferous

Sediments of Early Carboniferous age are included in the Rundle Group, Banff and Exshaw Formations.

Rundle Group: In the subsurface immediately east of the study area, the Pekisko, Shunda and Debolt Formations of the Rundle Group can be readily identified. Bamber and Macqueen (1971) indicate that a similar division (Pekisko, Shunda and Turner Valley Formations) is possible in the Narraway River south of Wapiti Lake. At Belcourt Creek, north of the Narraway River, the lowermost unit comprising skeletal limestone is recognizable, but facies changes to primarily dolomite in the overlying units preclude recognition of the threefold division. Northwest of Sukunka, the Rundle Group passes into an open-marine succession of cherty carbonates of the Prophet Formation.

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Banff and Exshaw Formations: The Banff Formation consists of argillaceous silty limestone and calcareous shale about 200 m thick. It conformably overlies either shales of the Besa River Formation in the west Sukunka River area or black shales of the Exshaw Formation in the Wapiti lake locality.

Upper Devonian

Upper Devonian sediments indicate the presence of the west flank of the Peace River Arch in the general Sukunka/Bullmoose area of the Foothills. At Wapiti Lake, approximately 330 m of limestone of the Palliser (Wabamun) Formation are present, and a similar thickness occurs at the Sundown a-10-A/93-P-10 well, 70 km east of Sukunka/Bullmoose. At the Sundown well, Wabamun and Winterburn sediments rest on early Paleozoic quartzites, whereas in the Front Ranges (the western valleys of the Sukunka and Burnt Rivers) argillaceous limestones of the Mount Hawk Formation and dark grey shales of the Perdrix Formation have been described by Taylor and Bamber (1970) and indicate a lower flank position on the arch.

Middle Devonian

At Pine Pass, dark grey dolomite and limestone about 330 m thick is assigned to the Pine Point Formation of Givetian age. The distribution and thickness southward is not known. In the same locality, Taylor and Bamber (1970) describe 510 m of finely crystalline dolomite, sandy dolomite and sandstone, with minor collapse breccias underlying the Pine Point carbonates. This interval is correlated with the Stone Formation and presumed to be of Eifelian age. The thickness of the Stone Formation decreases rapidly southward by onlap onto the southeast extension of the Peace River Arch. Major unconformities separate the Stone and equivalents from the underlying lower Paleozoic and from the overlying Middle Devonian Pine Point Formation.

Lower Paleozoic

A thick sequence of sediments of Cambrian and Ordovician age is present in the Murray Range west of the Sukunka/Bullmoose area. The terminology, lithology and thickness of the various units have been described by Slind and Perkins (1966) and Ziegler (1969). In excess of 1500 m of argillaceous limestone, quartzite, dolomite, and sandy dolomite is present in the Ordovician succession. Cambrian strata, comprising in the order of 2700 m of quartz sandstone, varicoloured dolomite, shale and grey dolomite overlie an unknown thickness of Precambrian sediments.

In summary, it is estimated that approximately 1100 m of Permian, Carboniferous and Devonian sediments underlie the Triassic in the Sukunka Field area. Seismic data indicate this thickness to be much greater. At approximately 2.7 seconds, a generally strong event is interpreted as representing the Paleozoic - Precambrian Shield - type Basement interface. The interval Carboniferous to Basement contains complex structural imbrications and thus does not represent true stratigraphic thickness. We speculate that the Devonian sediments rest on basement or on a relatively thin early Paleozoic sequence, under the Sukunka-Bullmoose area. At some point progressing westward from the field area to the Main Ranges, early Paleozoic and Precambrian metasediments must be present. A competent structural unit, the Rundle Group carbonates, is expected to underlie the field area, but the remainder of the Carboniferous and the Devonian probably contain a high percentage of incompetent shales.

REGIONAL TECTONICS

The Sukunka/Bullmoose area is centrally located within the Peace River Re-entrant, as defined herein (). It is relatively short, narrow segment of the western Canadian Laramide Orogen. Geographically, this area is situated in British Columbia between the Peace River and the Alberta border. From a tectonic point of view, the Peace River

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Re-entrant represents a constriction in the Laramide Fold Belt, between the main bulge formed by the Alberta Arc and a less pronounced arc centred in northeastern British Columbia. The origin of this feature is thought to be related to northeast-trending geofractures, which in turn were responsible, since Carboniferous time, for the development of the Peace River embayment where a substantial thickness of sediments was deposited and preserved.

In the southern Alberta fold belt, the Laramide compressive forces caused important eastward translation of the sedimentary skin through a series of imbricate structures above an undeformed west-dipping Shield-type basement. In British Columbia, the Laramide Orogen displays similar characteristics of compressional tectonics, mainly decollement folding and thrust faulting. However, because of variations in the sedimentary section, basement structure, and probably smaller compressive forces applied to the area, the

Regional Laramide tectonic setting.

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eastward translation of the competent sedimentary packages, along decollement zones and listric thrust faults, is thought to be much less than that observed in the Southern Alberta fold belt by Bally et al. (1966).

LOCAL STRUCTURE

In the Rocky Mountain Foothills of British Columbia, major gas reserves have been discovered in faulted anticlinal structures associated with the leading edges of thrust sheets involving upper Triassic carbonates.

The area is characterized at surface by a series of northwest-southeast trending folds and southwest-dipping thrust faults, involving Lower Cretaceous and Jurassic clastic sediments. A simplified map of the geology is shown in . Well and seismic data have been integrated with surface control to construct the regional cross section (). This section displays a series of asymmetric wave-like folds with some frontal imbrications at the leading edges of thrust sheets involving competent units. Progressing from east to west, the thrust faults cut deeper into the sedimentary section until Paleozoic units are exposed in the Front Range.

Surface observations indicate that numerous thrust faults, displaying short translation and lateral extent, are absorbed in the less competent Cretaceous units. Only the major Bullmoose, Nuisance, Gwillim and Tuscoola thrusts, which display important vertical displacement and can be followed at surface for distances up to 100 km, appear to affect most of the sedimentary section. These faults may originate within the deepest zones of decollement and thus be related to the early phase of Laramide deformations.

Seismic and well data indicate that fault planes that are relatively steep-dipping at surface flatten and converge into incompetent units such as the Fernie shales, the Charlie Lake anhydrites or the Banff - Besa River shales. Structural disharmony between surface and Triassic structures is attributed largely to the incompetent nature of the Cretaceous-Jurassic clastic sequence. Decollement zones in the Triassic anhydrite and shale units also cause disharmony between upper Triassic and Paleozoic carbonates. A deep seismic event, at approximately 2.7 seconds, is continous where data are of adequate quality. This event probably represents the lower Paleozoic - Shield - type 'Basement' interface, as no coherent events are recorded below. Overlying this 'basement' event, seismic data indicate the presence of converging low-angle events, suggesting intersection of bedding and fault planes.

STRUCTURAL DETAIL OF GAS FIELDS

The structural style affecting the competent upper Triassic carbonates is illustrated by wells drilled on the Bullmoose and Sukunka structures. The discovery well, Triad BP Sukunka a-43-B, displays frontal imbrication at the leading edge of thrust plate involving competent Pardonet-Baldonnel carbonates (). Detachment of these carbonates from the underlying Charlie Lake anhydritic units, and folding progressed until the threshold of rupture was reached and slippage occurred along fault planes. Three kilometres northwest of this discovery well, the b-65-B development well, () indicates an overturned anticlinal drag fold. The amplitude of this leading-edge fold has decreased from the a-43-B position, and slippage along fault planes is no longer evident in the core of the structure. This demonstrates the northward plunge of the fold. Another development well in the Sukunka structure, drilled in b-19-A, 3.5 km southeast of the a-43-B well, exhibits a pronounced imbricate structure, indicating a position of maximum amplitude of the Sukunka structure (). This section demonstrates clearly the incompetent nature of anhydrites in the Upper

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Regional surficial geology.

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Regional cross section, Rocky Mountains to Plains, Sukunka area.

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Charlie Lake Formation, and their role as a zone of decollement during a late phase of the structural evolution. A structural style similar to the Sukunka a-43-B well was encountered in the BP et al. Bullmoose d-77-E/93-P-3 discovery well (), where a vertical displacement of 1000 m represents a truly impressive leading edge.

A recent discovery, BP AEG West Sukunka c-45-J/93-P-4, drilled on a separate structural trend seven kilometres southwest of Sukunka, also displays a leading-edge style similar to a-43-B, with an overturned drag fold and minor lateral displacement along axial fault planes (). More than 600 m of relief are measured between the crest of the structure and the footwall.

An important aspect of this Sukunka/Bullmoose type of structure is fracture density. The highest density of fracturing is observed in the competent units in a position of maximum amplitude of the fold, before release of stress by faulting. Faulting occurred after the fold reached an overturned position. This corresponds to the highest density of fracturing that a given rock can support in the field of plastic deformation before entering the rupture field.

Structural details of Upper Triassic at a-43-B discovery, Sukunka structure.

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The BP Sukunka d-37-A/93-P-5 discovery well was drilled on a low-amplitude anticline with only minor faulting (). Cores from the Triassic in this well exhibit mainly subhorizontal slippage fractures and little or no vertical fracturing. This indicates that the fold has not reached the field of rupture deformation. A simplified Triassic structure map of the Field area is shown on .

We see the development of the Sukunka-type structure as a continuum of deformations proceeding from west to east, from which the following main events can be isolated:

1. Development of low-amplitude symmetrical folds.
2. Decollement.
3. Folding of competent package.
4. Overthrusting.
5. Development of asymmetrical folds with tight anticlines at the leading edge, on the east flank of low-amplitude synclines.
6. Fracturing of drag folds at the leading edge.
7. Thrust faulting resulting in frontal imbrication at the apex of the fold.

Structural details of Upper Triassic, north plunge of Sukunka structure.

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TRIASSIC RESERVOIRS

Stratigraphy and Lithology

The Pardonet, Baldonnel, Charlie Lake, Halfway and Doig Formations identified in Plains stratigraphy can be correlated readily into the Foothills, as illustrated in . The Pardonet Formation thins eastward as a result of pre-Jurassic erosion until it becomes truncated along an approximate north-south line as shown in .

The stratigraphy pertinent to the reservoirs is shown in . Subdivision of the Pardonet and Baldonnel Formations into informal units has been based on the signature of the gamma-sonic log. This subdivision facilitates correlation between fault slices and allows interpretation of facies changes. As can be seen, the units thus defined consist of relatively clean carbonate and argillaceous to silty carbonate. However, some highly radioactive stringers, rather than being shales, are clean carbonates containing some secondary uranium mineralization. Detailed lithological studies have not been completed on the Baldonnel and Pardonet Formations. The discussion that follows, therefore, represents our preliminary views.

Structural details of Upper Triassic, maximum crestal position, Sukunka structure.

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Structural details of Upper Triassic at d-77-E discovery, Bullmoose structure.

Structural details of Upper/Middle Triassic, West Sukunka, c-45-J discovery well.

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The Pardonet varies from 47 m at the north end of the Sukunka area (c-56-B) to 29 m at Murray River (d-48-I) 55 km to the south. This appears to reflect regional thinning (). The Pardonet Formation has been divided into four informal units P1 to P4 (in ascending order). P1 is highly radioactive and consists of dark grey to black, bituminous, dolomitic, pyritic, argillaceous siltstone. Unit P2 consists of an argillaceous, silty limestone and silty shale. Unit P3 is distinguished by the presence of relatively clean limestone or dolomite interbedded with silty, argillaceous limestone. The uppermost unit, P4, is characterized by clean arenaceous limestone or dolomite, and may be correlated with the Bocock Formation (Gibson, 1975).

The carbonates of the Pardonet Formation range from micropelleted to bioclastic and intraclastic wackestone and packstone. The matrix is dominantly very finely crystalline and contains silt and very fine grains of quartz dispersed or concentrated in thin laminae. Gradation to calcareous or dolomitic siltstone and sandstone is observed. Bituminous material, pyrite and phosphatized pellets are common accessories. The carbonates are occasionally oolitic or mottled. A brecciated texture showing syndepositional rip-up clasts is also observed ().

Structural details of Upper Triassic fold, East Sukunka structure.

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The Baldonnel Formation, about 80 m thick, provides most of the reservoir potential. The formation has been subdivided into four units, B1 to B4 (in ascending order), mainly on the basis of cycles of radioactive or argillaceous carbonates, and clean or arenaceous carbonates (). The carbonates are typically light grey or buff. Thin-shelled pelecypods, echinoids, crinoids, algal plates and filaments, unidentifiable bioclasts and oolites are present and form variable calcareous or dolomitic bioclastic wackestone, packstone or grainstone. The carbonates, mostly dolomite, are commonly silty or sandy. Laminae of sandstone are also present. Pyrite, euhedral quartz, amorphous silica and fluorite are present as minor constituents. Porosity is mostly of intergranular, intercystalline and biomouldic types. Pyrobitumen, dolomite and quartz crystals line some of the vugs and open fractures. Dark grey, thin-bedded and laminated argillaceous, bituminous and pyritic dolomites are present, most notably in Unit B3.

Representative rock types of the Baldonnel and Pardonet Formations are illustrated in . Although the thin sections are identified from specific cores of the Baldonnel or Pardonet, variants of the rock type as illustrated can be found in either formation.

The Halfway Formation has been penetrated in only two wells -- East Sukunka b-59-A/93-P-5 and West Sukunka c-45-J/93-P-4. It consists of fine-grained dolomitic sandstone. Original porosity has been destroyed by compaction and extensive silica cementation. Later leaching of unstable lithic grains or detrital carbonate created minor secondary porosity.

Top Triassic structure showing proven gas fields.

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Upper and Middle Triassic correlation, Foothills to Plains (see for line of section).

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Depositional Environments, Diagenesis and Pore Systems

Subsequent to deposition of the Charlie Lake evaporites, subsidence occurred along the eastern margin of the basin, believed to be near the site of the present-day Foothills. Subsidence allowed an eastward marine transgression and deposition of Baldonnel and Pardonet carbonate-clastic cycles on a very wide shelf in British Columbia and Alberta. Thin sections of selected cores of Pardonet-Baldonnel rock types (, ) provide a preliminary insight into the depositional history and diagenesis of these sediments.

The major depositional environments and facies are seen to be:

1. supratidal backshore or lagoonal with dolomitized algal wackestone facies ()
2. shoreline high-energy (oolitic grainstone and bioclastic packstone facies) (, )
3. foreshore environment (pelleted bioclastic and intraclastic packstone to wackestone (, , , )
4. deeper-water offshore (micropelleted mudstone to calcisiltite) (, )

Pardonet-Baldonnel isopach.

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Correlation of Upper Triassic Pardonet and Baldonnel units, Sukunka-Bullmoose area.

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The specific spatial relationship of Baldonnel and Pardonet facies types is not possible to determine. We speculate, however, that the cleaner carbonate units of the cycle represent high-energy, shallow-water, shelf and shoreline deposition, while the more argillaceous and silty carbonate units represent relatively deeper-water, low-energy deposition westward and offshore from the cleaner carbonates. The facies and major lithological components described above are similar to those outlined by Moore, Smitherman, and Allen (1972) for a Cretaceous carbonate beach sequence near Comanche, Texas. These authors describe the depositional environments as backshore (supratidal and lagoonal), beach (upper and lower foreshore), and offshore.

With regard to diagenesis and pore systems, the best porosity development is observed in the algal dolomites of the supratidal environment and in the bioclastic grainstone, packstone, and wackestone of the beach shoreline and foreshore environments. The most common porosity development is biomouldic and intercystalline. Similarities and differences between diagenetic changes in the Triassic carbonates and those described by Moore et al. (1972) for the Lower Cretaceous carbonates follow.

1. Good porosity is present in supratidal deposits of both sequences. In the Lower Cretaceous, Moore et al. (1972) believe that solution of aragonite and calcite is responsible for the dominant intercystalline pore system and, to a lesser degree, for mouldic porosity. They do not regard this system too highly because of relatively poor interconnection of pores.
2. Moore et al. (1972) found the best porosity to be present in the beach sediments. However, they note that very poor reservoir facies are present in the upper foreshore of the beach sediments whereas the best reservoir is present in the lower foreshore facies. The upper foreshore sediments have become silicified because of their position at the top of the phreatic zone where pH conditions were low. When uplift occurred, the upper foreshore sediments were impermeable and thus protected from meteoric-water circulation, whereas extensive solution and consequent biomouldic and intercrystalline porosity developed in the permeable lower foreshore sediments. Similar diagenetic conditions are interpreted for the Triassic carbonates, although original porosity of these sediments was much lower.
3. The offshore pelleted packstone and wackestone in the Lower Cretaceous of Comanche, Texas and the Upper Triassic carbonates of the Sukunka area form ineffective reservoirs.

Examples of Triassic reservoir rocks are illustrated in and . Porosity is typically biomouldic with or without intercrystalline and minor intergranular pores. The reservoir is described as low-grade which, by definition used herein, refers to porosities lower than 4 per cent and permeabilities less than 1 md. Although most of the reservoir is described as low-grade, zones with porosity of 5-10 per cent are not uncommon. A complex network of open fractures enhances porosity to some extent and permeability to a significant degree. As shown in , vertical and horizontal sets of fractures are often present. These fractures range from hairline to over a quarter of an inch in width. Some cores consist mainly of densely fractured rubble. A set of horizontal fractures is shown in , a photograph of a core of the Baldonnel from the East Sukunka b-59-A well. This well has permeability lower than most other Sukunka and Bullmoose wells -- a fact attributed to a lower density of fractures and (mainly) the absence of a vertical set.

Calculation of porosity/permeability by log analysis of the low-grade carbonate reservoirs through the gross-pay interval is difficult. Also, the quantitative contribution of micro- and macro-fractures to porosity is poorly known. In order to establish net pay, it has been necessary to integrate all data such as penetration rates, hydrocarbon mud logs, open-hole logs, drillstem tests, lithology and fracture evaluation. To qualify as net pay, an interval must be a relatively clean carbonate, indicate liberated gas and show porosity on logs or cores.

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a. Wackestone and Boundstone: (Baldonnel: b-19-A/93-P-5: 2838 m)

Abundant dolomitized algal crusts and filaments are present in a dolomicrite matrix. Minor constituents include floating silt-size quartz grains and opaque grains, probably sulphides or organic-rich material. Fenestral porosity (P) is well developed. Consistent with tidal and supratidal environment.

b. Oolitic Packstone to Grainstone: (Baldonnel: a-22-E/93-P-5: 3505 m)

The darker ooids and matrix on the left represent a silicification front with respect to the dolomitic matrix and ooids on the right. Intergranular porosity (P). Lithology characteristics of shallow-water high-energy beach deposits in a saline to hypersaline environment.

c. Dolomitized Pelleted and Bioclastic Packstone to Wackestone: (Baldonnel: d-77-E/93-P-5: 2686 m)

Platy bioclasts (probably mollusc or algal) in a dolomicritic and pelleted matrix. Biomouldic pores (P) partially infilled by sparry calcite. Scattered larger euhedral replacive dolomite crystals, and echinoderm or crinoid stems. Opaque grains, and darker matrix probably consists of sulphide and organic material. Biomouldic porosity and leaching indicative of shallow-water foreshore to emergent-beach conditions.

d. Pelleted Packstone: (Baldonnel: d-48-I/93-I-14: 1864 m)

Original pelletal packstone containing scattered mollusc and other skeletal fragments, dolomitized and leached. Biomouldic porosity (P).

e. Dolomitic Micropelleted Mudstone, to Siltstone: (Baldonnel: b-59-A/93-P-5: 3482 m)

The dolomitic mudstone is faintly micropelleted and laminated with evenly distributed floating silt-size quartz grains. A layer of quartz-sand packstone is present and large echinoderm and other bioclasts are scattered throughout. Opaque grains and partings may represent phosphatized organic material. Quiet depositional environment, probably subtidal to peritidal water depth of the lower foreshore on sheltered shelf.

f. Grainstone (Calcisiltite): (Pardonet: b-59-A/93-P-5: 3435 m)

Layer of dominantly dark-coloured micrite and micropelleted grainstone. Darker grains probably are organic material. Light-coloured layers of silt-size quartz grains.

g. Intraclastic Wackestone: (Pardonet: b-59-A/93-P-5: 3444 m)

Lithoclasts of pelleted packstone containing spicules and other bioclasts present in a carbonate mudstone matrix. Note shelter porosity (P) and some calcite infilling. The intraclastic fragments are probably syndepositional rip-up clasts and may have been deposited in a peritidal environment.

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a. Dolomitized Pelleted and Bioclastic Wackestone: (Baldonnel: d-48-I/93-I-14: 2150 m)

Platy bioclastic pelleted texture not easily recognized because of dolomitization. After leaching and enchancement of porosity, dolomitization stopped short of infilling all pores. Pores are not connected -- at least in the plane of this thin section. Vuggy porosity (P).

b. Dolomitized Pelleted and Bioclastic Wackestone: (Baldonnel: b-19-A/93-P-5: 2841 m)

Rock type is identical to that in (wackestone or boundstone). Good fenestral and intercrystalline porosity (P).

c. Fine Crystalline Dolomite: (Baldonnel: d-77-E/93-P-3: 2151 m)

Sediment completely dolomitized. Well-developed, vuggy porosity (P) with fair intercrystalline connected pores (P).

d. Fractured Calcisiltite (Pardonet: d-77-E/93-P-3: 2152 m)

Calcisiltite, faintly mircopelleted, illustrating a fracture system. Leaching followed by dolomite crystal growth and pyrobitumen lining of some fractures suggests an early fracture system. Fracture porosity (P).

e. Fractured Calcisiltite: (Pardonet: d-77-E/93-P-3: 2152 m)

Note what appears to be an early fracture system with partial infilling by dolomite (top) and a later fracture network (bottom and diagonal) devoid of infilling. Fracture porosity (P).

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a. Baldonnel: d-77-E/93-P-3

High density of open fractures displaying both horizontal and vertical sets.

b. Baldonnel: b-19-A/93-P-5

Complex pattern of open fractures, mainly subvertical. Fair matrix porosity.

c. Baldonnel: d-77-E/93-P-3

Vertical set of open fractures. Fair matrix porosity.

d. Baldonnel: d-59-A/93-P-5

Horizontal fracture set.

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Reservoir Data and Reserves

The b-65-B well, located on the north plunge of the Sukunka structure, was drilled into an overturned fold (). Perforations, net pays, drillstem tests and other relevant data are shown in . The well has excellent reservoir characteristics. Average porosity of 4 per cent does not include contribution from fractures. The b-65-B well has 92 m of net pay, mostly in the Baldonnel. The calculated absolute open flow for this well is 2 804 10³m³/d (99 MMcf/d).

The b-19-A well (, ), located four kilometres southeast of the Sukunka discovery well, penetrated five repeats of the Upper Triassic reservoir. Total net pay of 58 m has been established. After stimulation, the calculated absolute open flow is 760 10³m³/d (27 MMcf/d).

Similar illustrative data on the d-37-A well located on the East Sukunka structure are shown in . Different results obtained at this well are worth noting. The low-amplitude fold and minor faulting were previously mentioned and illustrated (). While normal pay thickness is present and a rate of 566 10³m³/d (20 MMcf/d) was recorded from drillstem testing, the calculated absolute open flow on this well is 340 10³m³/d (12 MMcf/d) after major stimulation.

The translation of reservoir data into pore-volume distribution for the Sukunka pool is illustrated in and . Total porosity-metres for the three wells range from 1.95 to 4.09.

At Bullmoose, it has not been possible at this time to separate and map individual thrust plates and, therefore, a narrow zone of fault repetitions is mapped (). The extent of Bullmoose reserves is not yet known, as the structure has not been completely delineated by drilling. An area of proven reserve has been established by well control and an additional area containing probable reserves was based on geological and seismic data.

A summary of reservoir data and proven reserve data is shown in . The proven reserves only of raw recoverable gas are 14 286 10⁶m³ (504 Bcf) while proven and probable reserves are 28 328 10⁶m³ (1.0 Tcf). The combined H₂S and CO₂ content ranges from 13 per cent at West Sukunka to 45 per cent at Bullmoose. This reduces the proven reserves to 8 868 10⁶m³ (313 Bcf) and the proven and probable to 16 544 10⁶m³ (584 Bcf) of sales gas. The excellent well producibility is evident from raw-gas deliverability potential of 6 034 10³m³/d (213 MMcf/d) against 8000 kPa (1140 psi) line pressure. This potential is from seven wells.

SOURCE, MIGRATION, TRAPPING MECHANISMS

Presentation of 'hard' data on the source, migration and trapping mechanisms of gas accumulation is notoriously difficult. Nevertheless, circumstantial evidence allows reasonable inferences to be made. These inferences are summarized in .

In northeast British Columbia, about 35 per cent of the gas and most of the oil reserves discovered to date are contained in Triassic sediments. As a number of oil and gas accumulations are present between sealing evaporite or shale units, it is evident that the source for at least some of the reserves is within the Triassic. Though limited source-rock analyses have been carried out at Sukunka, they nevertheless show that good hydrocarbon source beds, with 1 to 3 per cent total organic carbon, are present in the Pardonet, Baldonnel and overlying Fernie Formations. The organic matter is in the mature or dry-gas range, with thermal alteration index of 3+ to 4-.

The Pardonet and Baldonnel reservoirs have an overlying Jurassic Fernie shale seal and an underlying Charlie Lake evaporite seal. Migration of hydrocarbons or their percursors, from either the Pardonet, Baldonnel or overlying Fernie Formation, probably commenced soon after burial but, in any event, by early Cretaceous time after more than 1500 m of

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Net pay, producing potential and completion data, Sukunka b-65-B.

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Net pay, producing potential and completion data, Sukunka b-19-A.

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Net pay, producing potential and completion data, East Sukunka d-37-A.

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Sukunka -- lower fault plate complex, pore-volume map (porosity expressed as fraction).

Sukunka -- upper fault plate complex, pore-volume map (porosity expressed as fraction).

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sediments were deposited. Lateral migration was theoretically possible as reservoir carbonates were extensive. However, porosity and permeability in the oolitic, pelletoidal and bioclastic carbonates appear to be discontinuous. It is therefore probable that, although early migration did occur, it was only into locally porous and permeable facies.

The final important event was the Laramide Orogeny. In addition to providing structural traps, this series of events was critical in another sense -- it produced a network of open micro- and macro-fractures within the more competent Baldonnel/Pardonet carbonates. This network of fractures allowed re-migration of gas from previously isolated reservoirs and a gathering of that gas into closed structures at the leading edge of thrust sheets. Healing of these open fractures was prevented by the presence of gas. Formation of this fracture system was important in another way: it provided permeability enhancement which now accounts for the high deliverability rates on most wells.

EXPLORATION HISTORY

Nearly all facets of the oil and gas industry's operations are represented in the exploration history of the Sukunka/Bullmoose Gas Fields. These range from the early recognition of potential, the allocation of funds for geological and geophysical surveys, acquisition of land,

Bullmoose pore-volume map (porosity expressed as fraction).

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Reservoir, reserves and deliverability data, Sukunka-Bullmoose.

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Concepts on source and trapping mechanisms of Upper Triassic gas accumulations -- Sukunka area, Rocky Mountain Foothills, northeastern British Columbia.

Generalized top Triassic structure, 1966 interpretation of wide-angle reflection seismic data.

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farmouts, development of new technology, exploration and development drilling, compliance with the evolution of government regulations, and changing economic conditions, to marketing constraints.

In 1956, when Triad Oil Limited (now B.P. Canada Inc.) filed on acreage covering a broad region extending from the Pine River to near the Alberta border, the area was unmapped geologically and only a few shallow exploratory wells had been drilled. From 1956 to 1960, a number of surface stratigraphic and structural surveys were undertaken by Triad geologists to assess reservoir potential and to map structural configuration of the Cretaceous, Jurassic, Triassic and Paleozoic units. This work provided a reasonably good understanding of Cretaceous, Carboniferous and Permian stratigraphy and lithology, but only a limited understanding of the Triassic, as these units are poorly exposed. Structures of interest were mapped, including a Triassic outlier at Prairie Creek, the Bush Mountain anticline, and many other structures including the Sukunka/Bullmoose anticlinorium. A simplified version of this early mapping project is illustrated in . This work led to the drilling of the Prairie Creek a-24-H/93-I-10 in 1958, and the Bush Mountain a-24-A and b-33-A/93-0-10, exploratory wildcats in 1959. These wells were abandoned, but provided valuable information on stratigraphy and structural style. This work also helped in the planning of seismic surveys that started in 1959.

This early geophysical work, comprising seismic reflection and refraction, was largely ineffective. These results foreshadowed the problems that were to frustrate efforts of Triad and other companies for more than 20 years in their attempt to obtain usable seismic data in gravel-filled valleys or on higher ridges where the underlying strata are complexly folded and faulted. However, in 1963, a wide-angle reflection survey yielded useful data. This survey, a pioneering effort using helicopter transport, provided structural information on top of Triassic carbonates and was particularly useful in locating possible faults at this level. The generalized interpretation is shown in . Although the original target was Upper Carboniferous carbonates, seismic and geological data indicated that the Triassic would be a primary objective. Also, by this time Triassic gas discoveries had been made by other operators at Grizzly Valley, 80 km to the southeast of the Sukunka area, and at Falls Creek, 50 km to the north. The Sukunka a-43-B well was spudded in 1964 and completed in 1965 as a gas discovery in the Triassic Pardonet and Baldonnel Formations. The well encountered about 80 m of net pay over a 410 m interval and flowed gas at a stabilized rate of 950 10³m³/d (33.5 MMcf/d) against 15 053 klg (2183 psig) wellhead pressure.

It was with optimism, therefore, that Triad moved ahead to evaluate the extent of the accumulation. A location was selected in c-56-B, three kilometres to the northwest of the discovery well. The drilling of this well was followed in 1967 by a well in b-10-A, four kilometres to the southeast. Both wells were off-structure and were abandoned. It was not surprising, in view of these two dry holes, high drilling costs, and the price of gas at 11¢/Mcf, that the Company sought assistance through the farm-out route. In 1969 and 1970, a considerable amount of seismic work was done and an unsuccessful well was drilled under this program.

Except for the maintenance of a geological and land interest by B.P., the area languished until 1972, when the Company carried out a seismic reflection survey in another attempt to define the Sukunka and associated structures. This decision was based on the belief that common depth point shooting and improved field parameters and processing might prove useful. B.P. and partners decided to drill and evaluate a geological structure at Bullmoose Mountain that was prominently displayed on a version of the wide-angle reflection data (). The B.P. et al. Bullmoose d-77-E well was completed in 1975 as a significant Triassic gas discovery with 72 m of net pay, and a stabilized flow rate of 260 10³m³ (9.2 MMcf) of gas per day at 18 915 kPa (2743 psig).

The Bullmoose discovery marked a turning point, for it confirmed the presence of multiple gas-bearing structures with potential for large reserves. In addition, seismic reflection

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data started to provide useful clues as to details of structures. Acquisition of these data required innovative field methods involving surface energy sources, helicopter-portable equipment, and narrow line cutting to overcome the difficult topography and to minimize environmental damage. One line from the 1975 program is shown in . The difficulty in interpreting these data is obvious. However, integration with other control did provide useful information. Another line shot in 1976 at the north end of Sukunka is illustrated in . The improvement in quality is evident, as structural configuration at the Triassic level, faults, and near-basement events can be recognized readily.

The delineation and exploration drilling program since that time have gone well. A discovery on the East Sukunka structure, two successful development wells on the main Sukunka structure, a successful north step-out at Bullmoose and a new gas discovery at West Sukunka and at Murray River, 33 km south of Bullmoose, have been made. That complexities still exist is evident from the fact that a seven-kilometre step-out well on the Bullmoose structure, drilled in d-22-E, was unsuccessful, and that the extension at East Sukunka, b-59-A, found only marginal pay.

The costs of exploration and development in this region are high. For example, acquisition of seismic data range from $10,000 to $17,000 per kilometre. Moreover, complex structural geology requires greater attention to field methods, processing and interpretation, which also add to costs. Access to drilling sites is sometimes a major problem which, added to slow and difficult drilling, results in over-all costs of $4 to $6 million for a 3300 m well. Since 1956, B.P. and partners have invested over $120,000,000 in the Sukunka/Bullmoose area. Production started in early 1980.

Top Triassic structure, 1966 revision of wide-angle reflection seismic data.

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Sukunka -- seismic line "A", conventional, 600% (1975).

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Sukunka -- seismic line "B", surface source, 12000% (1976).

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