Chapter 36. Petroleum prospectivity of the Triassic Jurassic succession of Sverdrup Basin, Canadian Arctic Archipelago

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1 Chapter 36 Petroleum prospectivity of the Triassic Jurassic succession of Sverdrup Basin, Canadian Arctic Archipelago ASHTON EMBRY Geological Survey of Canada, Calgary, rd St NW, Calgary, AB, Canada, T2L 2A7 ( Abstract: The Sverdrup Basin of the Canadian Arctic Archipelago is an established petroliferous, rift/sag basin with 17 discovered oil and gas fields. Almost all the hydrocarbons occur in Triassic Jurassic shallow marine sandstones and were sourced from Middle to Upper Triassic bituminous shales. The discovered fields occur on the culminations of Palaeogene structures. Three prospective areas for future discoveries in the Triassic Jurassic succession include western Sverdrup, southeastern Sverdrup and the Fosheim Peninsula area. Most of the large structures in these prospective areas were mapped and tested in the initial round of hydrocarbon exploration. The largest potential play which has not been tested involves a stratigraphic component as part of the hydrocarbon trapping mechanism. Potential reservoir units are developed on the third-order sequence scale and 22 such sequences have been delineated in the Triassic Jurassic succession. Most of them contain a progradational, shallow marine sandstone unit which is in part porous within the prospective areas. These units are often truncated by unconformities on the basin margins and change facies to nonporous strata basinward. The pinchouts of these porous units in proper structural orientations provide good petroleum prospects because they were already present during the maturation and migration of the Triassic-sourced hydrocarbons. The Sverdrup Basin is a large rift/sag basin in the Canadian Arctic Islands, and it contains a thick succession of Carboniferous to Palaeogene strata. The initial workers, who defined the main stratigraphic units and mapped their distribution, recognized that the basin had a good petroleum potential. Industry drilled 120 wells in the basin from 1969 to 1986 and 17 oil and/or gas fields were discovered. The remoteness of the area and falling commodity prices left the hydrocarbons stranded. Recent predictions of future petroleum shortages and an overall assessment that the Arctic region contains major undiscovered hydrocarbon resources have sparked renewed interest in the hydrocarbon potential of the Sverdrup Basin. In this paper, I briefly review the geological development of the basin and the past hydrocarbon discoveries. The main theme of the paper is to qualitatively assess the potential for future hydrocarbon discoveries in the Triassic Jurassic strata of the basin. This succession was chosen for assessment because almost all of the discovered hydrocarbons occur in these strata and sufficient surface and subsurface control is available to allow a reasonable assessment to be made. Sverdrup Basin The Sverdrup Basin, in the Canadian Arctic Archipelago (Fig. 36.1), was first recognized and defined by Fortier et al. (1963) on the basis of field mapping by the Geological Survey of Canada in 1955 (Operation Franklin). The tectonic and depositional development of the basin has been recently summarized by Embry & Beauchamp (2008). The basin extends for about 1300 km in a NE SW direction and is up to 350 km wide. The basin axis trends southwesterly for the most part and turns westward and is truncated at the Arctic Ocean margin in the far SW (Fig. 36.1). Maximum sediment thickness is estimated to be c. 15 km and the basin fill consists of up to 5 km of Carboniferous and Permian strata with extensive carbonate and siliciclastic depositional systems and 10 km of Mesozoic to Palaeogene siliciclastics. Comprehensive descriptions for the Upper Palaeozoic stratigraphy are found in Davies & Nassichuk (1991) and Beauchamp et al. (2001, 2009). Embry (1991) summarizes the Mesozoic stratigraphy and deposition history and Ricketts (1994) and Ricketts & Stephenson (1994) provide excellent descriptions and interpretations of the Palaeogene strata of the basin. The basin formed by rifting over an Early Palaeozoic fold belt (Balkwill 1978; Embry & Beauchamp 2008) and the main siliciclastic sediment source areas were the immediately adjacent Franklinian Basin and the more distal Canadian and Greenland shield areas which lay to the south and east, respectively. Devonian clastic sediments are interpreted to have formed the bedrock of these source regions and it was not until Cretaceous that the Precambrian rocks were exposed and contributed detritus to the basin (Patchett et al. 2004). A smaller source region lay to the NW of the basin and was named Crockerland by Embry (1993a). It contributed sediment to the basin from Carboniferous to mid- Jurassic, although the amounts were far smaller than those derived from the extensive source areas to south and east. In mid- Jurassic times rifting of the proto-amerasia Basin began (Embry & Dixon 1994) and a rift shoulder (Sverdrup Rim) remained as a narrow land area separating the two basins (Fig. 36.2). Crockerland was dismembered by the opening of the Amerasia Basin and fragments of this former land area now lie buried on the shelves and slopes of NW Canadian margin as well as those of the East Siberian and Chukchi seas of Russia and the Chukchi Borderland. Rifting and high rates of subsidence dominated the basin history from early Carboniferous to mid-permian (Stephenson et al. 1987) and carbonate sedimentation, which included huge reefal edifices, was widespread (Davies & Nassichuk 1991; Beauchamp et al. 2001). Notably, a thick Carboniferous salt unit was deposited over the central part of the basin during this rifting phase of basin development. It subsequently gave rise to numerous diapiric and related salt structures and prominent thinning of the entire basin succession can be seen on seismic sections across the structures (e.g. Harrison 1995, figure 182; Jackson & Harrison 2006) (Figs 36.3 & 36.4). Salt movement probably began by Permian and is continuing today. From mid-permian to earliest Cretaceous, basin tectonics consisted mainly of thermal subsidence punctuated by relatively minor tectonic pulses which resulted in marginal unconformities and notable changes in depositional regime and subsidence pattern (Embry 1990, 2006). Mixed carbonate, chert and siliciclastic depositional systems occurred during the Permian with siliciclastic regimes completely dominating the Triassic and Jurassic. Renewed rifting occurred from Early Cretaceous (Hauterivian) to earliest Late Cretaceous (Cenomanian) and a thick succession of deltaic clastics was deposited. Also during this time interval, extrusive and intrusive basic volcanic rocks were emplaced in From: Spencer, A. M., Embry, A. F., Gautier, D. L., Stoupakova, A. V.& Sørensen, K. (eds) Arctic Petroleum Geology. Geological Society, London, Memoirs, 35, /11/$15.00 # The Geological Society of London DOI: /M35.36

2 546 A. EMBRY Fig Distribution of the large salt diapirs and salt-cored uplifts in Sverdrup Basin. The salt was derived from a thick Carboniferous salt unit (Otto Fiord Formation) which was deposited during the early rifting phase of the basin. Fig (a) Map of the Canadian Artic Archipelago with various geographic localities mentioned in the text. (b) Outline of Sverdrup Basin with basin axis. The red dots represent the 120 wells drilled in the basin. The main source areas lay to the south and east as indicated by arrows and Crockerland, a low-lying land area to the north, was a relatively minor source area. TH, Tanquary High. the basin and these are related to a mantle plume which lay north of Ellesmere Island in the Early Cretaceous. This plume was the source of a 30 km-thick basalt edifice which crosses the oceanic Amerasia Basin (Alpha Mendeleyev Ridge) (Embry & Osadetz 1988). Extrusive basalt flows occur only in the NE and the volume of intrusive, diabase sills and dykes progressively decreases to the SW. The basin was uplifted and deformed in late Palaeogene (Eurekan Orogeny) with the compression being mainly related to the counterclockwise rotation of Greenland away from North America (Harrison et al. 1999). The eastern portion of the basin (Ellesmere and Axel Heiberg islands) was substantially folded and cut by thrust faults and is now a mountainous terrain. To the west, uplift and deformation were much less and the fold amplitude progressively lessens to the SW. The salt structures were substantially amplified at this time and most of the anticlines in the western Sverdrup Basin are cored by Carboniferous salt. Past petroleum exploration and discoveries The early geological studies of the Arctic Islands indicated that the Sverdrup Basin, as well as the adjacent Lower Palaeozoic Franklinian Basin, had good petroleum potential (Douglas et al. 1963). In 1967, Panarctic Oils Limited, a partnership of private companies and the Canadian government, was formed to explore for petroleum in the Arctic Islands. In 1969, the first well spudded by Fig From Carboniferous through Early Jurassic, Sverdrup Basin received siliciclastic sediment from both the cratonic area to the south and Crockerland to the north (a). In Middle Jurassic the Amerasia ocean basin began to form by rifting and a narrow, intermittently exposed rift shoulder separated the two basins from then on (b). Yellow-shading indicates a dominance shallow water sandstone and green-shading indicates deeper marine shales and siltstones. Fig Stratigraphic thinning of Triassic to Cretaceous units, due to truncation and onlap, is apparent on the flanks of the Vesey Hamilton Salt Wall which is located just north of Melville Island (From Harrison 1995, Figure 182).

3 CHAPTER 36 TRIASSIC JURASSIC PROSPECTIVITY SVERDRUP BASIN 547 shortages and a general interest in Arctic petroleum provinces have led to renewed interest in the petroleum prospectivity of the Sverdrup Basin. In this paper prospective areas and potential trapping situations for the Triassic to Jurassic succession of the basin are discussed. Triassic Jurassic stratigraphy Fig Oil and gas fields of the western Sverdrup Basin (modified from Waylett & Embry 1993). Panarctic, Drake N-67, discovered a giant natural gas field on a gentle anticline on the southwestern margin of the Sverdrup Basin (Sabine Peninsula, Melville Island) (Waylett 1989) (Fig. 36.5). Between 1969 and 1986 Panarctic and other companies drilled a total of 120 wells in the Sverdrup Basin (Fig. 36.1) and discovered 17 oil and/or gas fields, all in the western portion of the basin (Embry et al. 1991; Fig. 36.5). The discovered fields occur on Cenozoic anticlines (Fig. 36.6) and most appear to have a salt core based on seismic data and early growth features. All of the substantial petroleum accumulations occur in Triassic and/or Jurassic sandstones and the in-place reserves are estimated to be m 3 of gas and m 3 of oil (Waylett & Embry 1993). The source rocks for the petroleum are interpreted to be mainly oil-prone, bituminous shales of Middle and Late Triassic age (Powell 1978; Brooks et al. 1992) with Jurassic marine shales and sapropelic coals as possible minor sources. There has been no drilling or seismic collection in the Sverdrup Basin since However, predictions of future petroleum Figure 36.7 illustrates a generalized NE SW stratigraphic chart for the Mesozoic succession of Sverdrup Basin. The succession consists of alternating intervals of sandstone-dominant units and shale/siltstone-dominant intervals and most sandstone units are confined to the basin flanks. However, some thick sandstone units extend across the entire basin. The Triassic/Jurassic succession is about 6.5 km thick in the basin centre and is truncated and overstepped on the basin margins by Cretaceous strata. The Triassic Jurassic succession can be subdivided into three first-order sequences on the basis of the occurrence of largemagnitude sequence boundaries of latest Permian, earliest Rhaetian, late Aalenian and base Hauterivian age (Fig. 36.8). Each of these first-order sequences is divided into a number of second-order sequences by prominent basin flank unconformities and their basinward correlative surfaces (Fig. 36.8). Notable changes in depositional and tectonic regime occur across the second-order sequence boundaries and they subdivide the succession into distinctive intervals which, for the most part, are the foundation for the formation-scale, lithostratigraphic nomenclature. Each of these second-order sequences has been subdivided into two or more third-order sequences. The boundaries of these smaller-magnitude sequences consist of basin-flank unconformities and prominent maximum regressive surfaces over much of the basin. Changes in depositional regime across the third-order boundaries tend to be subtle and there are no significant changes in tectonic regime across the boundaries. However, sequences of this magnitude contain the main potential reservoir intervals and their recognition is critical for play conception and evaluating petroleum prospectivity. In the following appraisal of the petroleum prospectivity of the overall succession, the facies relationships are discussed at a third-order level within a second-order framework. Areas of prospectivity Fig Section across the Balaena and Char fields which are located to the south of Ellef Ringnes Island. The traps are gentle anticlines which represent very distal deformation of the Eurekan Orogeny. Fractures over the fields, especially Balaena, has resulted in substantial vertical migration and escape of hydrocarbons (modified from Waylett & Embry 1993). Three areas of hydrocarbon prospectivity for the Triassic Jurassic succession have been outlined (Fig. 36.9). These areas are interpreted to have the best opportunities for discovering hydrocarbon pools in Triassic Jurassic strata and are delineated on the basis of two main constraints potential reservoir strata of the Triassic Jurassic succession are in the subsurface and the Triassic source strata are in the oil window over much of the area (Fig ). The main area of prospectivity is the western Sverdrup Basin where all the discoveries have been made. Much of this area is offshore and Lower Cretaceous strata are present at or very near the seafloor. The Triassic Jurassic succession subcrops or outcrops along the southern and northwestern margins of this area. To the east (Axel Heiberg/Ellesmere islands), high maturity, complex deformation, extensive dyke and sill intrusion, lack of porosity and widespread outcrop of the Triassic Jurassic succession greatly reduce any prospectivity. Two small areas of potential prospectivity have been outlined in the eastern Sverdrup Basin, one along the southeastern margin in the Norwegian Bay area (Fig. 36.1) and one on Fosheim Peninsula of west-central Ellesmere Island where the Triassic Jurassic succession is buried by Cenozoic and Cretaceous strata (Fig. 36.9). In both these areas the Triassic source rocks are interpreted to be present and in the oil window.

4 548 A. EMBRY Fig NE SW stratigraphic cross section of the Mesozoic strata of Sverdrup Basin (from Embry 1991). Structural prospects As noted above, all the discovered hydrocarbon fields belong to one general play type, anticlinal structures formed mainly during the Palaeogene (Eurekan Orogeny) with most having earlier, salt-related growth. Figure illustrates the productive structures as well as those which have not yet been tested, as delineated by the available seismic data (Waylett & Embry 1993). Many untested, small structures have been delineated in the both the western Sverdrup and southeastern Sverdrup prospective areas. Seismic coverage is such that it is doubtful that any large structures have gone undetected but the smaller ones may well be larger than those illustrated in Figure As discussed by Waylett & Embry (1993), some of the higher amplitude hydrocarbon fields in the Ellef Ringnes Island area are leaky, as demonstrated by zones of residual oil below the oil leg in some fields. It is likely that considerable amounts of hydrocarbons have escaped from these pools by way of fractures over the structure (Fig. 36.6). Potential reservoir strata occur over the entire western Sverdrup area and consist mainly of sandstones within the Heiberg Group Fig Triassic Jurassic stratigraphy, Sverdrup Basin with boundaries of first-, second- and third-order sequences indicated. Fig Prospective areas for hydrocarbon fields in Triassic Jurassic strata.

5 CHAPTER 36 TRIASSIC JURASSIC PROSPECTIVITY SVERDRUP BASIN 549 Romulus C-42 well. Good reservoir units occur in the Bjorne (Lower Triassic), Heiberg (Upper Triassic/Lower Jurassic), Sandy Point (Lower/Middle Jurassic) and Awingak (Upper Jurassic) formations. Given the abundance of sandstone in this basin margin area and the higher structural intensity, seal may have been a problem for the tested structures and seal integrity represents a significant risk factor for any remaining ones. It is likely that the past seismic surveys detected all the large structures in this relatively small area. Overall the prospectivity of this area for anticlinal plays is considered low. Prospects with a stratigraphic component Fig Thermal maturity zones for Middle Triassic strata (from Dewing & Obermajer 2011). which form the main reservoirs for most of the discovered fields. Upper Jurassic sandstones of the Awingak Formation occur in the southeastern portion of the area and porous sandstones are often present in the Lower Triassic, Middle Triassic and Upper Triassic intervals over portions of this area. It is reasonable to assume that a well drilled any where in this area will encounter some porous zones in the Triassic Jurassic succession. Chen et al. (2000) did a quantitative resource assessment for the structural play of the western Sverdrup Basin and concluded that natural gas pools larger than or equal to m 3 and between 7 and 9 crude oil pools greater than or equal to m 3 remain undiscovered. Additional and improved seismic data may well lead to such discoveries. Reconnaissance seismic data have outlined a number of structural culminations in the southeastern Sverdrup prospective area (Fig ) and reservoir strata are undoubtedly present in the Awingak and Heiberg formations over this entire area. Additional reservoir intervals may be present in Upper, Middle and Upper Triassic strata as well as in the Middle Jurassic interval. The main risk for the structural play in this area is the potential for the anticlines to be poorly sealed due to their higher amplitude. However, low-amplitude structures with adequate seals may well exist in this area. At this time, the Triassic Jurassic succession in this area has to be regarded as reasonably prospective with a moderate potential for a number of fields in excess of m 3 of gas or m 3 of oil. The Fosheim area of west-central Ellesmere Island is quite small and six wells have been drilled on structures in this area. Four of the wells had oil staining associated within various Triassic Jurassic sandstone units and a small amount of free oil flowed from the top of Upper Jurassic Awingak Formation in the Fig Mapped seismic closures for the western and central Sverdrup Basin. Numerous untested closures are present and more may exist. (modified from Waylett & Embry 1993). The first round of exploration in the Sverdrup Basin, led by Panarctic Oils, naturally focused on structural plays, in part to reduce risk, and also because the detailed sequence stratigraphic and facies relationships were not well known for much of the succession. In the first 10 years, onshore structures were tested and then, as technology was developed for offshore drilling, numerous structures were drilled in the inter-island areas. Exploration ceased with the collapse of petroleum prices in Over the past 25 years, detailed outcrop studies, as well as studies of all the well sections, has allowed the Mesozoic succession to be subdivided into first-, second- and third-order sequences (Embry 1988, 1991, 1993b, 1997). This sequence analysis provides a solid framework for determining the facies and truncation relationships at the third-order level. The extent of potential reservoir units has been broadly determined for each third-order sequence and the limits of these units are controlled by a combination of a basinward facies change to an impermeable facies and by landward erosional truncation. The disappearance of porous reservoir facies can form part of a petroleum trap when it occurs in a favourable structural orientation. Below, the general facies distribution for each of the third-order sequences is discussed in a framework of the eight second-order sequences present in the Triassic Jurassic succession. Comments are made on the potential for combination stratigraphic/structural traps within the third-order sequences. Lower Triassic sequence The Lower Triassic second-order sequence is characterized by a very thick succession of siliciclastics (up to 2000 m) of the Bjorne and Blind Fiord formations (Fig. 36.8). The sequence can be subdivided into three, third-order sequences and each contains a succession of fluvial, shallow shelf, deep shelf, slope and basinal facies which prograde basinward (Fig ). Figure illustrates the distribution of these facies for the uppermost, third-order sequence. In the western Sverdrup area, the basin-flank, fluvial facies, which consists mainly of porous, fine- to medium-grained sandstone, crops out on the southern basin edge. Notably, on western Melville Island, where the sandstones are unconformably overlain by Lower Jurassic shale, tar sand deposits are present (Trettin & Hills 1966). These deposits probably represent an exhumed oil field which was in part defined by the erosional edge of the Lower Triassic fluvial sandstones which were sealed by the overstepping Jurassic shale. The best opportunities for a stratigraphic component for traps involving Lower Triassic sandstones in the western Sverdrup would be associated with the basinward pinchout of the shallow marine facies of each of the three third-order sequences. As shown in Figure 36.13, such pinchouts are interpreted to occur just to the NE of the Lougheed Island area. Thick submarine fans are found in the Lower Triassic sequence in the central portion of the basin (Figs & 36.13). This facies has been studied in outcrop on eastern Axel Heiberg Island and in cores and chip samples in a well drilled on Cornwall Island. The

6 550 A. EMBRY Fig Stratigraphic cross section of Lower Triassic strata, east-central Sverdrup Basin. Three third-order sequences can readily be delineated from basin flank to basin centre and all three mainly consist of progradation siliciclastics which begin to fill the deep central basin. The fluvial and shallow marine facies contain porous sandstones and the pinchout edges of these units may help to trap hydrocarbons on the flanks of structures. sandstones of this facies were tight in all cases but it is possible that porous turbidite sandstones occur in some areas. In the SE Sverdrup area, the Lower Triassic fluvial sandstones outcrop on Bjorne Peninsula, Ellesmere Island but are truncated and overstepped in the subsurface to the west by Norian (Upper Triassic) shale and siltstone (Fig ). There would seem to be potential for the development of viable petroleum traps involving such truncation along the southern margin of this area. In the northern part of the area, there is a chance for the basinward pinchout of porous shallow shelf sandstones. In the Fosheim prospective area, the wells encountered thick fluvial to shallow shelf sandstones in the Lower Triassic sequence. These sandstones change facies to shale and siltstone westward and it is possible that a stratigraphic component will be a part of traps involving Lower Triassic strata in the western portion of the area. Middle Triassic sequence Fig The porous, fluvial sandstones of the Bjorne Formation (Lower Triassic) are truncated and overstepped by shales of the Barrow Formation (Upper Triassic) in the southeastern Sverdrup area. Such truncation can contribute to combination structural stratigraphic traps. shales are common in the sequence throughout the basin. The shales are part of the Murray Harbour Formation and are overlain by the sandstone-dominant Roche Point Formation (Fig. 36.8). The bituminous shales are excellent petroleum source rocks (Brooks et al. 1992) and they occur within all three prospective areas. Two third-order sequences can be correlated throughout the basin. These approximate the two stages of the Middle Triassic, Anisian and Ladinian. In the western Sverdrup area, only the lower, mainly Anisian part of the sequence is preserved on the basin flank below the Upper Triassic unconformity and the succession consists mainly of porous sandstone of fluvial to shallow marine origin. Basinward, additional Middle Triassic section is preserved and a porous, shallow marine Ladinian sandstone subcrops beneath the upper unconformity (Fig ). Traps involving this truncated sandstone may well occur along the southern margin of this area. Notably the Ladinian sandstone is also missing on the top of saltcored, Hazen structure (Fig ) and thus plays involving the truncated edge of the Ladinian sandstone are also possible on the The Middle Triassic second-order sequence is characterized by greatly reduced sediment input and bituminous, offshore marine Fig General facies distribution for the Spathian third-order sequence (modified from Embry 1991). The basinward edge of the shallow water facies is a potential fairway for stratigraphic/structural traps. Truncation traps may be present along the basin flank. Fig Stratigraphic cross section of Middle Triassic second-order sequence in the southwestern Sverdrup Basin (Melville Island Lougheed Island area). The sequence is divisible into two third-order sequences of Anisian and Ladinian age. The siliciclastic sediment was derived from the south. The truncated latest Middle Triassic (Ladinian) sandstone represents a potential prospect especially on account of the petroleum source rocks which lie directly below it.

7 CHAPTER 36 TRIASSIC JURASSIC PROSPECTIVITY SVERDRUP BASIN 551 Fig Correlation of Middle Triassic strata from the Hazen F-54 well located to the NW of Melville Island and drilled on the crest of a salt-cored structure to the Cape Norem A-80 on Mackenzie King Island and drilled well down the flank of a salt-cored structure. Note the absence of porous Ladinian sandstone on the crest of the structure. The down-dip pinchout edge of the sandstone may be prospective. flanks of basinward salt structures. Porous, upper Ladinian sandstone forms the main reservoir in the Roche Point gas field which occurs offshore Melville Island. A prominent unconformity also caps the Anisian sequence on the basin flanks and a truncated upper Anisian sandstone can be expected to be found at the basinward termination of this unconformity. Control is not sufficient to delineate the location of such a potential trapping situation which has to occur north of Lougheed Island where the Anisian unconformity is still present in wells in that area. The Middle Triassic shallow-water facies is replaced basinward by tight, offshore facies (Fig ) and pinchouts of porous shallow shelf sandstones on the flanks of highs can be expected. In the SE Sverdrup area, control for Middle Triassic stratigraphic analysis is very sparse. However, similar stratigraphic relationships as described for the western Sverdrup basin can be expected to occur here with the caveat that there was less sediment input. There is a reasonable possibility for the occurrence of traps involving the pinchout of porous Middle Triassic sandstones in this area and subcrop edges of the upper Anisian and upper Ladinian sandstones (Fig ). In the Fosheim area, the truncation of the porous, Ladinian sandstone beneath transgressive Carnian limestone can be observed in outcrop to the north and east of the area (Fig ). Such a relationship may also be preserved in the subsurface in the eastern Fig Distribution of latest Middle Triassic facies. The interpreted basinward edge of the Ladinian sandstone is prospective on the flanks of structures or in fault-related traps. Fig (a) Middle Triassic stratigraphy on northern Ellesmere Island. The Middle Triassic is completely missing on the Tanquary High (Yelverton section) and the truncation edge of the porous shoreface sandstone of the Ladinian occurs in the Sawtooth section. The sections lie basinward of any Anisian shallow water sandstone deposition. (b) Outcrop at the Sawtooth section showing the preservation of the porous (orange-weathering) shoreface sandstone of late Ladinian age. portion of the area. The basinward pinchout of the Ladinian shallow water sandstone unit due to facies change to tight offshore facies probably occurs in the western portion of the Fosheim area and presents play opportunities (Fig ). This area is too far basinward for the occurrence of a truncated Anisian sandstone. Carnian sequence The Carnian is the lowest of three stages of the Upper Triassic series and Carnian strata comprise a second-order sequence. The sequence consists of the limestone-dominant Gore Point Member, the shale/siltstone-dominant Hoyle Bay Formation and the sandstone-dominant Pat Bay Formation (Fig. 36.8). The Carnian sequence can be readily subdivided into three third-order sequences each bound by unconformities on the basin flanks. A major change in source area and subsidence rates occurred across the Ladinian/Carnian sequence boundary and sediment input greatly declined such that limestone deposition became widespread on the basin shelves. A prominent limestone unit, which is perhaps the best seismic reflector in the Mesozoic succession, forms the shallow water facies of the first third-order sequence and progrades far basinward over nonbituminous shales. Following this, Crockerland was the main source of the siliciclastic sediment. For most of the basin, the shallow marine sandstone facies is confined to the basin margins. However, in western Sverdrup Basin, Carnian sandstones prograded southward from

8 552 A. EMBRY Carnian shelf sandstones also pinch out basinward in this area, providing additional potential exploration opportunities. In the Fosheim area, the Carnian sequence contains mainly limestone at the base overlain by a thick shale/siltstone-dominant interval. A few shelf sandstones occur in the argillaceous succession and these would probably pinch out to the west. The base-norian unconformity extends over most of this area and a subcropping, uppermost Carnian sandstone is probably present within the subsurface in this area and represents a viable target. Norian sequence Fig Carnian (Upper Triassic) stratigraphy in the Lougheed Island Melville Island area of the southwestern Sverdrup Basin. The Carnian represents a second-order sequence which is divisible into three third-order sequences. In contrast to the underling Middle Triassic, the siliciclastics sediments were derived from the north (Crockerland). Crockerland almost completely across the basin and thus are thick and widespread in that area (Fig ). Most of the sandstones have little to no porosity due to widespread calcite cementation although the youngest sandstones in the sequence can have up to 10% porosity. These sandstones form a secondary reservoir in the Roche Point field (Fig. 36.5). Notably, on the southwestern flank of the basin, the middle third-order sequence (Eden Bay Member) consists of interbedded bituminous shale and limestone (offshore shelf), and is laterally equivalent to southerly prograding sandstones deposited to the north. These strata are important petroleum source rocks. This is the only area in the basin where Carnian hydrocarbon source rocks are present. In the western Sverdrup area, the shallow water sandstones change facies southward to offshore shale and siltstone and porosity pinchouts are possible in this area (Figs & 36.20). The uppermost Carnian sandstone is truncated by the basal Norian unconformity on the southern basin margin and subcrop trapping situations with enhanced porosity may occur in this area. In the SE Sverdrup area, the Carnian sandstones subcrop beneath the base-norian unconformity shales as illustrated in Figure and represent the best Carnian prospect in the area. The Norian sequence is bound by a second-order sequence boundary below and a large-magnitude, first-order boundary (base- Rhaetian) above. Once again, major changes in depositional and tectonic regimes occurred across the basal sequence boundary. Sediment supply greatly increased and entered the basin from Crockerland. Only minor sediment entered the basin from the shield areas to the east and south. In the east, the Norian sequence comprises the Barrow Formation and the Romulus Member of the Heiberg Formation. It is up to 1000 m thick and the sandstonedominant Romulus Member consists of repetitive coarseningupward cycles of shale to medium-grained sandstone of shallow marine, probably deltaic, origin. In the western Sverdrup, the sequence is usually less than 100 m thick and consists mainly of shale and siltstone of the Barrow Formation. In this area it can be subdivided into two third-order sequences (Fig ). The marine sandstones in the uppermost part of the sequence are assigned to the Skybattle Formation. In the western Sverdrup area, the sandstones of the Skybattle Formation are truncated by the major base Rhaetian unconformity both on the basin flank and over many structures (Fig ). There is good potential for the existence of combination structural/stratigraphic traps involving these sandstones. However, control is not sufficient to outline the fairways where such trapping conditions may exist, although on the flanks of salt structures is one prospective area for such a play. Notably shallow marine sandstones of the Skybattle Formation are the main gas reservoirs in the Maclean field, east of Lougheed Island (Fig. 36.5). In the southeastern Sverdrup and Fosheim areas, the thick Norian sequence, dominated by marine sandstones of Romulus Member, probably contains few, if any, opportunities for stratigraphic traps because the sandstone units mainly extend across the entire area. Fig Distribution of late Carnian facies. Most of the sand-dominated, nearshore shelf facies of the western Sverdrup was derived from the north. The basin margin truncation edge of the sandstone facies as well as the basinward edge of the sandstone facies are potentially prospective especially given their proximity to rich petroleum source rocks. Fig The Norian strata comprise a second-order sequence and in the western Sverdrup can be subdivided into two third-order sequences. A very widespread unconformity (first-order boundary) at the base of the Rhaetian truncates the upper Norian sandstones over structures and truncated sandstone edges down-dip and on the basin margin are prospective.

9 CHAPTER 36 TRIASSIC JURASSIC PROSPECTIVITY SVERDRUP BASIN 553 Rhaetian Sinemurian sequence This second-order sequence is very thick in the eastern and central portions of the basin, where it is completely dominated by a sandstone-rich fluvial delta plain facies (Fosheim Member, Heiberg Formation; Figs & 36.23). In the western Sverdrup Basin, shallow marine strata dominate the sequence and three third-order sequences, of Rhaetian, Hettangian and Sinemurian age, can be recognized (Figs & 36.24). Each sequence is characterized by a lower argillaceous portion (Grosvenor Island, Lougheed Island formations) and an upper sandstone-dominant portion (Maclean Strait and King Christian formations), and the sandstones of each sequence are truncated on the SW basin margin (Fig ) and change facies to offshore shale/siltstone in the far west (Embry 1982; Embry & Johannessen 1993). In the western Sverdrup Basin the sandstones of the Sinemurian sequence (King Christian Formation) are the main reservoirs in the numerous anticlinal fields in the Ellef Ringnes Island/Lougheed Island corridor (Fig. 36.5). Sandstones of the Rhaetian sequence (Drake Point Member, Maclean Strait Formation) are the main reservoirs in the giant Hecla and Drake Point fields on Melville Island. Figure is a cross section which is perpendicular to the southwestern basin margin and it illustrates the truncation of the sandstones of the three third-order sequences. The Rhaetian sandstone which forms the reservoir in the Hecla gas field is truncated south of the field (Fig ) and the truncation contributes to the trapping mechanism of the field. The sandstone of the overlying Hettangian sequence is truncated south of the North Sabine H-49 well at the north end of Sabine Peninsula, Melville Island and subcrops north of the Hecla and Drake Point gas fields (Fig ). Petroleum traps may occur along the truncation edge of the Hettangian sandstone, which extends from Melville to Prince Patrick Island (Fig ). The Sinemurian sandstone (King Christian Formation) is also truncated below the base- Pliensbachian unconformity along the SW margin and is also a potential target on the SW basin margin. As shown on Figure 36.26, the Sinemurian, shallow shelf sandstone changes facies to argillaceous outer shelf facies, as does the underlying Hettangian shallow shelf facies (Fig ) in the far western portion of the basin. Areas where these facies changes Fig Schematic basinwide cross section of the sandstone-dominant Heiberg Formation/Group. This stratigraphic unit consists mainly of the Rhaetian Sinemurian second-order sequence (Fosheim Member and equivalent formations) which can be subdivided into three third-order sequences in the western portion of the basin. In ascending order, these are of Rhaetian, Hettangian and Sinemurian age. The basal portion of the Heiberg Formation/Group consists of sandstones of the uppermost Norian sequence, and the uppermost Heiberg consists of the sandstones of the Pliensbachian sequence. Sandstones of the Heiberg are the main reservoirs in the many of the discovered fields. Fig Heiberg Formation at Yelverton Pass, Ellesmere Island, on the flank of the Tanquary High (Fig. 36.1b) northeastern Sverdrup Basin. The delta plain strata of the Fosheim Member (striped) (Rhaetian Sinemurian) are unconformably overlain by the massive shallow marine shelf sandstones of the Remus Member (Pliensbachian). and loss of porous sandstones occurs on the flanks of structures would be prospective. In the southeastern Sverdrup and Fosheim prospective areas, the Rhaetian Sinemurian sequence consists entirely of fluvial/delta plain facies and lacks opportunities for stratigraphic traps except for possibly large, incised valley fills. Pliensbachian Aalenian sequence The next second-order sequence comprises Lower to Middle Jurassic (Pliensbachian Aalenian) strata and, in contrast to the preceding sequence, sediment supply was low and shale and siltstone dominate the sequence throughout the basin. The sequence is subdivided into three third-order sequences Pliensbachian, Lower Middle Toarcian and Upper Toarcian Aalenian (Fig. 36.8). A major first-order sequence boundary of earliest Bajocian age caps the sequence and the unconformable portion of the boundary extends far basinward. The Pliensbachian sequence consists of shale and siltstone in the SW (Intrepid Inlet Member) and a very widespread, shallow shelf sandstone (Remus Member, Figs & 36.23), which caps the underlying Heiberg deltaic plain deposits. The Remus sandstones are important reservoir strata in anticlinal fields in the Ellef Ringnes Island area. The upper two sequences are similar, consisting mainly of offshore shelf shale and siltstone (Jameson Bay Formation) with an overlying, relatively thin, shallow to mid-shelf sandstone unit (Sandy Point Formation) which is truncated on the basin margins (Fig ). Some potential petroleum source strata occur near the base of the Toarcian portion of the Jameson Bay Formation but the extent and richness are not well established (Powell 1978). In the western Sverdrup area, the Sandy Point sandstones are almost invariably tight, burrowed, very fine- to fine-grained, silty sandstones of shelf origin (Fig ). These sandstones were deposited during a time of low sediment input and dampened energy within the basin. A narrow band of shoreface, potentially porous, sandstone probably occurs just basinward of the termination of both the Upper Toarcian and latest Aalenian unconformities and is a viable exploration target. The general position of such a prospective, upper Aalenian sandstone is illustrated in Figure In the Fosheim area porous Sandy Point sandstones are truncated by the sub-oxfordian unconformity and a truncation edge may occur in the eastern portion of the area. A similar situation may occur in the SE Sverdrup area.

10 554 A. EMBRY Fig Stratigraphic cross section of the Rhaetian Sinemurian second-order sequence on the southwestern basin margin. The sequence is divisible into three third-order sequences and each contains an upper sandstone which is truncated landward. Bajocian Callovian sequence The basal-bajocian unconformity is a first-order boundary and marks a major tectonic reorganization in the basin. This is interpreted to represent the onset of rifting in the adjacent Amerasia Basin (Embry & Dixon 1994). Sediment supply to the Sverdrup Basin decreased even more and the sequence is very thin (,150 m) and consists almost entirely of siltstone and shale over most of the basin (McConnell Island Formation). Sediment supply was higher only in the Prince Patrick western Melville Island area and widespread sandstone units (Fig ) were deposited (Hiccles Cove Formation) (Harrison & Brent 2005). The sequence can be subdivided into three, third-order sequences of Bajocian, Bathonian and Callovian age (Embry 1993b). Sediment supply increased for each succeeding sequence, being almost nonexistent for the Bajocian and still being relatively minor for the Callovian. A prominent unconformity (base Oxfordian) caps the sequence (Fig ) and has removed it over much of the southern and eastern basin flanks (Fig ). In the western Sverdrup, the Callovian sandstone may be involved in traps in the far SW where the sandstone changes facies to shale and siltstone on the flanks of structures. Elsewhere in this area, a thin, narrow band of porous shoreface sandstone may be present at the basinward termination of the basal-oxfordian unconformity (Fig ). However it is theoretically possible that such a sandstone may not exist, having been reworked and mainly eroded during the early Oxfordian transgression. Overall this sequence has very limited prospectivity in the western Sverdrup and has none in the SE Sverdrup and Fosheim areas where the sequence is completely absent. Oxfordian Valanginian sequence Fig Distribution of the late Hettangian facies of the central and western portions of the basin. The truncation edges of the sandstone facies in the west are prospective. This sequence is bound by the sub-oxfordian second-order boundary below and the base-hauterivian first-order boundary above. Sediment supply and subsidence rates significantly increased following early Oxfordian transgression. Three, third-order sequences are recognized in the Oxfordian Valanginian sequence. These smaller magnitude sequences are Oxfordian Kimmeridgian, Tithonian Berriasian and Valanginian in age. The Oxfordian Kimmeridgian sequence is up to 300 m thick and consists of a dark grey to black, lower shale siltstone unit (Ringnes Formation) which has minor petroleum source potential (Stewart et al. 1992). It is overlain by a sandstone-dominant interval characterized by prograding cycles of marine shale to

11 CHAPTER 36 TRIASSIC JURASSIC PROSPECTIVITY SVERDRUP BASIN 555 Fig Stratigraphic cross section of the Sinemurian third-order sequence, western Sverdrup Basin. The sandstone portion of the regressive systems tracts progrades westward and the western edge of the sandstone is prospective. The transgressive systems tract is thick near the deltaic input centre to the east and basinward edges of transgressive sandstones are also prospective. shoreface sandstone (Awingak Formation; Fig ). These shallow marine sandstones are important reservoirs in the Cisco and Whitefish fields near Lougheed Island and the Cape Macmillan field south of Ellef Ringnes Island (Fig. 36.5) and have oil and gas shows in other wells. The Awingak sandstones change facies to shale and siltstone of offshore shelf origin to the NW (Figs & 36.34) and the basinward edge of the Awingak is well established (Fig ). The Tithonian Berriasian sequence consists mainly of offshore siltstone and shale (Deer Bay Formation; Fig ) and shallow marine sandstone units are restricted to the southern basin margin. These sandstones are placed within the Awingak Formation and once again they form the upper portion of repetitive coarsening upward cycles of shallow marine origin. Gas occurs in one of these sandstone units in the Hecla field on Melville Island. The overlying Valanginian sequence also consists mainly of siltstone and shale of marine shelf origin (upper Deer Bay Formation) Fig Schematic cross section of the Toarcian and Aalenian third-order sequences which form the bulk of the second-order Pliensbachian/Aalenian second-order sequence. Both these third-order sequences contain regressive sandstones capped by unconformities and a strip of porous shoreface sandstone probably occurs at the basinward termination of the capping unconformities. Such narrow zones of truncated porous sandstone are prospective. Fig Outcrop of very fine grained, burrowed sandstone (Sandy Point Formation) which caps the Pliensbachian Aalenian second-order sequence on northern Axel Heiberg Island. The sandstone is conformably underlain by offshore shales and siltstones of the Jameson Bay Formation and conformably overlain by offshore shale of the McConnell Island Formation.

12 556 A. EMBRY Fig Distribution of Late Aalenian facies. The nearshore marine facies comprises mainly sandstone with the offshore shelf facies consisting mainly of shale and siltstone. Fig Outcrop of the Oxfordian Valanginian second-order sequence on the flank of the Lost Hammer Diapir on west-central Axel Heiberg. The formations which comprise the sequence include the Ringnes (R) Awingak (Aw), Deer Bay (DB) and basal Isachsen (I). The multiple fourth-order sequences of the Awingak Formation are well shown. Fig Outcrop of porous, shoreface sandstone of Callovian age unconformably overlain by Oxfordian siltstone, Marie Bay, northwestern Melville Island. The basinward edge of these Callovian sandstones are prospective in this area. and, in the central portion of the basin, units of delta front sandstone with interbedded shale and siltstone (basal Isachsen Formation) are preserved beneath the first-order, base-hauterivian unconformity. In the western Sverdrup area the basinward edges of individual Awingak sandstone units in both the Oxfordian Kimmeridgian and Tithonian Berriasian sequences are prospective on the Fig Stratigraphic cross section of the Oxfordian Kimmeridgian third-order sequence in the Lougheed Island area (western Sverdrup Basin) illustrating the basinward change of facies of the delta front/nearshore marine facies into the offshore shelf facies. Fig Distribution of Late Callovian facies. Over most of the basin sediment supply was low and the preserved nearshore sandstones are expected to be thin and of very limited extent. Fig Distribution of the late Kimmeridgian facies (upper Awingak/ Ringnes formations). The basinward edge of the sandstone-dominant shoreline to nearshore shelf facies is prospective.

13 CHAPTER 36 TRIASSIC JURASSIC PROSPECTIVITY SVERDRUP BASIN 557 There is also a good potential for the existence of combination stratigraphic -structural traps in the prospective areas. Stratigraphic pinchouts of porous facies within the Triassic Jurassic succession are associated mainly with either truncation by a basinmargin unconformity or by a basinward change to a nonporous facies. Such pinchouts in the proper structural orientation can contribute to sizable petroleum traps as evidenced by the truncation of the Maclean Strait sandstone as part of the trap for the giant Hecla field on Melville Island. Twenty-two, third-order sequences have been recognized in the Triassic Jurassic succession of Sverdrup Basin and most of these sequences contain a regressive, porous, shallow marine sandstone unit which is truncated on the basin margin and which pinches out basinward. Detailed mapping of such porosity pinchouts in combination with structural mapping will allow the delineation of numerous hydrocarbon prospects in the Triassic Jurassic succession of Sverdrup Basin. Fig Outcrop of channelized marine sandstone occurring the base of the Valanginian third-order sequence, Buchanan Lake area, east-central Axel Heiberg Island. Isolated sandstone units occur at this sequence boundary at various localities and are prospective. flanks of structures. The remainder of the sequence has little prospectivity except for perhaps sandstone units associated with the base-valanginian unconformity. The unconformable contact between the Tithonian Berriasian sequence and the Valanginian sequence is most often a shale-on-shale contact and can be difficult to recognize without close study. In a few outcrops on the NW flank of the basin, isolated shallow marine sandstones occur at this stratigraphic level and probably represent a combination of late regressive and/or early transgressive strata developed on one or both sides of the unconformity near its basinward termination (Fig ). Similar isolated sandstones form the reservoir facies and trap in the large Alpine oil field on the Alaskan North Slope (Houseknecht & Bird 2004) and thus offer a good target in the Sverdrup Basin. In the southeastern Sverdrup area these sequences have little, if any, prospectivity due to the high sand input in the area and the paucity of shale. In the Fosheim area, once again these sequences offer few, if any, opportunities for stratigraphically trapped hydrocarbons. Conclusions The Sverdrup Basin is a large rift/sag basin which contains a 15 km thick succession of Carboniferous to Palaeogene strata. An initial round of petroleum exploration from 1969 to 1986 resulted in the discovery of 17 gas and/or oil fields through the drilling of about 80 wildcat wells on viable prospects. The fields all occur in the western portion of the basin where favourable maturity of Triassic source rocks and very mild deformation occur. The fields are on Palaeogene structural culminations although most probably had earlier structural growth due to rising Carboniferous salt in their cores. The main reservoir strata are Triassic and Jurassic shallow marine sandstones. On the basis of the occurrence of mature source rocks and the presence of Triassic Jurassic reservoir strata in the subsurface, three areas of prospectivity have been outlined. These are (1) most of the western Sverdrup, (2) an area in the southeastern corner of the basin and (3) a small area on west-central Ellesmere Island. A number of small to moderate sized structural culminations remain to be drilled in these areas and many have good potential to contain hydrocarbons. I would like to thank my employer, the Geological Survey of Canada, for supporting my research on the geology of the Sverdrup Basin and for allowing publication of this paper. D. Sargent prepared the figures and his hard work and creativity are appreciated. References Balkwill, H. R Evolution of Sverdrup Basin. AAPG Bulletin, 62, Beauchamp, B., Harrison, J. C., Utting, J., Brent, T. A.& Pinard, S Carboniferous and Permian Subsurface Stratigraphy, Prince Patrick Island, Northwest Territories, Canadian Arctic. Geological Survey of Canada, Ottawa, Bulletin, 565. Beauchamp, B., Grasby,S.&Henderson, C Late Permian sedimentation in the Sverdrup Basin, Canadian Arctic: the Lindström and Black Stripe formations. Bulletin of Canadian Petroleum Geology, 57, Brooks, P., Embry, A., Goodarzi, F.& Stewart, R Organic geochemistry and biological marker geochemistry of Schei Point Group (Triassic) and recovered oils from the Sverdrup Basin (Arctic Islands, Canada). Bulletin of Canadian Petroleum Geology, 40, Chen, Z., Osadetz, K., Embry, A., Gao, H.& Hannigan, P Petroleum potential in western Sverdrup Basin, Canadian Arctic Archipelago. Bulletin of Canadian Petroleum Geology, 48, Davies, G. R.& Nassichuk, W. W Carboniferous and Permian history of the Sverdrup Basin, Arctic Islands. In: Trettin, H. P. (ed.) Geology of the Innuitian Orogen and Arctic Platform of Canada and Greenland. Geology of Canada, 3. Geological Survey of Canada, Ottawa, Dewing, K.& Obermajer, M Thermal maturity of the Sverdrup Basin, Arctic Canada and its bearing on hydrocarbon potential. In: Spencer, A. M., Embry, A. F., Gautier, D. L., Stoupakova, A. V. & Sørensen, K. (eds) Arctic Petroleum Geology. Geological Society, London, Memoirs, 35, Douglas, R., Norris, D., Thorsteinsson, R.& Tozer, E Geology and petroleum potentialities of northern Canada. World Petroleum Congress Proceedings, Frankfurt, Section 1, paper 7, Embry, A. F The Upper Triassic Lower Jurassic Heiberg Deltaic complex of the Sverdrup Basin. In: Embry, A.& Balkwill, H. (eds) Arctic Geology and Geophysics. Canadian Society of Petroleum Geology, Alberta, Memoirs, 8, Embry, A. F Triassic sea-level changes: evidence from the Canadian Arctic Archipelago. In: Wilgus, C. et al. (eds) Sea-Level Changes an Integrated Approach. Society for Sedimentary Geology, Tulsa, OK, Special Publications, 42, Embry, A. F A tectonic origin for third-order depositional sequences in extensional basins implications for basin modelling. In: Cross, T. (ed.) Quantitative Dynamic Stratigraphy. Prentice Hall, Engelwood Cliffs, NJ, Embry, A. F Mesozoic history of the Arctic Islands. In: Trettin,H. (ed.) 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