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Highlights of Petroleum and CrustalFramework of the Beaufort-MackenzieBasin: Key Results from BeaufortSPAN EastPhases I and II SurveysMenno DinkelmanGX Technology (GXT), a Subsidiary of ION Geophysical Corporation, Houston, Texas, USA

Naresh Kumar, James Helwig, Pete Emmet and James GranathConsultants, GXT, a Subsidiary of ION Geophysical Corporation, Houston, Texas, USA

Summary

The Beaufort-Mackenzie basin in Arctic Canada is a petrolif-erous province still in the early stages of exploration. Almostthree decades of exploration in the Mackenzie Delta onshoreand shallower water offshore have resulted in 48 conven-tional oil and gas discoveries. Although no significantproduction exists as yet, mean undiscovered resources in thebasin have been estimated at 16.8 billion barrels (2.7 millioncubic meters) by the Geological Survey of Canada (Chen et al,2007), inclusive of deep water (to 2,500 m), and at 14.5 billionbarrels of liquids (2.3 billion cubic meters) and 86.6 trillioncubic feet of gas (2.32 trillion cubic meters) by the UnitedStates Geological Survey (USGS, 2006), exclusive of “deep-water” (>1,000 m).

Despite major questions about the petroleum system, theresults of recent bidding demonstrate that industry recog-nizes the deep-water potential of the basin. In 2007, Imperialand Exxon-Mobil bid Can $585 million on one of the deep-water blocks; this was followed by a Can $1.18 billion bid byBP on an adjacent block in 2008 (Indian and Northern Affairs,Canada, 2007, 2008).

To expand the knowledge of the basin architecture, GXTacquired ~3,500 km of 2D long-offset seismic data in the areain late 2006 (Phase I). Building on the success of Phase I,another ~5,500 km of data was acquired in 2007 (Phase II). Athird phase of survey is taking place in the summer of 2008.Cognizant of the deep-water potential, we have attempted toextend the lines as far west and north in the open water as iceconditions and environmental considerations have allowedin each season. These programs have been designed to imagedown to the base of the crust with a 9-km long cable, 18-second recording, and final depth processing (prestack depthmigration) to 40 km. The new data are interpreted to region-ally map the ocean-continent boundary and the top ofMOHO discontinuity, then tied to existing well data to iden-tify the major stratigraphic sequences formed since theopening of the Canadian Basin.

Highlights of interpretation include:

1 ) The off s h o re Mackenzie Delta system contains almost 15km of sediments; in areas outside the delta system, the LateM e s o z o i c - Tertiary sedimentary wedge is still 8-10 km thickat the shelf edge and upper slope;

2 ) Complex folds (earlier interpreted as “diapirs”), faults andt h rusts formed due to interaction of compressive folding,w renching, extension, inversion and gravity-inducedloading, all operating at the same time in distinct segmentsof this region;

3 ) A series of large, late-Tertiary stru c t u res occur in theo ff s h o re part of the delta extending to water depths of>1,000 m;

4 ) Gas chimneys and amplitude anomalies in the seismicdata are widespread indicating a very active petro l e u msystem; and

5 ) The ocean-continent boundary and oceanic crust, with anextinct spreading center, are mapped beneath the deltasediments, but the nature of the crust underlying thedeepest part of the basin remains uncertain.

Introduction

Although exploration in the Beaufort-Mackenzie Basin beganalmost three decades ago, and despite a series of discoveries,c o m m e rcial production remains elusive. The GeologicalSurvey of Canada (Chen, et al, 2007) and the United StatesGeological Survey (USGS, 2006) have made recent resourceassessments of the area. They conclude that the potential issignificantly larger than the results to date imply. A single,very large discovery is needed to provide the economicincentive to build the infrastructure for oil production in thearea. Existing discoveries could then be tied to this “anchor”discovery (Johnston, 2007). The critical question, therefore, iswhether that supergiant field exists in the unexplored outershelf and upper slope.

The existing well data in the area confirm a working petro l e u msystem, but some aspects of the geological history of the basinhave not been fully defined; for example, the details of thec rustal stru c t u re, the nature and location of the ocean-conti-nent boundary, and the geometry and timing of events in theBeaufort fold and thrust belt. The legacy seismic data arel a rgely confined to the middle and inner shelf, and, althoughsome deep reflection and refraction profiles have beena c q u i red in the area (Stephenson, 1994), most data penetrate to“exploration depths” (12 km or so). A d d i t i o n a l l y, the off s h o rewells are located only in the shallow (<50 m) parts of theMackenzie Delta, and, because of the thickness of sediments,well penetrations into sequences older than Eocene are rare.

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The BeaufortSPAN East program extends along almost 800 km ofMackenzie Delta, the Canadian Arctic Islands margin and up to200 km offshore to provide a consistent regional image of thismargin (Figure 1). The Phase I data were acquired in Aug.-Oct.2006 with a shotpoint interval of 50 meters, receiver interval of 25meters and a sampling rate of 2 ms. The final processed data are90 fold and have undergone Kirchhoff PSDM (prestack depthmigration) to 40 km. The Phase II program was acquired in Aug.-Oct. 2007 with the same parameters. The third phase is expectedto be acquired during summer 2008 and will be processed byearly 2009. By the end of 2008, we expect to have acquired almost15,000 km of Canadian Beaufort Sea data with these parameters.

The data provide images down to the top of MOHO along mostof the lines, and, at the same time, provide stratigraphic ands t ructural details in the upper 12 km. Although our interpre t a-tions are regional, they shed light on the crustal framework, sedi-mentary-wedge geometry, and evolution of the Mackenzie foldand thrust belt, as well as primary sediment fairways. This allowsa more comprehensive evaluation of the petroleum sytem of thebasin than has been hitherto possible. Although detailed technicaldiscussion is beyond the scope of this paper, our objective is to listand display some of the highlights of the interpre t a t i o n .

Interpretation of SPAN Seismic Data

Figure 2 displays the major sequences identified in the Beaufort-Mackenzie Basin (Dixon, 1996) and the horizons we have inter-preted in the study area. The seismic events identified have beentied to the existing well data. Regional ties have been extendedbeyond the well control. There are no well ties available in theBanks Island area. In the Amundsen Gulf area, south of BanksIsland, we have relied on surface geology (Wheeler and others,1996) to identify the seismic events close to the sea floor. Asmentioned earlier, correlation of seismic reflectors older thanTaglu (52.2 m.y., Figure 2) are not based on well ties but areprimarily based on regional structure and stratigraphy and

published information. Interpretations of oceanic crust, crys-talline basement and the top of the MOHO are consistent withpublished work (Stephenson, et al., 1994) as well as potential-field data.

Although, contrasting views on the opening of the CanadianBasin have been presented (Lane, 2007, Grantz, 2007), SPAN dataoptimally support a rotational opening of the Canadian Basinand extension of the “paleo-spreading center” near the mouth ofthe Mackenzie River. We identify a passive margin boundary ofthe Canadian Basin, extending along the Tuk Peninsula andBanks Island, whereas the continuation of this passive margininto the Alaskan Beaufort Sea is masked by the immense thick-ness of Mackenzie Delta sediments (Figure 3). This figure showsan interpreted Continent-Ocean Boundary (COB), a potentialextension of Canadian Basin spreading center under theMackenzie Delta and a few lineaments observed on the gravitymap that could be interpreted as fracture zones.

As shown in Figure 4, towards the north, the ocean-continentboundary is well defined as a fault, and the interpreted MOHO,calculated from gravity data (red horizon at depth), is signifi-cantly deeper below the continental crust. Seismic events consis-tent with the gravity-derived MOHO are observed on SPANdata. The horizon colors approximately match the colors shownin Figure 2.

In the region of the Mackenzie Delta, as shown in Line 3700( F i g u re 5), the structural style changes dramatically to form the

Focus Article Cont’d


F i g u re 1. Location of BeaufortSPAN East Phase I (acquired in 2006), Phase II( a c q u i red in 2007) and Phase III lines (planned for 2008) shown on a bathymetricmap of the area. The data extend between 20-m and 2,000-m water depths.Although the wells are not shown on this map, the lines tie key exploratory wells inthe shallow (< 50 m) offshore. Note the location of recent bid blocks in deep water.Locations of lines 4500 (Figure 4) ,3700 (Figure 5), and a segment of 5600 (Figure6) are shown as well as on Figure 3. A significant number of active leases in theAlaskan Beaufort Sea also testify to industry interest in the are a .

F i g u re 2. Seismic horizons correlated and mapped within the BeaufortSPA Ndataset. The sequence on the continental crust includes Paleozoic and Pro t e ro z o i chorizons. The oldest sediments in the sequence on the oceanic crust consist ofValanginian-age and older, synrift sediments. Sediments following the1 3 6 . 4 _ Valanginian_BKUP unconformity are the “drift” sediments. SPAN datapermit recognition of all the major Mesozoic and Tertiary sequences offshore .Sediments overlying the 83.5_LateCRET horizon re p resent sedimentary wedges ofthe Mackenzie River and its delta. The Paleozoic and Pro t e rozoic sequences beneaththe rift sediments are identified along the Tuk Peninsula and Banks Island (Fig. 1).Sequence boundaries and absolute ages from Pyle and others (2006),ConocoPhillips (2004), and GTS Time Scale (2004).

Highlights of Petroleum and Crustal Framework …Continued from Page 22

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Beaufort Foldbelt. Gravity sliding and Brooks-Range compre s s i o nf rom northeastern Alaska have created folds and thrusts with amaster detachment level of almost 15 km in southern and westernparts of the area. A y o u n g e r, late-Te r t i a r y, compressional eventcaused structural inversion, folded the master detachment surfaceand created a large stru c t u re at the shelf edge which extends intodeeper waters. This inversion stru c t u re is located where we havepostulated the presence of a now-inactive spreading center underthe delta sediments and where others (Lane, 2002) have suggestedthe presence of the Continent-Ocean Boundary.

The SPAN data also show that, outside the Mackenzie Delta area,the total sedimentary wedge above the Breakup Unconformity isstill significantly thick. As shown in Figure 6 (located on Figures1 and 3), this wedge is considerably thicker than previouslybelieved. In addition, at the southern edge of Banks Island, thisline displays a major rollover feature, showing a landward dip ofall the Mesozoic and Tertiary horizons. As interpreted here, theContinent-Ocean Boundary is located almost 20-35 km landwardof this rollover feature. The full extent of this feature is notmapped, but it is almost 30 km wide, and the structural relief, atthe horizon marked as the top of “Mackenzie Bay” sequence, isalmost 800 m.

These examples show that the crustal structure of the CanadaBasin and its tectonic history are directly related to the structureand stratigraphy that is reachable through exploration.Therefore, any reconstruction of basin evolution needs to takeinto account the fundamental crustal structure and its influenceupon the geometry and timing of deformation from the initialrifting to the most recent tectonic events.

Conclusions

• Reflectors ranging in age from 5.3 million years to theP ro t e rozoic are identified in BeaufortSPAN East Phases I andII data.

• Along most of the margin of the Beaufort Sea, iterativeseismic- and potential-fields interpretations constrain the loca-tion of the Continent-Ocean Boundary along the CanadianA rctic margin. However, the crust under the central part of theMackenzie River sedimentary wedge is difficult to charac-terize due to the thickness and folding of the sediments, butthe crust does appear to include the buried spreading center ofthe Canada Basin.



F i g u re 3. Arc t i c S PAN surveys located on Free Air Gravity Map of the BeaufortSea, Mackenzie Delta, Banks Island and surrounding region (gravity data fro mA rctic Gravity Project, Kenyon and Forsberg, 2001). The COB is more or less coin-cident with that shown in Lane (2002) along Banks Island and farther south.H o w e v e r, the biggest differences between Lane’s and our interpretations are in thecentral Mackenzie Delta region. The segment interpreted here as the extension ofthe spreading center underneath the delta has been mapped by Lane (2002) as the“B Fracture Zone.” If his interpretation is correct, the delta region should be larg e l yunderlain by continental crust. . Also, should this interpretation be correct, a signif-icant part of the delta is underlain by oceanic crust. The oceanic crust, as inter-p reted here, is more expansive than interpreted by Grantz and others (2008).Definition of the crustal type in this area (outlined by a triangle), currently desig-nated as “anomalous crust,” is a current interpretation effort.

F i g u re 4. Cenozoic, Mesozoic and Paleozoic seismic horizons and the Havik wellshown on BeaufortSPAN East survey line. VE=~4. The vertical scale is 40 km andthe location is shown on Figures 1 and 3.

F i g u re 5. Deformation style and crustal profile for the Mackenzie Delta sedimen-tary wedge. The Eocene-Miocene Beaufort Foldbelt extends from “anomalous”crust northward across oceanic crust. The detachment surface is interpreted to belocated near the 136.4_Valangianian_BKUP surface. Late Tertiary inversion ofhalf-graben sediments below the Valanginian has formed an “outer high” in thefoldbelt. Line location is shown on Figure 3. Deep red line is MOHO calculatedf rom the gravity data, vertical scale is 40 km and the VE = ~3. Segment marked as“anomalous crust” is that part of the line which is inside the triangular area shownin Figure 3. Line location shown on Figures 1 and 3.


September 2008 CSEG RECORDER 25

• A syn-rift sedimentary wedge, consisting of 1 to 3 km of mixedpelagics and clastics, was deposited between 150 and 136million years ago.

• Multiple phases of thrusting and folding are evident; thephases were active from the Early Tertiary to the LateMiocene. The compression that was responsible for thet h rusting also produced wrench and extensional stru c t u res insome areas. The master detachment surface for the thru s t sappears to be located near the base of the sedimentary columnat a depth of approximately 15 km.

• Most of the reported “diapir-like” features in earlier literaturea re not true diapirs because steep internal layering can beobserved in the cores of anticlines in the SPAN data.

• Exploration opportunities exist in numerous fold stru c t u res inthe unexplored outer shelf and upper slope as well as in theuntested deeper parts of the sedimentary section under theinner shelf. In addition, there are ample opportunities forstratigraphic exploration in the area, although syntectonicg rowth facies and unconformities (Dixon, 1996) make it a chal-lenging endeavor. R

Acknowledgements

We gratefully acknowledge various scientists of the GeologicalSurvey of Canada, especially Tom Brent, Zhuoheng Chen,Ashton Embry, Chris Harrison, Larry Lane, and Kirk Osadetz forproviding data and information on the geology and petroleumpotential of Beaufort-Mackenzie basin and for discussing with usthe complexities of the Canada Basin history and tectonics.

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ConocoPhillips, 2004, Application for approval of the development plan for Parsons LakeField, Project Description: Geology, Geophysics and Petrophysics, Section 2.1, 18p.Available online at: h t t p : / / w w w. m a c k e n z i e g a s p ro j e c t . c o m / t h e P ro j e c t / re g u l a t o r y P ro c e s s / a p p l i c a t i o nSubmission/Documents/MGP_Parsons_DPA_Section_2.pdf

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F i g u re 6. Part of strike line 5600 extending west from the Banks Island shelf intothe Canada Basin. The total sedimentary sequence overlying the oceanic crust isi n t e r p reted here to be approximately 12 km. In the Mackenzie Delta area, thecomparable sequence is 30 to 50% thicker. Note also the “rollover” structure alongone of the major faults, seaward of where we have interpreted the Continent-OceanB o u n d a r y.
