Highlights of Petroleum and Crustal
Framework of the Beaufort-Mackenzie
Basin: Key Results from BeaufortSPAN East
Phases I and II Surveys
Menno Dinkelman
GX Technology (GXT), a Subsidiary of ION Geophysical Corporation, Houston, Texas, USA
Naresh Kumar, James Helwig, Pete Emmet and James Granath
Consultants, GXT, a Subsidiary of ION Geophysical Corporation, Houston, Texas, USA
Summary
The Beaufort-Mackenzie basin in Arctic Canada is a petroliferous province still in the early stages of exploration. Almost
three decades of exploration in the Mackenzie Delta onshore
and shallower water offshore have resulted in 48 conventional oil and gas discoveries. Although no significant
production exists as yet, mean undiscovered resources in the
basin have been estimated at 16.8 billion barrels (2.7 million
cubic meters) by the Geological Survey of Canada (Chen et al,
2007), inclusive of deep water (to 2,500 m), and at 14.5 billion
barrels of liquids (2.3 billion cubic meters) and 86.6 trillion
cubic feet of gas (2.32 trillion cubic meters) by the United
States Geological Survey (USGS, 2006), exclusive of “deepwater” (>1,000 m).
Despite major questions about the petroleum system, the
results of recent bidding demonstrate that industry recognizes the deep-water potential of the basin. In 2007, Imperial
and Exxon-Mobil bid Can $585 million on one of the deepwater blocks; this was followed by a Can $1.18 billion bid by
BP on an adjacent block in 2008 (Indian and Northern Affairs,
Canada, 2007, 2008).
To expand the knowledge of the basin architecture, GXT
acquired ~3,500 km of 2D long-offset seismic data in the area
in late 2006 (Phase I). Building on the success of Phase I,
another ~5,500 km of data was acquired in 2007 (Phase II). A
third phase of survey is taking place in the summer of 2008.
Cognizant of the deep-water potential, we have attempted to
extend the lines as far west and north in the open water as ice
conditions and environmental considerations have allowed
in each season. These programs have been designed to image
down to the base of the crust with a 9-km long cable, 18second recording, and final depth processing (prestack depth
migration) to 40 km. The new data are interpreted to regionally map the ocean-continent boundary and the top of
MOHO discontinuity, then tied to existing well data to identify the major stratigraphic sequences formed since the
opening of the Canadian Basin.
Highlights of interpretation include:
1)
The offshore Mackenzie Delta system contains almost 15
km of sediments; in areas outside the delta system, the Late
Mesozoic-Tertiary sedimentary wedge is still 8-10 km thick
at the shelf edge and upper slope;
2)
Complex folds (earlier interpreted as “diapirs”), faults and
thrusts formed due to interaction of compressive folding,
wrenching, extension, inversion and gravity-induced
loading, all operating at the same time in distinct segments
of this region;
3)
A series of large, late-Tertiary structures occur in the
offshore part of the delta extending to water depths of
>1,000 m;
4)
Gas chimneys and amplitude anomalies in the seismic
data are widespread indicating a very active petroleum
system; and
5)
The ocean-continent boundary and oceanic crust, with an
extinct spreading center, are mapped beneath the delta
sediments, but the nature of the crust underlying the
deepest part of the basin remains uncertain.
Introduction
Although exploration in the Beaufort-Mackenzie Basin began
almost three decades ago, and despite a series of discoveries,
c o m m e rcial production remains elusive. The Geological
Survey of Canada (Chen, et al, 2007) and the United States
Geological Survey (USGS, 2006) have made recent resource
assessments of the area. They conclude that the potential is
significantly larger than the results to date imply. A single,
very large discovery is needed to provide the economic
incentive to build the infrastructure for oil production in the
area. Existing discoveries could then be tied to this “anchor”
discovery (Johnston, 2007). The critical question, therefore, is
whether that supergiant field exists in the unexplored outer
shelf and upper slope.
The existing well data in the area confirm a working petroleum
system, but some aspects of the geological history of the basin
have not been fully defined; for example, the details of the
crustal stru c t u re, the nature and location of the ocean-continent boundary, and the geometry and timing of events in the
Beaufort fold and thrust belt. The legacy seismic data are
largely confined to the middle and inner shelf, and, although
some deep reflection and refraction profiles have been
acquired in the area (Stephenson, 1994), most data penetrate to
“exploration depths” (12 km or so). Additionally, the offshore
wells are located only in the shallow (<50 m) parts of the
Mackenzie Delta, and, because of the thickness of sediments,
well penetrations into sequences older than Eocene are rare.
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Figure 1. Location of BeaufortSPAN East Phase I (acquired in 2006), Phase II
(acquired in 2007) and Phase III lines (planned for 2008) shown on a bathymetric
map 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 in
the 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 (Figure
6) are shown as well as on Figure 3. A significant number of active leases in the
Alaskan Beaufort Sea also testify to industry interest in the area.
The BeaufortSPAN East program extends along almost 800 km of
Mackenzie Delta, the Canadian Arctic Islands margin and up to
200 km offshore to provide a consistent regional image of this
margin (Figure 1). The Phase I data were acquired in Aug.-Oct.
2006 with a shotpoint interval of 50 meters, receiver interval of 25
meters and a sampling rate of 2 ms. The final processed data are
90 fold and have undergone Kirchhoff PSDM (prestack depth
migration) to 40 km. The Phase II program was acquired in Aug.Oct. 2007 with the same parameters. The third phase is expected
to be acquired during summer 2008 and will be processed by
early 2009. By the end of 2008, we expect to have acquired almost
15,000 km of Canadian Beaufort Sea data with these parameters.
The data provide images down to the top of MOHO along most
of the lines, and, at the same time, provide stratigraphic and
structural details in the upper 12 km. Although our interpretations are regional, they shed light on the crustal framework, sedimentary-wedge geometry, and evolution of the Mackenzie fold
and thrust belt, as well as primary sediment fairways. This allows
a more comprehensive evaluation of the petroleum sytem of the
basin than has been hitherto possible. Although detailed technical
discussion is beyond the scope of this paper, our objective is to list
and display some of the highlights of the interpretation.
Interpretation of SPAN Seismic Data
Figure 2 displays the major sequences identified in the BeaufortMackenzie Basin (Dixon, 1996) and the horizons we have interpreted in the study area. The seismic events identified have been
tied to the existing well data. Regional ties have been extended
beyond the well control. There are no well ties available in the
Banks Island area. In the Amundsen Gulf area, south of Banks
Island, we have relied on surface geology (Wheeler and others,
1996) to identify the seismic events close to the sea floor. As
mentioned earlier, correlation of seismic reflectors older than
Taglu (52.2 m.y., Figure 2) are not based on well ties but are
primarily based on regional structure and stratigraphy and
Figure 2. Seismic horizons correlated and mapped within the BeaufortSPAN
dataset. The sequence on the continental crust includes Paleozoic and Proterozoic
horizons. The oldest sediments in the sequence on the oceanic crust consist of
Valanginian-age and older, synrift sediments. Sediments following the
136.4_Valanginian_BKUP unconformity are the “drift” sediments. SPAN data
permit recognition of all the major Mesozoic and Tertiary sequences offshore.
Sediments overlying the 83.5_LateCRET horizon represent sedimentary wedges of
the Mackenzie River and its delta. The Paleozoic and Proterozoic sequences beneath
the 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).
published information. Interpretations of oceanic crust, crystalline basement and the top of the MOHO are consistent with
published work (Stephenson, et al., 1994) as well as potentialfield data.
Although, contrasting views on the opening of the Canadian
Basin have been presented (Lane, 2007, Grantz, 2007), SPAN data
optimally support a rotational opening of the Canadian Basin
and extension of the “paleo-spreading center” near the mouth of
the Mackenzie River. We identify a passive margin boundary of
the Canadian Basin, extending along the Tuk Peninsula and
Banks Island, whereas the continuation of this passive margin
into the Alaskan Beaufort Sea is masked by the immense thickness of Mackenzie Delta sediments (Figure 3). This figure shows
an interpreted Continent-Ocean Boundary (COB), a potential
extension of Canadian Basin spreading center under the
Mackenzie Delta and a few lineaments observed on the gravity
map that could be interpreted as fracture zones.
As shown in Figure 4, towards the north, the ocean-continent
boundary is well defined as a fault, and the interpreted MOHO,
calculated from gravity data (red horizon at depth), is significantly deeper below the continental crust. Seismic events consistent with the gravity-derived MOHO are observed on SPAN
data. The horizon colors approximately match the colors shown
in Figure 2.
In the region of the Mackenzie Delta, as shown in Line 3700
(Figure 5), the structural style changes dramatically to form the
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Beaufort Foldbelt. Gravity sliding and Brooks-Range compression
from northeastern Alaska have created folds and thrusts with a
master detachment level of almost 15 km in southern and western
parts of the area. A younger, late-Tertiary, compressional event
caused structural inversion, folded the master detachment surface
and created a large stru c t u re at the shelf edge which extends into
deeper waters. This inversion structure is located where we have
postulated the presence of a now-inactive spreading center under
the delta sediments and where others (Lane, 2002) have suggested
the 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 is
still significantly thick. As shown in Figure 6 (located on Figures
1 and 3), this wedge is considerably thicker than previously
believed. In addition, at the southern edge of Banks Island, this
line displays a major rollover feature, showing a landward dip of
all the Mesozoic and Tertiary horizons. As interpreted here, the
Continent-Ocean Boundary is located almost 20-35 km landward
of this rollover feature. The full extent of this feature is not
mapped, but it is almost 30 km wide, and the structural relief, at
the horizon marked as the top of “Mackenzie Bay” sequence, is
almost 800 m.
Conclusions
•
Reflectors ranging in age from 5.3 million years to the
Proterozoic are identified in BeaufortSPAN East Phases I and
II data.
•
Along most of the margin of the Beaufort Sea, iterative
seismic- and potential-fields interpretations constrain the location of the Continent-Ocean Boundary along the Canadian
Arctic margin. However, the crust under the central part of the
Mackenzie River sedimentary wedge is difficult to characterize due to the thickness and folding of the sediments, but
the crust does appear to include the buried spreading center of
the Canada Basin.
These examples show that the crustal structure of the Canada
Basin and its tectonic history are directly related to the structure
and stratigraphy that is reachable through exploration.
Therefore, any reconstruction of basin evolution needs to take
into account the fundamental crustal structure and its influence
upon the geometry and timing of deformation from the initial
rifting to the most recent tectonic events.
Figure 4. Cenozoic, Mesozoic and Paleozoic seismic horizons and the Havik well
shown on BeaufortSPAN East survey line. VE=~4. The vertical scale is 40 km and
the location is shown on Figures 1 and 3.
Figure 3. ArcticSPAN surveys located on Free Air Gravity Map of the Beaufort
Sea, Mackenzie Delta, Banks Island and surrounding region (gravity data from
Arctic Gravity Project, Kenyon and Forsberg, 2001). The COB is more or less coincident with that shown in Lane (2002) along Banks Island and farther south.
However, the biggest differences between Lane’s and our interpretations are in the
central Mackenzie Delta region. The segment interpreted here as the extension of
the 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 largely
underlain by continental crust. . Also, should this interpretation be correct, a significant part of the delta is underlain by oceanic crust. The oceanic crust, as interpreted here, is more expansive than interpreted by Grantz and others (2008).
Definition of the crustal type in this area (outlined by a triangle), currently designated as “anomalous crust,” is a current interpretation effort.
Figure 5. Deformation style and crustal profile for the Mackenzie Delta sedimentary wedge. The Eocene-Miocene Beaufort Foldbelt extends from “anomalous”
crust northward across oceanic crust. The detachment surface is interpreted to be
located near the 136.4_Valangianian_BKUP surface. Late Tertiary inversion of
half-graben sediments below the Valanginian has formed an “outer high” in the
foldbelt. Line location is shown on Figure 3. Deep red line is MOHO calculated
from 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 shown
in Figure 3. Line location shown on Figures 1 and 3.
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Boundary.
•
•
A syn-rift sedimentary wedge, consisting of 1 to 3 km of mixed
pelagics and clastics, was deposited between 150 and 136
million years ago.
Multiple phases of thrusting and folding are evident; the
phases were active from the Early Tertiary to the Late
Miocene. The compression that was responsible for the
thrusting also produced wrench and extensional structures in
some areas. The master detachment surface for the thrusts
appears to be located near the base of the sedimentary column
at a depth of approximately 15 km.
•
Most of the reported “diapir-like” features in earlier literature
are not true diapirs because steep internal layering can be
observed in the cores of anticlines in the SPAN data.
•
Exploration opportunities exist in numerous fold structures in
the unexplored outer shelf and upper slope as well as in the
untested deeper parts of the sedimentary section under the
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Acknowledgements
We gratefully acknowledge various scientists of the Geological
Survey of Canada, especially Tom Brent, Zhuoheng Chen,
Ashton Embry, Chris Harrison, Larry Lane, and Kirk Osadetz for
providing data and information on the geology and petroleum
potential of Beaufort-Mackenzie basin and for discussing with us
the complexities of the Canada Basin history and tectonics.
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