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Titre2010 state of knowledge: Beaufort Sea seabed geohazards associated with offshore hydrocarbon development
TéléchargerTéléchargements
AuteurBlasco, S; Bennett, R; Brent, T; Burton, M; Campbell, P; Carr, E; Covill, R; Dallimore, S; Davies, E; Hughes-Clarke, J; Issler, D; Leonard, L; MacKillop, K; Mazzotti, S; Patton, E; Rogers, G; Shearer, J; White, M
SourceCommission géologique du Canada, Dossier public 6989, 2013, 340 pages, https://doi.org/10.4095/292616 (Accès ouvert)
Image
Année2013
ÉditeurRessources naturelles Canada
Documentdossier public
Lang.anglais
DOIhttps://doi.org/10.4095/292616
Mediaen ligne; numérique
Formatspdf
ProvinceRégion extracotière du nord
Lat/Long OENS-141.0000 -131.0000 71.2500 68.7500
Sujetshydrocarbures; capacité de production d'hydrocarbures; milieux marins; transport sous-marin; caractéristiques sous-marines; plate-forme continentale; talus continental; stabilité des pentes; glissements de pentes; bathymétrie; topographie du fond océanique; topographie du fond océanique; elements glaciaires; géologie marine; combustibles fossiles; géologie des dépôts meubles/géomorphologie
Illustrationslocation maps; tables; profiles; cross-sections; histograms; plots
ProgrammeGéoscience marine pour le développement économique de l'Arctique, Géoscience en mer
Diffusé2013 10 09
Résumé(Sommaire disponible en anglais seulement)
Introduction
Generic exploration drilling structures include floating and bottom-founded drilling platforms. Both types of exploration drilling structures require knowledge of seabed stability conditions or geohazards to ensure safe exploration drilling. In addition, knowledge of geohazards is required for subsea pipelines. This report covers the state of knowledge of Canadian Beaufort Sea seabed stability conditions that were investigated by the Geological Survey of Canada and its partners from 1969 through 1991 and from 2001 through 2010, including seabed scouring by ice keels, foundation conditions, submarine slumping, artificial islands, seabed features, subsea permafrost, hydrates, overpressure, earthquakes and tsunamis. Two survey platforms, the CCGS Nahidik and the CCGS Amundsen, have been used by the Geological Survey of Canada and its partners to collect geophysical, geotechnical and geological data to assess seabed geohazards. Survey equipment on the CCGS Nahidik included a single beam echosounder, multibeam echosounder, sidescan sonar, sub-bottom profiler, microprofiler, shallow seismic, and multi-channel seismic as well as piston and box corers. Survey equipment on the CCGS Amundsen included multibeam echosounder, sub-bottom profiler and piston and box corers. The Canadian Beaufort continental shelf covers an area of 50,000 km2 and stretches from the United States border at 141°W to the entrance of Amundsen Gulf at 128°W, and forms a shallow gently sloping plain from 68°N to 71.5°N, reaching the shelf break at 100m water depth. In addition, to the break in slope and bathymetry, the edge of the continental shelf is characterized by a number of morphological features, such as folds, faults, and pingo like features (PLF). Beyond the shelf break, the continental slope is a narrow area with a gradient of 0.5° to 5 or 6°. Water depth ranges from 100 to 1500m and width varies from 20 km to 100 km.
Stratigraphy and Structure
The seabed of the Beaufort Sea consists of two main areas, the Canadian Beaufort Shelf and the Beaufort Slope. The Canadian Beaufort Shelf is further divided into three main regions: the narrow Western shelf adjacent to the Canada - US border; the Mackenzie Trough; and the Eastern shelf. The Eastern Beaufort Shelf is a broad shallow shelf that lies between the Mackenzie Trough and Amundsen Gulf. It is 350 km long and 150 km wide with an average gradient of 0.03 degrees. The Eastern Shelf is divided at about 133.5°W longitude where there is a change in surficial sediment from clay over the central shelf to sandier sediments on the eastern shelf. The upper 100 m of sediment on the Eastern Beaufort Shelf has been sub-divided into five stratigraphic units: A; B; C; D; and E. Unit A is composed of a horizontal sequence of recent marine silt and clay deposited subsequent to the last sea level rise. This unit grades downward into Unit B, which consists of a transgressive sequence of littoral, deltaic, and lagoonal sand, silt and clay deposited in a transitional environment that existed during the last sea level rise. This unit unconformably overlies Unit C which is composed of silts, clays, sand, and gravel originating from former continental (glacial, fluvial, and eolian) and transitional environments (littoral, deltaic). Unit D, underlying Unit C comprises silt and clay up to 40 m thick representing a delta- margin or inner-shelf marine depositional environment. Unit E, a medium to fine grained sand of unknown thickness is below Unit D. Data indicates this unit was deposited during alternating sub-aerial delta plain and near-shore marine environments similar to Unit C. The Western Shelf is less than 60 km wide by 80 km long, and slopes seaward at a gradient less than 0.05 degrees. The Western Beaufort Shelf has been divided into three units. The lowermost unit, Unit III, is a massive clay with exotic clasts of granitic and dolomitic composition. Unit II overlies Unit III and is composed of clay. A major erosional unconformity separates Unit II and the youngest unit, Unit I, laminated silty clay that outcrops on the seabed in the outer shelf and reaches thicknesses up to 70 m on the shelf edge. Unit I is overlain by a discontinuous thin veneer of late-Wisconsinan / Holocene sediments. The Mackenzie Trough is a paleo-valley which is partially infilled with more than 300 m of Quaternary sediments. The trough was excavated to its maximum depth by glacial erosion, leaving behind the glacial till (Unit MT5). A thick sand sequence, Unit MT4, was deposited with the retreat of the glacier. Speculation exists that there may have been a second ice advance into the trough, partially eroding Unit MT4. Unit MT3 represents a deformation structure caused by the ice or possibly a moraine deposit. The silty-clays of Unit MT2 are a result of subglacial drift, also deposited by the ice. Deglaciation was associated with Unit MT1 which was formed by the deposition of deltaic sediments, followed by transgression and deposition of marine clays which reach up to 125m in thickness in the central Trough area. Geohazard Conditions
Ice Scour
Ice scouring is the process whereby sea-ice pressure ridge keels contact the seabed forming long, linear gouges in the seafloor sediments. From a geohazard perspective the potential for ice keel- subsea pipeline interaction needs to be mitigated. During the ice scouring process, in marine clays, the base of the ice keel initially parallels the sea surface as it incised the seabed. With continued movement into shallower water the base of the ice keel feature rises up and eventually parallels the seabed slope. The process of sub-scour disturbance is known to occur within the sediments beneath scour troughs as observed on sub-bottom profiles of the seabed. Seabed repetitive mapping surveys of the same sector of the seabed were conducted from 1978 through 1990 and from 2001 to 2008 on an annual to semi-annual basis. New scours were identified, characteristics measured and compiled in a digital database which now contains 17,047 scours, 5881 of which have complete morphological data including depth, width, orientation etc. and form the basis of database analysis. Scour depths range up to 5.0 m and average 0.55 m. The majority of scours occur in water depths ranging from 5 to 30 m. Average scour width is 28 metres with the widest scour being multi-keeled at 1,993 m. Spatial scour frequency represents the number of new scours per line kilometre. Peak spatial frequencies of ? 40.0 events per kilometre occur in water depths of 7 - 14 m. Temporal scour frequencies of 18-19 scours/km.year occur in water depths of 18 - 20 m. Seabed disturbance rate over a 100 year period is 100% for water depths > 5 and < 14 m. Seabed disturbance steadily decreases as water depth increases. Extreme scours are defined as scours having an original scour depth of ? 2.0 metres. The extreme scour population (n = 290) accounts for approximately 1.7% of the total population. The spatial distribution of extreme scour events is controlled by sea-ice regime, bathymetry and geotechnical zonation of the shelf. Relict ice scours are those occurring in water depths greater than 60 metres and may have formed by a more extreme sea-ice regime compared to the modern day regime. The relict scour database contains 522 scours. These scours occur in water depths reaching 150 m, have depths ranging up to 4.4 m, widths of up to 210m, and are predominantly single-keeled, morphologically smooth scours.
Foundations Conditions
Seabed stability conditions in terms of soft or low strength sediments may pose a geohazard by providing inadequate support for engineering structures placed on or in the seabed. There have been about 300 boreholes drilled on the Beaufort Shelf since 1970. The physical properties of surficial seabed sediments (Unit A) across the shelf have been compiled for seabed stability assessment and to enhance the understanding of ice scour processes. The laboratory testing of sediment samples included index property measurements, consolidation testing and triaxial testing. Geotechnical properties measured for Unit A sediments from the boreholes and samples include plastic limit, liquid limit, moisture content, bulk density, clay %, silt %, sand %, undrained shear strength, over-consolidation Ratio (OCR) and compression index (Cc). Geotechnical data is sparse in water depths greater than 75 m. Water content, bulk density, and undrained shear strength measurements have been made in these surficial sediments.
Submarine Slides
Limited seabed mapping using high-resolution multibeam sonar and sub-bottom profiler survey systems of the central Beaufort upper slope area provides an insight into processes active on the seabed over the last 10,000 years. These data indicate that both stable and unstable areas of the seabed exist down slope. Subbottom profile data indicate thick sequences of stratified sediments, in excess of 70 m exist down slope. The lack of disturbance in this sequence indicates sediments in these areas have been stable for a long time, possibly several thousand years. Conversely, an area of submarine slides (under water landslides) has been identified in which sediment was dislodged to form scars approximately 25 km wide (E-W) and 20 km long (N-S) has been mapped. The age of this slumping activity is unknown. Submarine slides are considered geohazards because such events can damage engineering structures such as drilling systems and pipelines placed on or below the seabed.
Artificial Islands
A total of 37 artificial islands were constructed in the Canadian Beaufort Sea between 1975 and 1989 in water depths ranging from 1 m to 45 m. After temporary use as drilling platforms, the islands were deserted, were subject to erosion and are now below sea surface. The submerged islands are a geohazard to marine traffic. During 1990 to 2008, repetitive surveys have been conducted on artificial islands to assess the island stability over time and to document the changes that have occurred at the island sites. The initial study of sediment transport at thirteen artificial island sites within the Beaufort Sea was completed in 1994. Between 2001 and 2008 eight of the thirteen island sites were resurveyed, analyzed, and reported. Results show that the artificial islands eroded to the water line within a few years, at which time the erosional processes have continued sub-sea surface but have decreased over time and appear to have stabilized at wave base 4 to 5 m below sea level. The processes causing erosion include wave action, current action, slumping and ice keels. Island sediment migration trends in a south- easterly direction which is parallel to the dominant direction of wave action.
Seabed Pingo-like Features
Positive-relief conical mounds, commonly referred to as "pingo-like features" (PLFs) were first discovered on the Beaufort Shelf in 1969 as geohazards to navigation. Since then over 1000 such features have been identified across the Beaufort and Yukon shelves. These features may need to be avoided by hydrocarbon development activity as they may indicate localized unstable seabed conditions. It is currently believed that submarine PLFs fall into five categories of origin: submarine-formed true pingos, mud volcanoes, diapirs, relict topography or slump features. Conical shaped PLFs have diameters up to 980 m, and elongated PLFs have lengths up to 2100 m and widths up to 311 m. Recorded PLF heights range up to 50 m. Some PLFs have been studied in detail, including Kopanoar, Admiral's Finger, and the PLF field at Garry Knolls at the eastern edge of the Mackenzie Trough. The location of these features seems to be controlled by the presence of subsea permafrost and the migration of fluids/gas. Three large PLFs identified beyond the shelf break are thought to be mud volcanoes. Seven PLF features have been observed to be actively venting methane.
Gas Vents and Oil Seeps
Pockmarks are roughly circular shaped depressions in the seafloor caused by the venting of fluids/gas from the sub-surface into the water column. Pockmarks have been identified on the seafloor of the Beaufort Sea during past geophysical surveys, and have been found to be abundant in the Kugmallit Channel, in the eastern Mackenzie Trough physiographic region and at the edge of the Beaufort Shelf. Gas has been observed in the water column venting from pockmarks. Oil has been found in surficial seabed sediments at one location on the central shelf. From a geohazard perspective gas vents and oil seeps may indicate the presence of unstable conditions below seabed that need to be considered while planning drill activities.
Subsea Shallow Gas
Shallow gas occurs below seabed in many areas within the Beaufort Sea. Results from regional geophysical surveys show that shallow gas is present shelf-wide as discontinuous, localized deposits within sediments from the seabed to depths greater than 500 m. Gas observed venting from the seabed through PLFs and pockmarks can indicate the presence of shallow gas deposits at depth. The presence of shallow gas at depth below seabed is identified as a geohazard because these gas deposits, if overpressured can lead to well-bore instability during drilling activities.
Faults
Despite extensive mapping of the Beaufort Shelf, faulting within the upper 100 m is infrequently observed. Displacement of strata within unconsolidated sediments has been identified where thick sequences of laminated sediment occur, most notably in areas where PLFs occur. Possible faulting near seabed has been observed along the shelf edge on sub-bottom profiler data. A fault feature has been identified in the northern Kringalik Plateau. The most recent apparent movement along the fault took place about 7,500 years age; more recent movement is possible although it would have been obscured by recent ice-scouring activity. A graben-like fault structure on the seafloor with two 200 m wide, parallel fault-like features spaced approximately 1600 m apart has been mapped. The seafloor between the two fault-like features is approximately 3 to 5 m lower. As a geohazard the presence of faults may indicate the normal integrity of the seabed has been disrupted which could lead to well-bore instability during drilling activity.
Subsea Permafrost
Shallow shelf ice-bearing permafrost consists of hummocky permafrost, stratigraphically controlled permafrost and marginally or partially ice-bonded sediments. The distribution and occurrence of shallow ice-bearing permafrost in the upper 100 m of shelf sediment is spatially discontinuous and is a product of long periods of subaerial exposure during low sea level associated with glaciation. Deep shelf ice-bearing permafrost (IBPF) is elongated in a southwest-northeast orientation, with a more abrupt decrease in thickness towards the Mackenzie Bay compared to the Eastern Beaufort Shelf (distribution based on wells drilled in the offshore area). The significant westward decrease in thickness and the eastward elongation of the IBPF body is due to the distribution of the sand layers at depth. In the region west of 136°W longitude it likely comprises very thin, marginally ice-bearing coarser-grained layers and the occurrence of thin frozen sand and silt layers at shallow depth. The presence of ice within subsea sediments is a geohazard. Heat generated by drilling activity or the production of higher temperature hydrocarbons may result in the melting of the ice and reduction of strength of sediments required to support the well-bore.
Subsea Gas Hydrate
Gas hydrate, a solid ice-like substance composed of water and natural gas molecules has been inferred in offshore wells drilled in the southern Beaufort Sea. Gas hydrate forms in geologic environments under conditions of low temperature and high pressure where there is a sufficient supply of gas, pore water and a suitable host porous media environment. To date no samples of gas hydrate bearing strata have been collected from sediments beneath the Beaufort Shelf. For the most part, estimations of gas hydrate occurrence have been made on the basis of examination of well log response (intervals with high seismic velocity, electrical resistivity and porosity) and drilling response (indications of subsurface gas, borehole instability, over pressure, or excess mud volume). Knowledge of the occurrence of marine gas hydrates beneath the Beaufort slope and deeper waters offshore is limited. However, given that annual bottom temperatures on the slope are near 0°C, conditions may exist for gas hydrate formation where water depths are greater than ~250m. Some of the first data suggesting possible gas hydrate occurrence on the Beaufort slope was observed in a deep water piston core. The decomposition of hydrates through thermal instability and/or pressure reduction that may result from drilling activity has been identified as a geohazard with similar impact to the degradation of permafrost discussed in the previous section.
Overpressure
Overpressure is the term used to describe fluid pressure that exceeds hydrostatic pressure. Like shallow gas these abnormal high pressure conditions constitute the geohazard. Overpressure is associated with undercompacted sediments in the Beaufort-Mackenzie Basin. Multi-parameter data (well log, mud weight, pressure test, and temperature data) were used to pick the depth to top of overpressure at well locations throughout the Beaufort-Mackenzie region. Based on the analysis of approximately 250 wells, 105 of which penetrated the overpressure zone, the depth to overpressure was mapped. There is evidence for vertical migration of overpressured fluids at some well locations in the south in close proximity to deep seated faults. In the western Beaufort Sea, there is evidence for gas chimneys where deep gas migrates upward towards the seabed. Overpressure is restricted to offshore regions and the central Delta area. The top of overpressure cuts across progressively younger strata to the north. Overpressure is associated with structural highs.
Earthquakes and Tsunamis
The Beaufort Sea seismic (earthquake) zone has substantial earthquake activity over a 250 km wide area, with on average one magnitude ~ 4 earthquake per year, with one magnitude 6.5 earthquake in 1920. The origin of seismicity in this area is unknown because of data limitations preventing modern analysis. Seismicity in the Beaufort-Mackenzie area is monitored with distant observation stations. This distance limits the ability to determine the location and depth of historical earthquakes. To the south of the Beaufort Sea, the Richardson Mountains seismic zone has high seismicity, including three magnitude ~6 earthquakes in the last 60 years. Two additional earthquake source zones can be postulated, the Eskimo Lakes fault zone and the front of the modern Mackenzie Delta. Tsunamis are high amplitude ocean waves that impact the shelf and coastal zone usually caused by earthquakes or submarine slumping of the seabed. There is no historic evidence of tsunamis in the recent geological record, or within traditional knowledge. Both earthquakes and tsunamis constitute geohazards because of their potential for damaging engineering structures.
Conclusion
The gaps in the understanding of seabed geohazards include:
- a lack of spatial and temporal knowledge of geohazard conditions from 100 m to 1000 m below seabed for the Canadian Beaufort Shelf
- a lack of spatial and temporal knowledge of geohazard conditions from seabed to 1000 m below seabed for the outer shelf and upper slope of the Canadian Beaufort Sea Given appropriate resources the technology exits to conduct the research required to determine the distribution, severity and frequency of known seabed geohazard knowledge gaps as well as to map as yet unidentified geohazards.
GEOSCAN ID292616