Interactions between buried mass-transport complexes and subsequent slope failures on a passive margin

Submarine Mass Movements and Their Consequences

A probabilistic model is proposed to describe the probability distributions of MTC hazards. Existing deposits appear to validate the model to the degree possible. The model reproduces deposit structures, and also identifies model inputs that are most likely to produce hazardous MTCs. Introduction A mass transport complex (MTC) can present significant hazards to certain offshore structures and activities. Specifically, the integrity and operations of underwater cables, pipelines, moorings, and other structures can be threatened by MTCs. An effective and manageable use of these structures motivates a study of MTC hazards. In general, MTC hazards are revealed by field studies of existing MTC events (Orange et al., 1999; Tappin et al., 2001, 2003; von Huene et al., 2004). Field studies are complimented by numerical models developed to evaluate MTC hazards. These include various sediment stability models (e.g., Wright and Rathje, 2003), mass transport models (e.g., Imran et al., 2001; Syvitski and Hutton, 2003; Niedoroda et al., 2003), and probabilistic models (e.g., Watts, 2003, 2004). Of these different techniques, probabilistic models have perhaps received the least attention, despite their many advantages. In this work, MTC hazards are found by combining 1) stability analyses and 2) sediment motion into a single hazards assessment model (HAM). The HAM is a probabilistic model that provides probability distributions for most MTC hazards of interest. Hazard Assessment Model The HAM presented here is based in part on the probabilistic model of Watts (2003), although the HAM is significantly more sophisticated. HAM inputs include slope morphology, sediment strength, sedimentation rate, water pressures, gas hydrate pressure and temperature, seismic parameters and other slope stability factors. The stability of any given slope may be dominated by only a few model inputs (Watts, 2004). The frequency of MTCs is controlled by the rate of occurrence of storm waves, earthquakes, gas hydrate phase change, oversteepening, sedimentation events and other MTC triggering mechanisms. The HAM performs two distinct computations. Stability analyses of sediment structures evaluate MTC failure planes. Sediment motion post failure describes MTC velocities and deposition. There are several important differences between our earlier work (Watts, 2003, 2004) and the HAM. First, HAM computations are carried out explicitly on a yearly basis, directly providing return periods of practical interest. Second, HAM outputs can occur at any distance from the initiation of mass failure. Third, HAM outputs focus on deposit hazards rather than tsunami hazards. Fourth, slope stability is treated by a method of slices with a variety of failure plane shapes (Turner and Schuster, 1996). Fifth, gas hydrates influence slope stability in the HAM. Fig. 1: Region offshore Santa Barabara, CA(Available in full paper) Uses for Uncertainty The slope conditions that trigger hazardous MTCs are found by running the HAM multiple times with randomized inputs. The HAM uses probability distribution functions to address geological uncertainty, with the understanding that these uncertainties may have a greater impact on sediment deposits than the errors in the slope stability or sediment motion models used.

Integrated Ocean Drilling Program (IODP) Expedition 308 in the northern Gulf of Mexico collected geotechnical and petrophysical properties of several mass transport complexes (MTCs) in the upper 600 meters below seafloor. We present a core-log-seismic integration of the largest MTC in the area. The MTC is identified in seismic data by semi-transparent to chaotic seismic facies. Close inspection of this facies in the area of Site U1324 reveals intact blocks of undeformed material. The base of the MTC is recorded in logging-whiledrilling (LWD) data as a characteristic sharp offset from highto- low values in the LWD resistivity, LWD bulk density, and sonic velocity, which is imaged as a strong reflector in seismic. The internal deposits are characterized by high LWD resistivity, LWD bulk density, and sonic velocity. At the core scale the internal deposits are composed of faulted, folded, and tilted mud with rare silt lamina. At Site U1322, core through this MTC was much more deformed than at Site U1324. This observation may record different deformation intensity and/or transport distance despite being in the same MTC unit and ~10 km apart. The characteristic signature in the resistivity log and seismic data of this MTC allows for correlation to nearby industry wells where shallow logs exist. Introduction Mass transport complexes (MTCs) are ubiquitous on continental margins where they re-shape the seafloor, transport large volumes of sediment, threaten offshore facilities, and produce tsunamis that threaten coastal communities. MTCs are generally defined as the deposits of seismically resolvable mass movement events that include deposits of slumps, slides, and debris flows1,2. MTCs are potential drilling hazards and can act as seals for hydrocarbon reservoirs. Several recent studies have focused on the 3-D seismic geomorphology of MTCs3,1,4,5,6,7. These studies and others have shown that MTCs are a major component of deepwater environments. A comprehensive study of MTCs in the Amazon Fan showed that MTCs have a high degree of consolidation relative to their burial depth8. This property of MTCs has been related to the slow penetration time of jetted conductors and suction anchor piles9. Integrated Ocean Drilling Program (IODP) Expedition 308 in the Ursa Region, northern Gulf of Mexico, cored, logged, and sampled, several prominent MTCs in the upper 600 meters (~2000 feet) below seafloor. A high-resolution 3-D seismic volume shot specifically for hazard analysis provides detailed images of the MTCs. The goal of this paper is a detailed analysis of the top, base, and internal deposits of the largest MTC by integrating core, log, and seismic data at Sites U1324 and U1322. We document lateral differences in this MTC from site-to-site. We then close by correlating this unit to two nearby industry wells. Geologic Setting The Ursa Basin lies 210 km (~125 miles) southeast of New Orleans, Louisiana (USA) on the continental slope in water depths ranging from 800-1500 meters (2600-4900 ft.) (Figure 1). IODP Expedition 308 Sites U1324, U1323, and U1322 are located in Mississippi Canyon blocks 897, 898, and 855, respectively. Sedimentation rates were very high in this area during the Late Pleistocene, averaging 10mm/yr at Site U1324 and 3.8 mm/yr at Site U132210. The Blue Unit is a ponded sand-prone unit infamous for causing shallow-water flow problems in this area11,12.

3D exploration seismic data were interpreted to investigate the locations and characteristics of submarine slope failures along the continental slope in the offshore Carnarvon Basin on Australia’s North West Shelf. Seisnetics™, a patented genetic algorithm was used to process the 3D seismic data to extract virtually all trough and peak surfaces in an unbiased and automated manner. The extracted surfaces were combined in the 3D visual database to develop a seafloor digital terrain model that extends from the continental slope to the Exmouth Plateau. The 3D data were used to map the subsurface extent and geometry of landslide failure planes, as well as to estimate the thickness and volumes of slide deposits. This paper describes the geomorphic characteristics of five of the survey areas. Geomorphic mapping shows the presence of slope failures ranging from small (20 km across) mass transport complexes (MTC). The features are associated with debris flow chutes, turbidity flow channels, and debris fields. Analysis of failure planes show prominent grooves or striations related to the mobilisation of slide material down both the continental slope and Exmouth Plateau and into the Kangaroo Syncline. Submarine slope failures can occur at the continental shelf break in about 200–300 m of water and run out to the Exmouth Plateau surface in about 1,100–1,400 m water depths. The largest individual slides in the survey areas have widths of 30 km and minimum run-out lengths of 75 km, though associated turbidity flow deposits likely extend much further. The subsurface expression of the large MTCs illustrates a history of sediment accumulation along the mid-slope followed by repeated slope failure and debris run-out. Sediment accumulation and slope failure processes are actively occurring along the continental slope and submarine landslides thus are a major driver of hazard to subsea infrastructure development. Smaller more frequent slides may pose a greater hazard than large infrequent MTCs.

The drilling areas in Shenhu and Dongsha, South China Sea, studied from 2007 to 2015, reveal great heterogeneity in the spatial distribution of the gas hydrate reservoir. Various types of mass-transport complexes (MTCs) were developed in the study areas, which served as ideal reservoirs. To conduct exploration in these areas, it is necessary to study the different types of MTCs and the corresponding gashydrate accumulations. By integrating seismic reflection and log coring data, we classified three types of MTCs according to their stress distribution: the tension, extrusion, and shear types, and their corresponding gashydrate accumulation patterns. The results show that the accumulation of the gas-hydrate varies with the type of MTC and stress distribution depending on the MTC’s position (e.g., in the headwall, translational, or toe areas). Owing to this variance of the MTC’s position, the corresponding kinemics situation in the MTCs also varies. Accordingly, we determined the corresponding location in which the gashydrate develops for various types of MTCs. Based on the bottom simulating reflectors (BSRs) and the hydrate core and image logging data, the gashydrate reservoir shows an obvious heterogeneity in various types of MTCs. The gashydrate in the tension-type MTCs are mostly borne in the toe and the headwall parts. In extrusion-type MTCs, the translational and toe parts constitute an ideal hydrate reservoir. In shear-type MTCs, the headwall and toe parts’ coarse-grained sediments show an obviously hydrate response. After comparing the gas-hydrate saturation and MTCs morphology statics data, we were able to quantitatively prove that the main factors determining gashydrate accumulation in the different types of MTCs are the fault displacement, sedimentary rate, and flow erosion rate.

Mass transport complexes (MTCs) within the Quaternary section of the ultra-deep water Nile Fan, offshore Egypt, form a significant portion of the Quaternary sedimentary section. This indicates the importance of mass-failure processes in deep water fan progradation. Regional mapping within the western platform of the Nile Fan reveals MTCs generally comprise 30%, but locally up to 94% of the Quaternary sedimentary column. Interpretation of the Quaternary section (upper 1 km of sediment) of a recently acquired, large, 3D seismic survey on the Nile Fan reveals the presence of five large mass transport complexes (MTCs) on the mid-slope of the western platform of the Nile Fan. These MTCs are on average 150 ms thick, but can obtain thicknesses of up to 357 ms. One of the MTCs is 175 km in length from the upper to the lower slope and comprises approximately 670 km3 of sediment. Introduction Slope failure deposits are common features of continental margins around the world1. Despite the fact that they are considered destructive sedimentary processes, their relevance to margin progradation is becoming increasingly recognized. Margin progradation is perhaps best studied in areas where, it is accelerated in high sedimentation rate environments, such as the deep water fan systems. The Nile Fan is the one of the largest deep water fans in the world. In ultra-deep water, the Quaternary section is in excess of 1 km thick. It is the objective of this study to investigate the architecture of the Quaternary section of the deep water Nile fan, and to understand the role of mass transport processes in the construction and progradation of the fan system. Slope failure is generally recognized by identifying escarpments, unconformities, and sedimentary deposits with certain characteristics suggesting mass transport (e.g., hummocky surfaces and incoherent internal reflections). Without referring to a specific physical process or depositional style, these deposits are referred to as mass transport complexes (MTCs). Large mass transport complexes are considered to be those that are generally greater that 2500 km2 in area and up to several hundreds of metres thick. The deep water continental slope environment has been studied for some time, but it is only recently that the oil and gas industry has become interested in these areas for their enormous hydrocarbon potential. This interest, combined with seismic imaging improvements and widespread availability of 3D seismic data, has resulted in large 3D seismic surveys of continental slope margins worldwide. From an industry perspective, research conducted using these datasets, has greatly improved understanding of turbidite channel and levee systems. Other important elements of deep water depositional system, such as the role and significance of MTCs, have been overlooked. A large 3D seismic data set has been acquired over the Nile Fan (Figure 1) and will provide the foundation for this investigation.

Submarine landslides pose significant risks to offshore infrastructure, such as seafloor telecommunication cables and oil and gas pipelines. To address geohazards associated with mass transport processes, it is crucial to understand the origin and behaviour of ancient mass transport complexes (MTCs). This study investigates the evolutionary stages and kinematics of a giant fossil MTC in the Taranaki Basin, off the West Coast of North Island, New Zealand. The submarine landslide occurred during the Pleistocene, covering an area of ~ 21,856 km² and evacuating 3,713 km³ of sediment in a NW direction. The landslide has been mapped in this study in greater detail, using a regional grid of 2D seismic reflection lines, allowing us to define its extent more accurately.The MTC consists of four distinct failure events (A-D), each characterized by distinct headwall, translational, and toe domains. MTC A, B, C, and D span areas of 16,512 km², 2,318 km², 1,287 km2 and 1,277 km² respectively. The MTC A is characterized by disintegrated extensional blocks and debris flow with an extensive runout of 328 km. MTC D is a frontally emergent slide complex with a shorter runout of 55 km. Both MTC A and MTC D are slope-attached failures, and mobilised 700 to 900 meters thick sediments near the headscarp region, whereas MTC B and MTC D mobilized 100-200 m thick sediments downslope.A 3D prestack depth migrated seismic volume provides insight into the internal architecture of the MTC D. It is a faulted coherent slide block, which features thrusts, pop-up blocks and fault inversion zone, located behind a frontal ramp. The basal shear plane lies within a turbidite layer, sandwiched between two pre-existing MTCs. 3D seismic analysis reveals that, during sliding, part of the underlying older MTC was eroded and remobilized, due to shear softening, and was incorporated into the overlying MTC D. The remobilized MTC above the basal shear plane shows linear zones of thinning and stratal welding, where fault blocks became attached to the basal shear plane, creating high-friction pinning areas that inhibited further translation. Slide cessation is evidenced by transformation of earlier extensional faults into thrusting, stratal folding, and formation of backthrust.In our study, we document for the first time the complex interaction between an older MTC and a more recent submarine landslide, highlighting its role in halting the slide. The insights gained from the study have important implications for geohazard assessments, emphasizing the need to account for the interplay between older and newer MTCs to better constrain the risk of submarine landslides.

3D seismic geomorphology of mass transport complexes in a foredeep basin: Examples from the Pleistocene of the Central Adriatic Basin (Mediterranean Sea)

Submarine landslides are significant geohazards, capable of displacing large volumes of sediment from continental margins to deposit mass transport complexes (MTCs) and generate offshore tsunamis. However, the reactivation of MTCs after their initial failure has long been overlooked. By analyzing high-quality three-dimensional seismic reflection data and seismic attribute maps, as well as comparing the geometry of different MTCs, we investigate the development of long-term slope instability and its hazardous consequences on the northwest flank of the Storegga Slide on the Norwegian margin. Our results demonstrate that the reactivation of MTCs can deform both their inner structure and overlying strata, promoting the formation of sinuous channels and local slope failures on the seafloor. These findings further reveal the MTCs that are underconsolidated or comprise slide blocks may remain unstable for a long time after their initial failure, particularly when affected by slope undercutting and a corresponding reduction in lateral support. This study shows that MTC-prone sequences are more likely to comprise regions of continental slopes with long-term instability and recurring marine geohazards.

Large-scale carbonate submarine mass-wasting along the northwestern slope of the Great Bahama Bank (Bahamas): Morphology, architecture, and mechanisms

Mass-transport complexes (MTCs) are one of the most sedimentologically and seismically distinctive depositional elements in deep-water depositional systems. Seismic reflection data provide spectacular images of their structure, size, and distribution, although a lack of borehole data means there is limited direct calibration between MTC lithology and petrophysical expression or knowledge of how they may act as hydrocarbon reservoir seals. In this study, we evaluated the lithological and petrophysical properties and seismic characteristics of three deeply buried (>2300 m below the seabed) Pleistocene MTCs in the northern Gulf of Mexico. We show that (1) MTC lithology is highly variable, comprising a mudstone-rich debrite matrix containing large (4.5-km3), deformed, sandstone-rich blocks; (2) MTCs are generally acoustically faster and are more resistive than lithologically similar (i.e., mudstone-dominated) slope deposits occurring at a similar burial depth; (3) MTC velocity and resistivity increase with depth, likely reflecting an overall downward increase in the degree of compaction; and (4) the lowermost 15–30 m of the MTCs, which represent the basal shear zones, are characterized by relatively high P-wave velocity and resistivity values, likely caused by shear-induced overcompaction. We conclude that detailed analysis of petrophysical data, in particular velocity and resistivity logs, may allow recognition of MTCs in the absence of high-quality seismic reflection data, including explicit identification of the basal shear zone. Furthermore, the relatively thick basal shear zone, rather than the overlying and substantially thicker MTC itself, may form the primary permeability barrier and thus seal for underlying hydrocarbon accumulations.

Due to high well costs, one of the primary risks in developing deepwater fields is the lack of understanding of reservoir compartmentalization at the initial development stage. Observed pressure compartmentalization in a field in deepwater North West Borneo, currently undergoing field development planning, has been interpreted as being due to a combination of faulting and stratigraphy. Direct mapping of the reservoir sands from seismic is not possible as they are below seismic resolution and the sand thickness or character varies along the field. Stratigraphically, the distribution of Mass Transport Complexes (MTCs) in Deepwater Sabah provides a strong control on the distribution of reservoir and seal. The ability to identify MTCs is therefore key to understanding the distribution and correlation of sand bodies. An integrated approach in understanding the distribution of the MTCs will be presented in this paper. This involves sedimentological analysis of core, borehole image, dipmeter and wireline data combined with detailed seismic mapping. As the core coverage is limited to one interval in a single well, the MTCs are identified mainly through borehole image and dipmeter interpretation, with the MTCs typically exhibiting high angle chaotic dips. In intervals with no borehole image/dipmeter data, the MTCs can be identified via the density log, as they are denser than the overlying and underlying sediments. MTCs generate strong seismic markers due to their increased density, and hence acoustic impedance, compared to their surrounding sediments. Therefore the seismic reflectors can be used to accurately tie the MTCs between wells. This has resulted in a more confident well correlation than one attempted through correlation of sand bodies, and the ability to incorporate the risk of compartmentalization into the field development planning. Introduction One of the key subsurface risks in developing deepwater fields is the lack of understanding of structural and stratigraphic compartmentalization at the initial development stage. Structural elements such as faults and stratigraphic elements such as reservoir sand continuity can be interpreted on seismic to explain the cause of compartmentalization provided that they are within the seismic resolution. A recent deepwater discovery in the North West Borneo basin was found to be compartmentalized when wells drilled along its elongated structure encountered different pressure regimes. The interpreted fault patterns are not able to explain all the pressure differences observed between wells as they are mostly parallel to the field orientation and the quality of the available seismic data maybe limiting the ability to image other potential faults or features. Mapping the reservoir sands seismically is not possible as they are below seismic resolution and well data shows the sand thickness or character varies along the field. Instead of focusing on the reservoir intervals to answer the compartmentalization issue, a different approach was taken to understand the distribution of MTCs regionally and locally as they are believed to provide strong control on the distribution of reservoir and seal.

The Tarfaya-Agadir Basin in offshore Morocco is a frontier exploration area with no deepwater industry well penetrations and limited well control in the updip shelfal equivalent. Recent detailed 3D seismic mapping and study by Shell has identified the presence of two large mass transport complexes (MTCs) in the Tertiary interval, in addition to a number of smaller and younger MTCs. The large MTCs, named Tejas A and Tejas B, exhibit distinct seismic characteristics that may be significant to understanding the evolution of the basin during the Tertiary. The basal Tejas A is characterized by numerous kilometerscale, coherent transported blocks. The size and quantity of transported blocks observed in Tejas A is unique in Morocco and the Atlantic Margin. The MTC in the younger Tejas B is a thick chaotic unit with distinctive downdip erosional lobes. This deposit is overlain by high amplitude, parallel seismic facies. The chaotic seismic facies is often sharp-edged, with well-defined pressure ridges. Only a few small transported blocks and erosional remnants have been observed in Tejas B. Failure deposits account for a significant percentage of the Tertiary stratigraphic interval in the 3D survey area. Introduction Exploration for hydrocarbon in offshore Morocco Atlantic Margin began in the late 1960s and continued intermittently until 1990. This early phase of exploration focused exclusively on the shelf, relying on 2D seismic data, with mixed results and no commercial discoveries. Following a period of no activity, interest in the area increased again in the late 1990s, especially in the deepwater area. Shell is the operator of two deepwater concessions in the Tarfaya-Agadir Basin (Fig. 1) and has carried out a detailed subsurface evaluation based largely on recently acquired 3D seismic data. The availability of 3D seismic data has allowed for a detailed interpretation of the paleogeography and depositional systems of the study area in the Tarfaya- Agadir Basin (Fig. 1). The results of this study highlighted the presence and the significance of mass transport complexes (MTCs) in the southern part of the basin within the Tertiary stratigraphic interval. Two main Tertiary MTCs, named Tejas A and Tejas B, have been identified, in addition to several smaller and younger MTCs, including one at the present day seafloor. These different MTCs exhibit distinct seismic characteristics. 3D seismic semblance slices have been used to clearly illustrate the many interesting features observed, including kilometer-scale transported blocks, arcuate ridges near flow terminations, and possible basal glide tracks. The identification of MTCs in the Tertiary interval has important implications on its prospectivity in the southern part of the Tarfaya-Agadir Basin. On the basis of 2D seismic data, Tejas A was interpreted by earlier explorationists to be a deepwater turbidite fan with channel-levee complexes. The present study has conclusively shown that Tejas A is in fact a MTC, with very little likelihood of containing producible reservoirs. However, the extensive chaotic seismic facies within Tejas A may act as very effective regional seals.

Characterization of blocks within a near seafloor Neogene MTC, Orange Basin: Constraints from a high-resolution 3D seismic data

Erosion and ponding of Thunder Horse deepwater turbidites by mass transport complexes in Mississippi Canyon based on image log sedimentology