Mass Transport Complexes Research Articles

Styles of slope instability on a Quaternary sub-arctic continental margin: The northwest flank of the Storegga Slide

Submarine landslides are widely recognised around the Northeast Atlantic margin, some of which forming vast slide complexes. On the mid-Norwegian margin, the Storegga Slide complex has been active since the early Pleistocene, first as a regional event (Slide W) at 1.7 Ma, and later culminating in the Storegga Slide sensu stricto at 8.2 ka. By interpreting high-quality seismic data, this work investigates slope instability styles, and their geneses, on the northwest flank of the Storegga Slide complex. Seismic and borehole data prove: i) the presence of two new landslides formed prior to Slide W, ii) that mass-transport complexes occurred during the evacuation of sediment oozes, iii) the existence of a series of cracks on the seafloor, denoting modern slope instability. Fluid accumulation in glacial-marine deposits was a primary factor promoting early instability; fluid pipes increase in number below the oldest landslide deposits, with most pipes terminating at their glide planes. Furthermore, mathematical models show that vertically stacked intervals with weak layers, and older landslide deposits, are able to promote further instability. This work thus suggests episodic fluid flow as the primary factor promoting long-term instability near the Storegga Slide complex. As a corollary, we reveal the stability of the continental slope to still be precarious, at present, on the northwest flank of the Storegga Slide.

Quantitative and geomorphologic parameterization of megaclasts within mass-transport complexes, offshore Taranaki Basin, New Zealand

Mass-transport complexes (MTCs) in sedimentary basins reflect the gravitational transport of sediments from the shelf edge to the abyssal plain. Megaclasts, large sedimentary blocks of hundreds of meters long within MTCs, can record kinematic and sedimentary information deemed essential to understanding source-to-sink systems. Yet, deformation structures in such megaclasts remain poorly understood. This study uses high-quality, three-dimensional (3-D) seismic reflection data from the deep-water Taranaki Basin offshore New Zealand to analyze the morphological character of 123 megaclasts and propose a new classification scheme based on their morphometric properties. The megaclasts are up to 400 m tall, 1900 m long, and 1200 m wide. In the study area, they are high- to moderate-amplitude features owing to their different lithologies and continuous-to-contorted seismic facies. The megaclasts can be classified as undeformed, rotated, deformed, and highly deformed based on their internal deformational styles. Two different kinds of morphological depressions observed on their basal shear zones further indicate whether the megaclasts are transported or formed in situ. Our study demonstrates that quantitative parameterization of the megaclasts provides important information about their deformational processes and a more complete understanding of megaclast emplacement along continental margins.

Evolution of submarine channel morphology in intra-slope mini-basins: 3D-seismic interpretation from offshore Niger delta

The morphological characteristics of a submarine channel system, within buried intra-slope mini-basins in the offshore Niger Delta have been investigated on 3D seismic data. The studied submarine channel named Amaku Channel Levee System (ACLS) initiated after avulsion of a feeder channel called Amaku Major System (AMS). The ACLS runs across three folds (A, B, and C), related to mobile shale deformation, being underlain by mass transport complexes (MTCs), lobes, and levees. From three defined channel reaches (upper, middle, and lower reaches), we have used seismic facies description, channel architecture characterization and quantitative analyses to decipher the spatial and temporal evolution of the channel levee system. Our results reveal that the western channel margin was controlled by structural barriers associated with mobile shale deformation, while the eastern channel margin was influenced by depositional barriers linked to levees and mass transport complexes (MTCs). We argue that the depositional barriers are in some cases more efficient than the structural barriers in controlling channel pathways, since pre-channel sediments confine the channel elements and divert them against positive reliefs. Seismic observations show that channels migrate from folds to structural lows causing flow stripping processes. Downstream variations of morphological parameters (such as sinuosity, width, and levee width), indicate that pre-existing deposits prevented these basin-ward migrations, driving straightening and narrowing of the channel system. This study reveals that submarine channels can undergo dramatic morphological and stratigraphic changes in structurally complex areas because of the configuration of sidewall reliefs, which is controlled, in turn, by the interplay between structural deformation and pre-channel deposition.

High-Precision CA-ID-TIMS U-Pb Zircon Geochronology of Felsic Rocks in the Finlayson Lake VMS District, Yukon: Linking Paleozoic Basin-Scale Accumulation Rates to the Occurrence of Subseafloor Replacement-Style Mineralization

Abstract Felsic igneous complexes and associated volcano-sedimentary rocks in continental back-arc environments host large-tonnage and/or high-grade volcanogenic massive sulfide (VMS) deposits. The emplacement mechanisms, style, and preservation of these deposits is thought to be partially dependent on depositional rates of the host lithofacies (i.e., discrete volcanic eruptions) relative to the setting of massive sulfide genesis on the seafloor as mounds and/or via subseafloor replacement of existing strata. The localization and occurrence of subseafloor replacement-style VMS deposits is therefore strongly influenced by the characteristics of the volcano-sedimentary facies in the hosting basin and the rates of their emplacement; the latter are poorly constrained in the literature due to the difficulty of obtaining high-precision dates that make this possible in Phanerozoic and older rocks. New high-resolution U-Pb geochronology and detailed regional stratigraphic investigation indicate that Devonian-Mississippian volcanic rocks and associated VMS mineralization in the Yukon-Tanana terrane in the Finlayson Lake district, Yukon, Canada, were erupted or emplaced during distinct time periods (ca. 363.3, 362.8, and 355.2 Ma) in two discrete submarine basins: the Kudz Ze Kayah formation and the Wolverine Lake group. The VMS deposits in both settings are contained within intrabasinal rocks that accumulated at rapid rates of ~350 to 2,000 m/m.y. over 0.6 to 1.4 m.y. Locally, these rates reach peak rates up to 7,500 m/m.y. in the Wolverine Lake group, which are interpreted to reflect facies deposition by mass transport complexes or turbidity currents. These new dates indicate that rapid accumulation of volcanic rocks in the back-arc basins was critical for localizing subseafloor replacement-style mineralization and the development of the Zn-enriched GP4F, Kudz Ze Kayah, and Wolverine VMS deposits. Rapid depositional processes observed in these deposits and their host basins are interpreted to have an important role in developing highly porous and permeable, water-saturated lithofacies that provide optimal conditions for enhancing zone refining processes and subsequent preservation of massive sulfide mineralization, which are key in the development of high-grade and large-tonnage VMS deposits. It is herein suggested that quantitative basin-scale accumulation rates, as a result of new U-Pb geochronological methods and increased precision combined with detailed stratigraphic and facies analysis, may provide important perspectives on the formation of continental back-arc basins and the localization of VMS deposits in other continental margin environments globally.

Assessing the sealing quality of submarine mass transport complexes and deposits

Mass transport complexes and their associated mass transport deposits—both referred to here as submarine failure deposits—are virtually ubiquitous in the modern and ancient sedimentary record of many deepwater basins. As exploration expands to new frontiers, submarine failure deposits are being identified as components of the petroleum system in numerous prospects. Although such deposits were historically considered facies to avoid from a viewpoint of petroleum exploration, it is now recognized that they can act as source, reservoir, or seal elements. In this paper, we set out to investigate the role of submarine failure deposits as effective seals in the petroleum system using published data and propose a methodology to risk some first-order factors at a macro-, meso-, and micro-scale that influence the seal quality of these submarine failure deposits. We accomplish this by discussing the properties intrinsic to submarine failure deposits that affect the seal quality at different scales. Based on published literature, at least six offshore fields from the Gulf of Mexico and NW Borneo are reported to have a submarine failure deposit as an effective seal. These fields combined account for ∼0.9 billion barrels of oil equivalent of cumulative discovered reserves globally and prove the potential of submarine failure deposits as effective seals and a future undervalued play concept. We use three case studies to illustrate our methodology. Our methodology can be further customized using datasets from industry and public records. The seal risk matrix presented here is based on more than a decade of research and has been already used by several exploration companies with encouraging feedback. We acknowledge the limitations of the methodology, but future interdisciplinary research and integration of new datasets and results will improve the de-risking of submarine failure deposits in exploration. Additionally, this methodology can potentially be applied to assessing seal potential of submarine failure deposits for carbon capture sequestration and storage projects.

Giant paleo-seafloor craters and mass wasting associated with magma-induced uplift of the upper crust

Giant seafloor craters are known along many a continental margin with recurrent mass-wasting deposits. However, the impact of breakup-related magmatism on the evolution of such craters is barely understood. Using high-quality geophysical datasets, this work examines the genetic relationship among the location of magmatic sills, forced folds and the formation of giant paleo-seafloor craters underneath an ancient mass-transport complex in the Møre and Vøring basins, offshore Norway. The data reveal that forced folding of near-seafloor strata occurred because of the intrusion of several interconnected magmatic sills. Estimates of 1-dimensional uplift based on well data show that uplift occurred due to the intrusion of magma in Upper Cretaceous to Lower Eocene strata. Our findings also prove that subsurface fluid plumbing associated with the magmatic sills was prolonged in time and led to the development of several vertical fluid flow conduits, some of which triggered mass wasting in Neogene to Recent times. The repeated vertical expulsion of subsurface fluids weakened the strata on the continental slope, thereby promoting mass wasting, the selective cannibalization of the paleo-seafloor, and the formation of elongated craters at the basal shear zone of the mass-transport complex. Significantly, the model presented here proves a close link between subsurface magmatic plumbing systems and mass wasting on continental margins.

Structural evolution and sediment depositional system along the transform margin- Palar–Pennar basin, Indian east coast

Continental transform margins are characterized by sub-vertical strike-slip faults and rapid change in thermal gradient across the margin from continental to oceanic crust. The continental crust terminates abruptly along such sheared margins. Tectonic evolution associated with sedimentation in continental slope of the Palar–Pennar transform margin is inadequately understood due to limited seismic data and/or poor imaging with few well controls. Exploratory drilling attempts for hydrocarbon presence have resulted in some promising outcomes in the Palar–Pennar basin.The Palar–Pennar graben in onland is bound by steep normal rift faults, which reactivated by strike-slip movement during late syn-rift time as Antarctica separated along the NNW to ∼ N-trending Coromandal transform margin in the offshore part. The pre-existing Precambrian shear zones and lineaments in the basement have played a governing role in the basin development. The onland drainage pattern is structurally controlled as observed from its rectangular drainage pattern, tectonic index and sinuosity index. In offshore area of the basin, seismic interpretation identifies distinct pull-apart rift basin along the Coromandal transform margin with syn-rift deposits in the lower continental slope portion. Strike-slip movement uplifted the fault blocks in the southern part of the pull-apart basin. This has also reactivated some of the rift faults in the syn-rift section. Such reactivation results in early post-rift seal breach. This tectonic event has implication in the depositional sequences, which has been portrayed by seismic stratigraphic analysis and construction of chronostratigraphic section across the transform margin in this work. Thickening of post-rift deposition towards E -SE, major hiatus, large scale mass transport complexes and erosional events have been identified from such analysis. On this basis, seven distinct tectonic sequences, M1 to M5 in Mesozoic and T1 and T2 in Tertiary, have been identified. The chronostratigraphic section can help in deciphering the relative sea level changes based on the nature of stratal terminations, extent of erosion and non-deposition. This study emphasizes the tectonic-controlled drainage system and geometry as well as sedimentation pattern in onland and offshore deepwater area of Palar – Pennar basin alongside the Coromondal transform margin.

Identification of submarine landslides in the Colombian Caribbean Margin (Southern Sinú Fold Belt) using seismic investigations

Submarine landslides can be triggered by several processes and involve a variety of mechanisms. These phenomena are important sediment transport processes, but they also constitute a significant geohazard. Mapping of the southwestern Caribbean Sea using 3D seismic data has allowed identification of several submarine landslides in the Colombian Margin in the area dominated by the Southern Sinú Fold Belt (SSFB). A poststack depth-migrated seismic cube survey with a 12.5 by 12.5 m bin spacing was used to identify landslides in an area covering 5746 km2. Landslides were interpreted using a seafloor morphologic parameter identification process and the internal deformation of the slope-forming material, as seen from seismic data. A total of 93 landslides were identified and classified based on their movement styles as follows: 52 rotational, 29 translational, and 12 complex landslides. In addition, 12 distinct deformational zones and a zone of mass transport complex (MTC) were identified. Five different ground condition terrains were interpreted based on landslide type and distribution as well as in geologic structures and seismic reflection analysis. Two main processes seem to influence landslides in the study area. First is the folding and faulting involved in the SSFB evolution. This process results in oversteepened slopes that start as deformational zones and then fail as translational or rotational slides. Those individual landslides progressively become complex landslide zones that follow geologic structural orientation. Second is the continental shelf break erosion by debris flows, which fills in intraslope subbasins and continental rise with several MTCs. According to the results, risk of damage by landslides increases in distances shorter than 4 km along structural ridge foothills in the study zone.

Taming the Thunder Horse With Axes and Vectors

The Thunder Horse Field targets Middle Miocene deepwater turbiditic reservoirs. Despite being prolific, the mapping of the ~180 m thick, partly amalgamated reservoir sandstones is challenging. Seismic quality is reduced by the presence of salt structures. The salt overburden and high formation pressure require the use of heavy mud weights and oil-based drilling fluids, which limit the resolution and interpretation potential of borehole image logs (BHI). Halokinetic movements caused significant post-depositional deformation of the already complex gravity-driven sediment stack, and the reservoir beds drape against an E-W oriented salt wall. Consequently, the assessment and removal of the structural dip component are not trivial, and the evaluation of paleo-transport directions is considerably more complicated compared to undisturbed deepwater reservoirs. This paper illustrates the potential of eigenvector methods to BHI from Ruehlicke et al. (2019) for reconstructing the depositional slope and the architecture of mass transport complexes in the case of chaotic depositional settings and uncertain structural dip. Figures from Henry et al. (2018) are used wherein part axial analysis was performed on data from a group of Thunder Horse wells and presented in more detail.

Evolution of a Fault-Controlled, Deep-Water Sub-Basin, Tabernas, SE Spain

The Neogene Tabernas Basin, SE Spain, provides important evidence at outcrop for the interplay between tectonic deformation of the sea floor, slope instability and turbidity current behaviour. Dextral-oblique strike-slip faults and associated folds propagated along the basin axis to deform the palaeo-sea floor, creating structurally-controlled depressions in which turbidity currents were trapped and ponded. EW-trending syn-depositional faults define a narrow sub-basin that subsided asymmetrically as a negative flower structure. The sub-basin contains an expanded succession (>300 m of ponded turbidite sheets, debrites and slumps) along its northern margin flanked by the principal fault strand defined by a wide zone of sheared and calcite-veined marl. A narrower fault zone with a smaller displacement marks the southern margin of the sub-basin and the fill close to it is thin with internal discordances, evidence of local failure and southward thinning of sandstone sheets. Both northern and southern faults ‘died’ at the same stratigraphic level and were overstepped by basin floor turbidites showing evidence of weaker and longer-range topographic confinement. As turbidites healed and aggraded out of the sub-basin to progressively onlap the southern margin of the basin, major gravity failures occurred emplacing thick (>100 m) mass-transport complexes. The first initially reworked the southern part of the sub-basin fill together with the early onlap wedge, the second remobilised the onlap wedge, and the third records failure of the upper part of the slope well above the wedge. The first two were toe-confined failures, the third and furthest travelled was confined by basin axis topography. All three failures are lateral to or directly overlain by ‘megabed’ sheets on the basin floor, implying either a common trigger (earthquakes) or slope instability following reflection of large volume flows. Tabernas turbidites highlight the role of basin tectonics (as opposed to up-dip supply and sea level fluctuations) in directly impacting on deep-water processes and stratigraphy. Small deep-water transtensional sub-basins opened up along long transfer faults accommodating regional extension.

Submarine landsliding in carbonate ooze along low-angle slopes (Inner Sea, Maldives)

Multichannel seismic and hydroacoustic data from the Maldives carbonate platform in the northern Indian Ocean document 4–8 km wide and are 70–170 m thick submarine landslides at water depths greater than 500 m, potentially associated with extensive fluid escape and the occurrence of gas hydrates. The mobilized sediments are part of giant drift bodies that consist of fine-grained carbonate ooze. Their downslope extent is located adjacent to large gateways that connect the semi-enclosed inner basin of the carbonate platform with the open Indian Ocean. Exposed at the sea-floor are headwall and translational domain of the landslides, showing different kinematic indicators typical for mass transport complexes. The submarine landslides evolved at slightly inclined slopes (<1°) along a listric basal shear surface (BSS) by downslope progradation and retrogressive failure. Styles of motion include sediment creeping, sliding and slumping. Where the BSS is flat, sediments translate as a large body. Where the BSS is irregular, folded, thrusted and rotated blocks/ridges occur, associated with secondary listric shear faults. Characteristically for the headwall domain are arcuate depressions which are related to rollover structures in the displaced slide block. The distal toe domain of the submarine landslides is located at the steep (>20°) part of the platform slope which faces the Indian Ocean. Data show that landslide activity is ongoing.The likely trigger of slope failure is a combination of gravitational forces and the reduction of sediment shear strength moderated by fluid ascent and the destabilization of gas hydrates. Gas hydrates are suggested to play an important rule, because the top of the gas hydrate stability zone coincides with a significant steepening of the platform slope from 1-3° to 10–20°. It is therefore proposed that the dissociation of the gas hydrates, for example during sealevel lowstands might have been an additional trigger of slope failure. During sealevel highstands, gas-hydrate coating of sediment grains may facilitate low-velocity plastic deformation of the strata.

Giant meandering channel evolution, Campos deep-water salt basin, Brazil

AbstractSubmarine channels are conduits for sediment delivery to continental margins, and channel deposits can be sandy components of the fill in tectonically active salt basins. Examples of salt-withdrawal basin fill commonly show successions of sandy channelized or sheet-like systems alternating with more mud-rich mass-transport complexes and hemipelagites. This alternation of depositional styles is controlled by subsidence and sediment-supply histories. Salt-basin fill comprising successions of largely uninterrupted meandering-channel deposition are less commonly recognized. This begs the questions: can sediment supply be large enough to overwhelm basin subsidence and result in a thick succession of channel deposits, and, if so, how would such a channel system evolve? Here, we use three-dimensional seismic-reflection data from a &amp;gt;1500 km2 region with salt-influenced topography in the Campos Basin, offshore Brazil, to evaluate the influence of salt diapirs on an Upper Cretaceous–Paleogene giant meandering submarine-channel system (channel elements &amp;gt;1 km wide; meander wavelengths several kilometers to &amp;gt;10 km). The large scale of the channels in the Campos Basin suggests that sediment discharge was large enough to sustain the meandering channel system in spite of large variability in subsidence across the region. We interpreted 22 channel centerlines to reconstruct the detailed kinematic evolution of this depositional system; this level of detail is akin to that of recent studies of meandering fluvial channels in time-lapse Landsat satellite images. The oldest channel elements are farther from salt diapirs than many of the younger ones; the centerlines of the older channel elements exhibit a correlation between curvature and migration rate, and a spatial delay between locations of peak curvature and maximum migration distance, similar to that observed in rivers. As many of the younger channel centerlines expanded toward nearby salt diapirs, their migration pattern switched to downstream translation as a result of partial confinement. Channel segments that docked against salt diapirs became less mobile, and, as a result, they do not show a correlation between curvature and migration rate. The channel migration pattern in the Campos Basin is different compared to that of a tectonically quiescent continental rise where meander evolution is unobstructed. This style of channelized basin filling is different from that of many existing examples of salt-withdrawal minibasins that are dominated by overall less-channelized deposits. This difference might be a result of the delivery of voluminous coarse sediment and high discharge of channel-forming turbidity currents to the Campos Basin from rivers draining actively uplifting coastal mountains of southeastern Brazil. Detailed kinematic analysis of such well-preserved channels can be used to reconstruct the impact of structural deformation on basin fill.

Seismic interpretation workflows for deep-water systems: A practical guide for the subsurface

Effective exploration, appraisal and development of deep-water reservoirs requires systematic mapping, description and characterization of the depositional systems that host them. Success in these areas requires that there is a firm understanding of hydrocarbon volumes and uncertainties, facies distributions to locate exploration and appraisal wells, and a framework to build reservoir models. We present a practical seismic interpretation workflow methodology which will be applicable in most cases and with most seismic data sets. The workflows are for channels, sheets and mass transport deposits (MTDs). Each step of the workflow is illustrated with subsurface and/or outcrop examples. The channels workflow guides the interpreter through identification of three hierarchical levels of channel architecture and the recognition of six key seismic facies: channel axis; channel margin; basal lags; slumps; thin-bedded facies (internal levees and terraces); and external levees. The sheet sandstone workflow emphasizes a series of steps including: understanding the regional and local context; understanding the evolution of sea-floor topography at basinal and depositional scales; and generating detailed amplitude maps. Mass transport complexes are rarely reservoirs but can have a major impact on reservoir distribution. The workflow encourages enquiries into these impacts which include: potential erosion of underlying reservoirs; ponding on and around the MTD; diversion and deflection of subsequent reservoir-hosting depositional systems; and their potential as sealing facies. The workflows are designed to be efficient and practical, and will provide an appropriate characterization encompassing exploration and appraisal to early development phases.

Genesis and evolution of large-scale sediment waves in submarine canyons since the Penultimate Glacial Maximum (ca. 140 ka), northern South China Sea margin

Sediment waves are common on the seafloor, but large-scale ones developed in submarine canyons are rarely reported. In this study, we document for the first time the Quaternary deep-water canyon-confined large-scale sediment waves developed on the northern South China Sea margin based on the analysis of a high-quality 3-D seismic reflection volume combined with a 2-D multichannel seismic reflection profile and three wells. Results show that the onset of these canyon-confined large-scale sediment waves can be dated back to the Penultimate Glacial Maximum (PGM)(ca.140 ka). This was the last period for the Pearl River Delta to prograde over the shelf edge. And these sediment waves have continued to develop and migrate upslope until present. The large-scale sediment waves pertaining to the PGM have a dominant down-slope asymmetrical 2-D morphology with wavelengths of 1.059–6.090 km and wave heights of 3.3–32.5 m. The present large-scale sediment waves show an up-slope asymmetrical 2-D morphology with dimensions of 0.667–5.628 km wavelength and 2.7–14.0 m wave height, which are smaller relative to those at the PGM. Grain sizes of the sediment waves at the PGM and present are interpreted to be coarse, inferred from seismic reflection data by high amplitude reflections (HARs). After a comprehensive analysis of the types of sediment-laden flows, the sediment provenances, the seafloor topography and the features of the sediment waves, these large-scale sediment waves might be interpreted as cyclic steps generated by the down-slope flowing supercritical turbidity currents and associated internal hydraulic jumps along high gradient (approximately greater than 1.28°) canyon thalwegs characterized by numerous slope breaks. The evolution of the large-scale sediment waves from partially depositional at the PGM to erosion-dominated at the present is controlled by variations of the sediment supply and the submarine slope accommodation along the canyon thalweg, which is manifested in the co-evolution of the sediment waves with the canyon. The findings of this study tell us that in addition to axial channels and mass-transport complexes, large-scale sediment waves can also develop on the canyon floor, which helps to improve our understanding of the complex canyon processes. The ‘large-scale sediment waves’ described here may be a new potential deepwater-reservoir element for the deep-water hydrocarbon exploration associated with submarine canyons.

Lithological, petrophysical, and seal properties of mass-transport complexes, northern Gulf of Mexico

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 evaluate the lithological and petrophysical properties, and seismic characteristics of three deeply-buried (>2300 m/7546 ft below the seabed), Pleistocene MTCs in the northern Gulf of Mexico. We show that: (i) MTC lithology is highly variable, comprising a mudstone-rich debrite matrix containing large (4.5 km3/1.08 mi3), deformed, sandstone-rich blocks; (ii) MTCs are generally acoustically faster and are more resistive than lithologically similar (i.e. mudstone-dominated) slope deposits occurring at a similar burial depth; (iii) MTC velocity and resistivity increase with depth, likely reflecting an overall downward increase in the degree of compaction; and (iv) the lowermost 15-30 m (49-98 ft) of the MTCs, which represent the basal shear zones, are characterised by relatively high P-wave velocity and resistivity values, likely due to shear-induced over-compaction. 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.

Gas hydrate identification and distribution in the Mexican Ridges, Mexico

We have mapped gas hydrates, free gas, and bottom-simulating-reflector (BSR) distributions in an area of the Mexican Ridges, central Gulf of Mexico, Mexico, revealing the relationship between these three elements and the tectonostratigraphy. The three elements are more visible when the host rock is a high-porosity sandstone because there is a large seismic impedance contrast between the solid gas hydrates above and the free gas below, which manifests itself on the seismic as a BSR. Gas hydrates are identified in the well as higher resistivity sandstone layers with a strong positive amplitude. When the host rock has a higher shale content with lower porosity, the impedance contrast is lower and the BSR is weak or not visible. The study area in Mexican Ridges is an anticline where gas hydrates and free gas are trapped on the crest after migrating through the dipping layers and faults from synclines that are generated in calcareous shale. The main seal is mass transport complex deposits from the Pliocene; when they are not deposited at the crest of the anticline, there is gas escape to the seafloor in the form of a gas chimney. In this way, we established a complete petroleum system for gas hydrates and free gas on the Mexican Ridges.

Submarine landslide origin of a tsunami at the Black Sea coast: Evidence based on swath bathymetry and 3D seismic reflection data

Tsunami waves were observed along the Bulgarian Black Sea coastline on 7 May 2007. The maximum rise and fall of the sea level were 1.2 and 2.0 m, respectively, with wave oscillations between 4 and 8 min. At first, submarine landsliding and then later on atmospheric disturbance were suggested as the cause of the tsunami. Numerical modeling, assuming a landslide displacing 30–60 million m3 material on the slope with a thickness range of more than 20–40 m, could reproduce the main characteristics of the recorded tsunami. In this early model, the landslide initiated on the shelf at a water depth of 100 m with a runout of approximately 20 km into 1000 m water depth. Subsequent and recent numerical modeling suggested that the failure may have initiated on the slope, anywhere between 200 and 1500 m seafloor depth. The runout of the transported sediments in these latest model was at 1850 m water depth. Just a few years after the tsunami, OMV and its joint venture partners, TOTAL and Repsol, acquired modern deepwater data sets in the same area where the submarine landsliding was assumed to occur. These data sets included multibeam swath bathymetry area and 3D reflection seismic data. These data sets offer a possibility to establish the presence of speculative submarine landslide responsible for the tsunami, with its geometry and nature. Our results provide direct evidence for the occurrence of large nonseismic catastrophic sediment failures along the Bulgarian coast. In this study, we illustrate Quaternary submarine landslides on 3D seismic reflection data immediately below the one responsible for the 2007 event; we also briefly point out the potential interpretation pitfall related to sediment waves and mass transport complexes.

Integrated characterization and failure mechanism for a mid-Pleistocene mass transport complex-dominant interval in the Mars Ursa Basin, northern Gulf of Mexico, USA

The economic and operational risks associated with mass transport complexes (MTCs) in deepwater hydrocarbon exploration act as a principal motivation to investigate their depositional elements using industry data. There is a lack of extensive seismic and well data coverage that limits the understanding of the processes associated with the evolution of MTCs within deepwater sedimentary basins. This study leverages a unique integrated data set to evaluate the depositional character and potential failure mechanisms of seven identified MTCs preserved in a synkinematic mid-Pleistocene MTC-dominant interval that spans the hydrocarbon-bearing Mars Ursa Basin in the northern Gulf of Mexico. Through seismic interpretation and attribute extraction methods using a 3D prestack depthmigrated seismic survey, we have determined the kinematic indicators and preserved the morphodomain geometries of the identified MTCs. The MTC-dominant interval covers an area of 631 km2, a volume of 392 km3, and a maximum thickness of 549 m in minibasin centers. The interval is penetrated by 15 boreholes that provide stratigraphic and lithologic calibration of the morphometric analyses. The lithologic composition of the MTC-rich interval is claystone/mudstone-dominant with a few interbedded, thin sandstones. The identified kinematic indicators and geometric extent of the identified MTCs are a function of the local salt tectonics extrabasinal controls. The stratigraphic framework presented in this study constrains the timing of failures to a period of high sediment deposition related to a major increase of glacial input into the Quaternary Mississippi Fan. This study offers borehole-calibrated MTC morphometrics preserved in an MTC-dominant interval whose failure is triggered by local salt inflation, but ultimately it is a consequence of loading following increased sediment supply into the basin. The results from this robust data set build upon past integrated seismic-well studies that strive to improve the understanding of MTC processes and their implications in hydrocarbon exploration across salt sedimentary basins.

Formation Evaluation Workflow Assesses Mass-Transport Complex Reservoirs

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 203016, “Formation Evaluation in Mass-Transport Complex Reservoirs,” by Ulises Bustos, Schlumberger; Carlos Duran, Petróleos Sudamericanos; and Alvaro Chapellin, Schlumberger, et al., prepared for the 2020 Abu Dhabi International Petroleum Exhibition and Conference, Abu Dhabi, held virtually from 9-12 November. The paper has not been peer reviewed. Mass-transport deposits (MTDs) are sedimentary, stratigraphic successions remobilized after initial deposition but before substantial lithification and transported downslope by gravitational processes as non-Newtonian rheological units. In the complete paper, the authors present an openhole advanced formation-evaluation approach that enables assessment of tight-matrix and natural-fracture systems at a level not previously accomplished in these types of geological formations. Introduction The considered wildcat project by Petroleos Sudamericanos is in the Lower Magdalena Valley hydrocarbon province in Colombia. From a stratigraphic point of view, the targets belong to tertiary deposits from the lower Neogene. Gravity-driven processes are complex and include creep, slide, slump, debris flow, and multiphase granular flows. The remobilized sedimentary deposits resulting from these processes are called MTDs or mass-transport complexes (MTCs) and are the main target job. Overlaying a crystalline basement, four MTC cycles were identified (although the present work only covers three MTC cycles) deposited in shallow marine environments. Each cycle consists of quartzite; phyllite; and schistose metamorphic rocks, largely gneiss with strong milonitization effects and foliated tremolitic marble in the top of the sequence. A summary of these types of rock is provided in the complete paper. The non-Archie nature of these rocks represents a challenge for formation evaluation. For reducing these uncertainties, a volumetric model with spectros copy dry-weight elements and nuclears was created that enabled solution of the total porosity, which was then benchmarked against the lithology-independent total nuclear magnetic resonance (NMR) porosity. The saturation computation was achieved with the fast neutron cross section method (FNXS) for the gas component and with total carbon for the liquid hydrocarbon fraction. The natural fracture system was analyzed with borehole image logs and with radial sonic-based dispersion analysis. The integration of matrix and natural-fracture assessment provided a robust formation evaluation that enabled identification of the main interest zones across the MTC cycles. Multifunction Spectroscopy for Matrix Analysis The multifunction spectroscopy tool is 1.72 in. in diameter and 18.3 ft long, consisting of a pulsed neutron generator and four detectors. The first detector is the compact neutron monitor, sensitive to fast neutrons and located adjacent to the high-output pulsed neutron source, measuring neutron output with high accuracy and precision. The second (near) and third (far) gamma-ray detectors are used for inelastic and capture spectroscopy measurements. The fourth and farthest-spaced detector (deep) uses an yttrium aluminum perovskite scintillator, which is involved in gas detection and assessment. The nuclear spectroscopy measurement is performed in energy and time domains; both aspects are described in detail in the complete paper.

Structure of the Offshore Obducted Oman Margin: Emplacement of Semail Ophiolite and Role of Tectonic Inheritance

AbstractThe northern Oman margin is a key area for understanding the emplacement of the Semail Ophiolite and obduction processes in general. This study uses a grid of 2D‐multichannel seismic lines tied to well data to characterize the offshore domain of the Semail Ophiolite and reappraises the obduction and post‐obduction history of the Oman margin. West of Muscat, in the Sohar basin, the late Cretaceous to Paleogene tectonic mega‐sequence records syn‐ to late‐obduction stages and the deposition of erosional products of the autochthonous Arabian sediments, including a major mass transport complex. Syn‐obduction thrusting is documented in this sector only, as a major fault emplacing a distal basement high (likely volcanic) onto Campanian sediments over &gt;10 km. To the east, the Hatat and Tiwi basins are characterized by a less‐copious Maastrichtian‐Paleogene sequence. These basins developed above a domain characterized by the northern equivalent of the Saih Hatat dome and later extensional faults. This sector distinctively records the extensional phase associated with the exhumation and erosion of the subducted continental margin. The dichotomy between the two sectors is linked due to a structural high located offshore, in the continuation of the Semail Gap transfer fault. We propose that this transfer fault, coincident with a major Pan‐African structure, affected the architecture of the passive margin during both rifting of the Neotethys and later ophiolite emplacement, that is, during (continental) subduction and obduction.