High-resolution 3D Seismic Data Research Articles

Deposition of cenomanian – Turonian organic-rich units on the mid-Norwegian margin: Controlling factors and regional implications

On the mid-Norwegian margin, extensive rifting and subsequent deposition of thick Cretaceous and Cenozoic sediments have buried the traditional Upper Jurassic organic-rich shales too deep. Consequently, these organic-rich shales are overmature and spent in the deep basins on the mid-Norwegian margin. The absence of well-control, variable seismic quality and in particularly, the great burial depth, makes it difficult to identify alternative Upper Jurassic and Lower Cretaceous organic-rich units. By combining high-resolution 2D seismic data, well logs, and Rock-Eval data, this study documents the presence of alternative organic-rich units in the Cretaceous succession on the Halten Terrace and the Vøring Basin. Multiple seismic horizons which correspond to regional flooding surfaces and define a series of seismic sequences have been mapped across the study area. The regionally extensive upper Cenomanian horizon is associated with wireline log signals and Rock-Eval parameters which imply the presence of a potential source rock unit. Source rock evaluation indicate that this unit contains mainly kerogen Type III on the Halten Terrace, suggesting an organofacies with significant contribution from terrestrial sources. In the Vøring Basin, the unit is sparsely drilled but appears to be mature, thus displaying a relatively limited potential. One well from the Vigrid Syncline demonstrate somewhat higher potential, with Rock-Eval data indicating a kerogen Type II composition. As such, more prolific units seems to exist in the Vøring Basin, albeit exhibiting a patchy distribution. We speculate that the deposition and preservation of this upper Cenomanian organic-rich unit record the development of an extended oxygen minimum zone attributable to increased primary production and sluggish water circulation, linked to the global Oceanic Anoxic Event 2 (OAE 2). However, local physiographic conditions, such as high sedimentation rates, erosion by gravity flows and periodically oxygenated conditions hindered preservation of a significant quantities of organic matter, thus limiting the thickness and quality of the upper Cenomanian organic-rich unit on the mid-Norwegian margin.

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

Submarine mass wasting processes in deepwater settings can incorporate large blocks, which may play a key role in deepwater geological processes and geohazard assessment. However, there is a limited understanding of the deformation style arising from the interaction between submarine blocks and structural/bathymetric barriers such as ramps. The deformation and kinematic history of several submarine blocks (with a thickness of up to 150 m) within a near seafloor mass transport complex in the Orange Basin are documented using seismic geomorphic methods. The interpreted blocks are preserved in three discrete fault-bounded morphological terrains within a Neogene mass-transport complex. These terrains vary in lengths from 2 km to 6 km; they have heights of 60 m to 150 m and are characterized by discrete and localized structural highs on the present-day seafloor. Block sizes vary across terrains suggesting differences in the block evolution process. Blocks near the ramp appear in seismic profiles comprising (a) Chaotic and transparent seismic reflections and (b) parallel to sub-parallel, continuous, low to moderate amplitude reflections. This variability in seismic facies of the blocks reflects the degree of their interaction and translation over a ramp at the basal shear zone of the mass transport complex, evidenced by the difference in the block features on the upslope portion of the ramp versus the downslope part. Notably, the deformation styles recorded in the blocks show the impact of the ramp during mass flow, which has broader implications for understanding the internal mechanisms of blocky mass transport complexes in many continental margins.

Coral reef development and sea-level changes over the past 50,000 years: new evidence from the north-west shelf of Australia

Understanding of global sea-level changes and coral reef development is poorly constrained during Marine Isotope Stage 3 (MIS 3; ~ 60 to 30 ka). Australia’s North West Shelf (NWS), at depths of ~ 50 to 120 m below present sea-level (mbsl), represents an ideal natural laboratory to address these knowledge gaps. In this study, the authors investigate a unique suite of sea-bed rock drill (PROD) cores recovered as part of a geotechnical survey from the NWS ~ 150 km south-east of Ashmore Reef. Twenty cores, penetrating to 28 m below sea floor, were collected from the top of the now drowned platform complex in similar water depths (74.8 to 81.6 mbsl), forming two transects ~ 17 km apart. High-resolution 3D seismic and multibeam bathymetry data reveal three distinct, multigenerational platforms that are rimmed by smaller reef terraces and bisected by deeper channels, placing the core transects into a robust, regional geomorphic context that includes a succession of linear palaeo-shorelines and tidal-estuarine channel systems on the adjacent shelf between ~ 90 to 110 mbsl. The authors have completed detailed logging, high-spatial resolution hyperspectral scanning, petrologic, mineralogic and sedimentary facies analysis of these cores, including a precise palaeoenvironmental reconstruction based on coral, algal and larger benthic foraminifera assemblages; and extensive radiometric dating. The authors have observed a complex suite of lithologies including in situ coralgal reef frameworks, well-lithified to friable grainstones, packstones and coralline algal floatstone facies separated by at least two major palaeosol horizons. Together with thirty 14C-AMS and closed-system U/Th ages spanning 10.7 to > 50 ka, the authors define a complex but consistent record of four distinct chrono-stratigraphic units (Units 1 to 4), representing a repeated succession of shallow reef to deep reef-slope depositional settings as the platforms experienced repeated sea-level oscillations (interstadial/stadial to glacial/deglacial) over the last 75,000 yr. Two distinct phases of shallow-water, high-energy reef development are defined. The age of the older, diagenetically distinct reef unit (Unit 3) is unknown but interpreted to have developed before the MIS 4 lowstand (~ 65 ka). However, firm chronological constraints on the MIS 3 development of the younger reef unit (Unit 2), place the position of relative sea-level (RSL) between ~ 63 to 75 + 1.8 mbsl by 45.95 to 39.23 + 0.2 ka, consistent with other predictions and observations for the region. Following its exposure and demise due to sea-level fall to the Last Glacial Maximum (LGM), the platform system was unable to re-establish fully as it was reflooded during the subsequent deglacial sea-level rise. Deeper reef slope (Unit 1) facies dominate the core tops between ~ 13.2 to 10.7 ka, representing a major hiatus in shallow-water reef development on the platforms. Deglacial sea-level rise was either too fast and/or other environmental conditions inadequate (i.e. massive riverine sediment flux due to the strengthening Australian summer monsoon and/or reworking of shelf sediments) to allow re-establishment of shallow-water coral reef development on the platforms apart from a few isolated and distal locations (i.e. Ashmore, Cartier, Adele and Scott Reefs).

Structural and fluid-migration control on hill-hole pair formation: Evidence from high-resolution 3D seismic data from the SW Barents Sea

Hill-hole pairs are subglacial landforms consisting of thrust-block hills and associated source depressions. Formed by evacuation of material where ice sheets have been locally frozen to the substrate, they give insights into paleo-ice-sheet dynamics. The aim of this study was to document the relationships between ancient hill-hole pairs identified on a buried glacial unconformity with the structure of the underlying sedimentary deposits, and then to determine if the basin geology and glacial fluid migration pathways promoted local subglacial freeze-on during the hill-hole pair formation. The study is based on seismic geomorphological interpretation of four high-resolution 3D seismic cubes covering an area of 800 km2 in the SW Barents Sea, and fluid seepage data from 37 gravity cores.The seismic datasets allowed the identification of 55 hill-hole pairs along the buried unconformity. The hills are characterized by chaotic to homogenous seismic facies forming up to 19 m high mounds, each covering areas of 2000–644,000 m2. The holes form depressions between 1 and 44 m deep and 2000–704,000 m2 in areal extent, which cut into preglacial Mesozoic bedrock and later infilled by glacial till. The holes are often found above fault terminations. High-amplitude reflections identified along the faults and in the strata below the holes are interpreted as shallow gas migrating upward towards the glacial unconformity. Geochemical data of the seabed sediment cores further indicates an association between hill-hole pair occurrence and present-day thermogenic hydrocarbon seepage.The hill-hole pairs geometries were also used to identify five paleo-ice-flow directions along the glacial unconformity. These ice flows exhibit polythermal regimes, and four of them are parallel to ice-stream flow sets interpreted from glacial lineations. The integrated interpretation supports localized fault-related basal freezing of the Barents Sea Ice Sheet which resulted in the formation of hill-hole pairs when the ice sheet moved. In this context, the faults functioned as migration pathways for deep thermogenic fluids, possibly sourced from leaking Jurassic reservoirs.This study highlights the importance of the underlying geology for ice-sheet dynamics: While hill-hole pairs above glacial till appear to be commonly associated with dispersed gas hydrates, hill-hole pairs above bedrock additionally indicate a link to underlying fault systems and hydrocarbon reservoirs. Freeze-on of underlying bedrock to the basal ice along the strike of faults in sedimentary bedrock explains deeper hill-hole pairs with smaller extents along the glacial unconformity compared to areally larger but shallow hill-hole pairs detected above glacial till on modern seabeds. Such close association between paleo-thermogenic gas seepage and the location of hill-hole pairs strongly support that hill-hole pairs are excellent markers revealing exit points of fluid migration pathways in petroleum system models.

Geodynamic generation of a Paleocene–Eocene landscape buried beneath North Bressay, North Sea

Histories of vertical lithospheric motions provide important clues about geodynamic processes. We present evidence of an ancient ( c. 58–55 Ma) landscape that probably underwent rapid uplift and subsidence during the initiation of the Icelandic plume. Now buried beneath c. 0.4–0.8 km of rock in the North Bressay region in the North Sea, this landscape is located within a sedimentary basin on the margin of the North Atlantic Ocean. We use high-resolution 3D seismic reflection data to map this ancient surface. Correlation of stratigraphy with a survey in the Bressay region constrains the age and depositional environment. The landscape contains excellent evidence of meandering fluvial channels, some of which record avulsions, that terminate against a coastline to the east where deltaic landforms are identified. The landscape was depth-converted and decompacted to generate a digital elevation model from which river profiles were extracted. Their geometries indicate that the landscape was generated by three phases of uplift. This history of uplift and subsidence is analogous to similar-aged landscapes in the Judd area c. 400 km to the west and Bressay c. 30 km to the south, and appears to be another manifestation of lithospheric motions generated by the passage of warm thermal anomalies away from the Icelandic plume.

Tunnel valley formation beneath deglaciating mid-latitude ice sheets: Observations and modelling

The geological record of landforms and sediments produced beneath deglaciating ice sheets offers insights into inaccessible glacial processes. Large subglacial valleys formed by meltwater erosion of sediments (tunnel valleys) are widespread in formerly glaciated regions such as the North Sea. Obtaining a better understanding of these features may help with the parameterisation of basal melt rates and the interplay between basal hydrology and ice dynamics in numerical models of past, present, and future ice-sheet configurations. However, the mechanisms and timescales over which tunnel valleys form remain poorly constrained. Here, we present a series of numerical modelling experiments, informed by new observations from high-resolution 3D seismic data (6.25 m bin size, ∼4 m vertical resolution), which test different hypotheses of tunnel valley formation and calculate subglacial water routing, seasonal water discharges, and the rates at which tunnel valleys are eroded beneath deglaciating ice sheets. Networks of smaller or abandoned channels, pervasive slump deposits, and subglacial landforms are imaged inside and at the base of larger tunnel valleys, indicating that these tunnel valleys were carved through the action of migrating smaller channels within tens of kilometres of the ice margin and were later widened by ice-contact erosion. Our model results imply that the drainage of extensive surface meltwater to the ice-sheet bed is the dominant mechanism responsible for tunnel valley formation; this process can drive rapid incision of networks of regularly spaced subglacial tunnel valleys beneath the fringes of retreating ice sheets within hundreds to thousands of years during deglaciation. Combined, our observations and modelling results identify how tunnel valleys form beneath deglaciating mid-latitude ice sheets and have implications for how the subglacial hydrological systems of contemporary ice sheets may respond to sustained climate warming.

The formation and development of avulsions and splays of submarine channel systems: Insights from 3D seismic data from the northeastern Bengal Fan

Avulsions and splays of submarine channels are important autogenic responses affecting sediment distribution in deep-water areas; however, their formation and development are still not well understood. Using high-resolution 3D seismic data and spectral decomposition red–green–blue (RGB) color blends, two seismically well-imaged submarine channel systems (SCS1 and SCS2) on the northeastern Bengal Fan are used to document the formation and development of avulsions and splays. The results suggest that avulsions occur mainly during the early evolution stage of submarine channel systems. SCS1 and SCS2 are respectively accompanied by one and four avulsions and accordingly produce two and five independent submarine channels. During this early phase, these channels collectively incise into the underlying stratigraphy but lack levee deposits. Splays, in contrast, develop consistently during the late evolution stage of the submarine channel systems. Three crevasse splays and three overbank splays develop on the overbank areas of SCS1 and SCS2. During that late stage, channel and levee deposits build over the site of former erosion, thus forming the aggradational phase of channel evolution. Crevasse and overbank splays are two different types of splays observed on this part of the Bengal Fan. The former are created by flow breaching of the adjacent levees, producing large-scale elongated shapes; they are related to relatively stable sediment gravity flows, a high-degree of channel instability, and steep levee topography. Overbank splays, however, are created by flow overtopping adjacent levees and produce small-scale, fan-like shapes; they are related to gradually-weaking sediment gravity flows, a low-degree channel unstability, and gentle levee topography.

Deformation Styles and Multi-Stage Evolution History of a Large Intraplate Strike-Slip Fault System in a Paleozoic Superimposed Basin: A Case Study From the Tarim Basin, NW China

The complex deformation styles of large intraplate strike-slip fault systems in the multi-stage superimposed basin are hot topics worldwide. This article proposes structural models and evolution processes for such strike-slip fault systems in the Tarim Basin based on high-resolution 3D seismic data and deep wells. Our analyses reveal that strike-slip fault in the Tarim Basin formed with different structural styles in five tectonic layers from the Sinian to the Permian that accompanies the Sinian rift systems and uplift, the Lower–Middle Cambrian reversed faults and salt tectonics, the Ordovician fault-karst systems, the Silurian to the Carboniferous en-echelon transtensional faults, and the Permian volcanic structures. Influenced by the multi-tectonic layers and complex evolution history, the strike-slip faults performed as multi-layer flower structures and various fault types. The evolution history of paleo-uplifts also influenced the distribution characteristics of strike-slip faults, such as X, diamond, and V shapes in the Tabei uplift and T shapes in the Tazhong uplift. The strike-slip faults formed in late Cambrian stage were associated with unconformities, inverted structures, and growth strata in deep layers. The different tectonic evolution models of the Tabei and the Tazhong uplift were built, which shows pre-existing structures, the lithological combinations from the rift basin to the marine basin, and the change of regional tectonic stress from the Cambrian to Permian are controlling factors of the strike-slip fault systems. These models provide a new interpretation method for intraplate strike-slip fault systems worldwide.

Tracing Lower Cretaceous organic-rich units across the SW Barents Shelf

On the Barents Shelf, the northernmost and least explored hydrocarbon province of the Norwegian Continental Shelf, Upper Jurassic organic-rich shales have traditionally been given most attention as these represent the most prolific source rock unit of the region. However, in the western frontier areas of the Barents Shelf, the Upper Jurassic is too deeply buried. By combining high-resolution 2D seismic data, well logs, and digitalized Rock-Eval data, this study documents the lateral distribution and variability of alternative source rock units within the Lower Cretaceous succession on the SW Barents Shelf. Negative high-amplitude anomalies are traced from shallow basins on the platform westward into deeper basins along the western shelf margin. The anomalies are tied to intervals of increased total organic carbon contents in several exploration wells, and we thus establish the presence of four potential source rock units; these are the (1) upper Hauterivian, (2) Barremian, (3) lower Aptian, and (4) upper Cenomanian units. Based on the distribution of the associated seismic anomalies, we infer that the deposition and preservation of these organic-rich units are coupled to localized, fault bounded depocenters, mainly controlled by Late Jurassic – Early Cretaceous rifting and local reactivation events. The lower Aptian stands out as the most significant source rock unit, particularly in the Fingerdjupet Subbasin, where it displays a kerogen Type II composition. The distribution and development of this oil-prone source rock unit is linked to an early Aptian fault reactivation event. Due to increased sediment influx in combination with high subsidence rates during the Albian to Cenomanian, potential pre-Aptian source rock units appear to have undergone too deep burial in the Tromsø and Bjørnøya basins to be presently generative. Furthermore, organic matter dilution due to increased sedimentation rates seems to have reduced the overall potential of the upper Cenomanian unit.

Control of mechanical stratigraphy on the stratified style of strike-slip faults in the central Tarim Craton, NW China

The large-scale (tens to hundreds of kilometers long) but small-displacement (<2 km) strike-slip faults in the Shunbei region of the Tarim Basin, NW China, provide an opportunity to document the role of mechanical stratigraphy in an intracratonic strike-slip situation. Uniaxial compression test results and dipole acoustic logging data illustrate that the lower Paleozoic mechanical profile in the Shunbei region consists of 2 incompetent layers (middle Cambrian and Upper Ordovician) and 3 competent layers (Sinian–lower Cambrian, upper Cambrian–Middle Ordovician, and Silurian). Borehole and high-resolution 3D seismic data illustrate that the 3 competent layers are characterized by low-amplitude folds and isolated brittle faults. The discrete element method (DEM) modeling results indicate that mechanical stratigraphy is an important factor controlling the soft-linked faulting style that is independent of multiphase deformation. A pre-existing structure promotes the formation of narrow and localized fault zones. The early–middle Silurian evolution of the Shunbei No. 1 fault zone reflects a classic stratigraphically decoupled deformation model: the middle fault arrays (upper Cambrian–Middle Ordovician layers) and en echelon normal faults (Silurian layer) synchronously experienced left-lateral slip. These two styles of faulting have different geometries (fault dips, slips, and arrangements) but are kinematically coupled and genetically related. This proposed deformation model provides mechanical support to document deeply buried structures by characterizing genetically related shallow structures, which may be widely employed as multiphase inheritance structures with mechanical stratigraphy. We also point out that vertical fault connectivity is a key research aspect of hydrocarbon exploration in the Shunbei oilfield.

Sediment deformation triggered by underlying magma intrusion

Magma intrusion often uplifts and deforms overlying strata to produce forced folds. How the amplitude changes of forced folds through time reflect the transverse thickness changes in the deformed sedimentary strata, and can thus shed light on the processes driving deformation. For example, we expect the original morphology of intrusion-induced forced folds to reflect the shape and emplacement history of the underlying magma body. However, in sedimentary basins, the geometry of forced fold above ancient, buried magma body is a function of intrusion-induced uplift and post-emplacement compaction. Few studies have attempted to separate such syn- and post-intrusion deformation, meaning we poorly understand how burial-related compaction can modify forced fold geometry. This paper deals with the forced folds which are presently buried to depths of 2350–2600 m and caused by underlying magma intrusions, using high-resolution 3D seismic data and well data from the Pearl River Mouth Basin, northern South China Sea. The contribution of differential compaction (compaction-related deformation) was estimated by calculating sediment compaction curve, which allows the reconstruction of original syn-emplacement fold amplitude. This study shows that during magma emplacement, overlying strata greatly thin due to shear compaction. Moreover, burial can increase the amplitude of forced fold, improving their potential for capturing and storing fluids, and making it attractive site for hydrocarbon accumulation or carbon sequestration.

Application of multi-attribute matching technology based on geological models for sedimentary facies: A case study of the 3rd member in the Lower Jurassic Badaowan Formation, Hongshanzui area, Junggar Basin, China

Regarding high drilling costs, an effort should be made to substantially reduce the drilling operation. To achieve this goal, exploration and development stages should be carried out precisely with maximum information acquired from the reservoir. The use of multi-attribute matching technology to predict sedimentary system has always been a very important but challenging task. To resolve the challenges, we utilized a quantitative analysis method of seismic attributes based on geological models involving high resolution 3D seismic data for sedimentary facies. We developed a workflow that includes core data, seismic attribute analysis, and well logging to highlight the benefit of understanding the facies distribution in the 3rd Member of the Lower Jurassic Badaowan Formation, Hongshanzui area, Junggar Basin, China. 1) Data preprocessing. 2) Cluster analysis. 3) RMS attribute based on a normal distribution constrains facies boundary. 4) Mapping the sedimentary facies by using MRA (multiple regression analysis) prediction model combined with the lithofacies assemblages and logging facies assemblages. The confident level presented in this research is 0.745, which suggests that the methods and data-mining techniques are practical and efficient, and also be used to map facies in other similar geological settings.

Structural Control on Hill-Hole Pair Formation: Evidence from High-Resolution 3d Seismic Data from the Sw Barents Sea

Structural Control on Hill-Hole Pair Formation: Evidence from High-Resolution 3d Seismic Data from the Sw Barents Sea

Structural inheritance and its control on overpressure preservation in mature sedimentary basins (Dongying depression, Bohai Bay Basin, China)

Overpressure in sedimentary basins has practical commercial importance for petroleum exploration as the steady maintenance of pore-pressure in reservoirs is important for CO2 sequestration. Traps associated with faults comprise >30% of hydrocarbon discoveries in the Dongying depression, Bohai Bay Basin. However, the way structural inheritance, together with fault reactivation and its development, influence pore-pressure development and overpressure preservation remain poorly understood. This research uses high-resolution 3D seismic data combined with well-log data to investigate the structural evolution in the Lijin and Boxing sags in the Dongying depression and elucidate the influence of structural inheritance on overpressure development. The interpreted data reveal that: 1) five fault families in the Lijin Sag influence overpressure development, 2) three phases of faulting occurred in the Boxing Sag, with fault reactivation predominating in the early Eocene and late Oligocene; and 3) pressure coefficients (Pc) in Eocene strata vary from 0.90 to 1.96 in the Lijin and Boxing sags. In the Boxing Sag, Pc values vary from 0.91 to 1.54 in the Eocene strata, with 99% of overpressure coefficients below 1.50. In comparison, pressure coefficients in the Lijin Sag are significantly higher than in the Boxing Sag, with Pc values varying from 0.9 to 1.96, and 30% of the Pc values are over 1.50. This study concludes that inherited structures significantly control the development of overpressure in sedimentary basins. Systematic fault reactivation often leads to fluid leakage, which has a negative influence on overpressure preservation. In basins with border faults controlling their evolution, high-magnitude overpressure tends to be generated and maintained, favoring the formation of pressure compartments, conditions less ideal for carbon sequestration.

Glacitectonic evidence of ice sheet interaction and retreat across the western part of Dogger Bank (North Sea) during the Last Glaciation

High-resolution 2D seismic data from the western side of Dogger Bank (North Sea) has revealed that the glacigenic sediments of the Dogger Bank Formation record a complex history of sedimentation and penecontemporaneous, large-scale, ice-marginal to proglacial glacitectonism. The resulting complex assemblage of glacial landforms and sediments record the interplay between two separate ice masses revealing that Late Devensian ice sheet dynamics across Dogger Bank were far more complex than previously thought, involving the North Sea lobe of the British and Irish Ice Sheet, advancing from the west, interacting with the Dogger Bank lobe which expanded from the north. The active northward retreat of the Dogger Bank lobe resulted in the development of a complex assemblage of arcuate thrust-block moraines (≤15 km wide, >30 km long) composed of highly folded and thrust sediments, separated by sedimentary basins and meltwater channels filled by outwash. The impact of the North Sea lobe was restricted to the western margin of Dogger Bank and led to deep-seated (100–150 m thick) glacitectonism in response to ice-push from the west. During the earlier expansion of the North Sea lobe, this thrust and fold complex initially occupied a frontal marginal position changing to a more lateral ice-marginal position as the ice sheet continued to expand to the south. The complex structural relationships between the two glacitectonic complexes indicates that these ice masses interacted along the western side of Dogger Bank, with the inundation of this area by ice probably occurring during the last glaciation when the ice sheets attained their maximum extents.

Controls of Deep-Seated Faults and Folds on Hydrocarbon Fluid Migration and Accumulation in Sedimentary Basins: A Case Study from the Northwestern Sichuan Basin, China

Deep-seated faults and folds of foreland basin systems have become important exploration targets in the recent years because they are crucial in controlling fluid migration and hydrocarbon accumulation. In this study, we analyzed the characteristics and formation history of these structures in the northwestern Sichuan Basin using recently acquired two-dimensional (2D) and three-dimensional (3D) seismic data. The seismic interpretation revealed that the thrust sheets, tectonic wedges, and foredeep were well developed in the northwestern Sichuan Basin from the mountain to the basin. Forward thrusts, fault-bend folds, and wedges are the main types of structures in the thrust sheets and tectonic wedges. The deep-seated faults and folds were easily recognized in the high-resolution 3D seismic data. The imbricate thrust faults that merged into detachment layers of the Lower Cambrian are the main types of structures in the foredeep, and they show a prominent strike-slip influence in the horizontal direction. The formation of these structures in the foredeep in the northwestern Sichuan Basin mainly endured two stages of thrusting, including those during the Middle-to-Late Triassic and Cenozoic. Based on the tectonic evolution and seismic data, we infer that these deep-seated faults and folds in the foredeep may have formed earlier than the northern Longmen Shan fold-and-thrust belts and they may have been initially active in the late of Early Triassic and reactive during the Cenozoic. Furthermore, evaporites in the Lower and Middle Triassic were crucial in forming these structures. The petroleum exploration data suggested that the deep-seated faults can facilitate hydrocarbon accumulation. The thrust faults in the foredeep were more likely to act as migration pathways for fluids instead of sealing barriers along the horizontal direction. The interconnected reservoirs of deep-seated folds possess a great potential to allow large-scale hydrocarbon accumulation. Our study provides a good example for evaluating the hydrocarbon exploration potential in the deeply buried area in the sedimentary basin.

Thrust Faults Promoted Hydrocarbon Leakage at the Compressional Zone of Fine-Grained Mass-Transport Deposits

Fine-grained mass-transport deposits (MTDs), especially their compressional toe zones, are traditionally considered as effective seal in constraining the vertical fluid migration underneath. However, this study documents thrust faults at the compressional toe zone of fine-grained MTDs that could disaggregate the seal competence and promote vertical fluid flow. The investigated MTD referred to as MTD-a lies directly over a large hydrocarbon reservoir that is located within the Central Canyon of northern South China Sea, which is examined by using high-resolution 3D seismic and borehole data. Thrust faults and irregular blocks composed of coarse-grained sandstones are observed in the compressional zone of the MTD-a’s toe. More importantly, seismic evidence (e.g., enhanced seismic reflections) suggests that a large amount of hydrocarbons from the underlying reservoir penetrated through the MTD-a along these thrust faults and charged into the coarse-grained sandstone blocks. This clear evidence of thrust faults compromising the MTD’s seal effectiveness and thus facilitating the vertical fluid flow through the non-permeable strata demonstrate the importance of reassessing the seal capacity of MTD.

Effect of tectonic inversion on supra-salt fault geometry and reactivation histories in the Southern North Sea

High-resolution 3D seismic and borehole data in the Broad Fourteens Basin, Southern North Sea, are used to investigate the relationship between lithology and fault throw distribution, as well as to understand the reactivation and growth histories of faults developed due to tectonic inversion. Two (2) distinct tiers of faults are identified, and their geometry analysed in detail. Tier 1 faults comprise closely spaced sets of normal faults that resulted from the progressive buckling and stretching of Upper Mesozoic strata during Late Cretaceous to Paleogene tectonic inversion. They have been reactivated, but still show net normal throw separations, even though they were formed during a period of regional compression. Tier 2 faults comprise densely spaced sets of normal faults in Paleogene strata with a broad range of strikes, forming a polygonal pattern. These faults relate to early diagenesis but still record the effect of the Paleogene inversion episode. An important characteristic is that Tier 1 faults are highly segmented, and show differences in throw distribution between shale-rich and sandy intervals. These faults are more segmented with relatively small throw maxima of 14 ms (17.7 m) in shale-rich intervals, while sandy intervals are less segmented with larger throw maxima of 32 ms (40.3 m). Discrepancies in fault throw distribution and segmentation increase the chances of compartmentalisation or localised fluid flow through fault linkage zones, presenting at the same time significant risks when injecting CO2 in subsurface traps. Recognising the effect of tectonic inversion on supra-salt fault geometry, and reactivation histories, can be crucial to the characterisation of faulted hydrocarbon and carbon capture and storage (CCS) reservoirs in tectonically inverted basins such as the Southern North Sea.

Different avulsion events throughout the evolution of submarine channel-levee systems: A 3D seismic case study from the northeastern Bengal Fan

As very common deep-water phenomena, submarine channel avulsions have been paid considerable attention recently. The differences among them are, however, rarely noted, nor their relationship to the evolution of submarine channels. A seismically well-imaged channel-levee system from the Pliocene Bengal Fan, named target channels (TCs) and consisting of three independent channels (Tc1-Tc3), has been selected as a case to address these issues. Using high-resolution 3D seismic data along with spectral decomposition and RGB color blending techniques, this study images TCs from initiation to abandonment and recognizes three different types of avulsion events: regional avulsion, local avulsion and bank crevasse. In the early evolution stage of TCs, the regional avulsion and the local avulsion successively occurred, promoting TCs to obtain a stable flow course and then undergo a significant evolution. Moreover, coupled with those regional and local avulsion events, TCs were growing respectively in the form of individual channel and amalgamated stacking. In the late evolution stage of TCs, only bank crevasses occurred and led to the formation of crevasse splays in the overbank setting, which coincided with the aggradational stacking of TCs developing. This new evolution model of channel-levee systems, which acknowledges different avulsion events, may provide insights on the understanding of analogue deep-water deposits and in turn can be applied to guide the hydrocarbon exploration and development.

Seismic chimney characterisation in the North Sea – Implications for pockmark formation and shallow gas migration

Fluid-escape structures within sedimentary basins permit pressure-driven focused fluid flow through inter-connected faults, fractures and sediment. Seismically-imaged chimneys are recognised as fluid migration pathways which cross-cut overburden stratigraphy, hydraulically connecting deeper strata with the seafloor. However, the geological processes in the sedimentary overburden which control the mechanisms of genesis and temporal evolution require improved understanding. We integrate high-resolution 2D and 3D seismic reflection data with sediment core data to characterise a natural, active site of seafloor methane venting in the UK North Sea and Witch Ground Basin, the Scanner pockmark complex. A regional assessment of shallow gas distribution presents direct evidence of active and palaeo-fluid migration pathways which terminate at the seabed pockmarks. We show that these pockmarks are fed from a methane gas reservoir located at 70 m below the seafloor. We find that the shallow reservoir is a glacial outwash fan, that is laterally sealed by glacial tunnel valleys. Overpressure generation leading to chimney and pockmark genesis is directly controlled by the shallow geological and glaciogenic setting. Once formed, pockmarks act as drainage cells for the underlying gas accumulations. Fluid flow occurs through gas chimneys, comprised of a sub-vertical gas-filled fracture zone. Our findings provide an improved understanding of focused fluid flow and pockmark formation within the sediment overburden, which can be applied to subsurface geohazard assessment and geological storage of CO2.