High-resolution Seismic Data Research Articles

Late Weichselian and Holocene behavior of the Greenland Ice Sheet in the Kejser Franz Josef Fjord system, NE Greenland

To better understand the past retreat patterns and chronologies of major marine-terminating outlet glaciers, the Late Weichselian and Holocene glacial history of a NE Greenland fjord system is reconstructed using new and previously published swath bathymetry and high-resolution seismic data, supplemented with multi-proxy analysis of sediment cores. The investigated area extends more than 190 km, including Fosters Bugt, Kejser Franz Josef Fjord, Nordfjord and Moskusoksefjord, the drainage routes for Waltershausen Gletscher, one of the largest outlet glaciers in the NE sector of the Greenland Ice Sheet. A complex of moraine ridges identified in Fosters Bugt show former stillstand positions during this period, and, alongside radiocarbon ages, supports the theory of a maximum ice front position in the outer coastal areas during the Younger Dryas-Preboreal period. The retreat moraines distributed within the Kejser Franz Josef Fjord system provide evidence of a stepwise retreat during the last deglaciation, whereby rapid retreat was interrupted by episodes of stillstands and/or readvances of the grounding line. The fjord system was mostly deglaciated prior to 7.8 ka cal BP, with an estimated average retreat rate ranging between 33 and 96 m a−1 since the Younger Dryas. Following this, only minor ice advances in the inner fjords have been identified. The sediment supply from Waltershausen Gletscher dominated the Kejser Franz Josef Fjord, Nordfjord and outer Moskusoksefjord throughout the Holocene period, whereas in the middle and inner Moskusoksefjord the sediment deposits reflect a more local catchment area. The estimated average retreat rate is in the same order of magnitude as the rates from fjords of northern Fennoscandia, indicating the same overall control for the deglaciation, ice melting from increased Northern Hemisphere summer insolation that peaked in Early Holocene.

Early stage diapirism in the Red Sea deep-water evaporites: Origins and length-scales

Rayleigh-Taylor models for diapirism predict that diapirs should develop with characteristic spacings, whereas other models predict varied spacings. The deep-water Miocene evaporites in the Red Sea provide a useful opportunity to quantify length scales of diapirism to compare with model predictions. We first review the stratigraphy of the uppermost evaporites in high-resolution seismic data, revealing tectonic growth stratigraphy indicating that halokinetic movements occurred while the evaporites were being deposited. In some places, movements continued after the Miocene evaporite phase. The S-reflection marking the top of the evaporites is an erosional surface, in places, truncating anticlines of layered evaporites. In others, reflections within the uppermost evaporites are conformable, suggesting a lack of erosion. The top of the evaporites therefore had relief at the end of the Miocene. We select for numerical analysis 14 long profiles of topography of the S-reflection. Variograms derived from them after detrending reveal minor periodicity, though with varied wavelength, and varied roughness of the surface. However, an average variogram computed from these profiles is nearly exponential, indicating that the evaporite surface is mostly stochastic with no uniform scale of diapirism. An exponential model fitted to that average variogram suggests a spatial range over which the S-reflection topography becomes decorrelated of 3 km, which is comparable with the mean vertical thickness of the evaporite body. Power spectra of the evaporite surface are flatter at long wavelengths, which we interpret as due to weakness of halite preventing large surface relief from developing. The results suggest only modest periodicity, so the Rayleigh-Taylor model does not explain deformation in the Red Sea evaporites studied here. Their topography may turn out to be useful for suggesting the vertical scales and lengthscales of relief to expect of early stages of other salt giants, such as that of the Santo Basin.

Seismic monitoring for subsurface uncertainties at the Endurance CO2 store

The development of the Endurance structure in the southern North Sea as a CO2 store offers a potentially large decarbonization opportunity for industry along the east coast of England. The Bunter Sandstone Formation in the Endurance structure has been assessed as an excellent candidate for CO2 storage due to its position, depth, reservoir properties, and extensive seal. Monitoring CO2 during active injection and after closure is vital to confirming containment and conformance of the CO2 within the store. There are two key subsurface uncertainties: compartmentalization and vertical permeability. Current seismic resolution does not provide imaging of thin, low-permeability layers that may create low relative vertical permeability, and the high net to gross of the sandstone limits imaging of any minor faulting. However, the strong CO2 response on 4D seismic likely will illuminate baffles and barriers to flow along with variations in reservoir permeability. In 2020, four two-dimensional high-resolution (2DHR) seismic data test lines were acquired to assess the best technology for imaging and monitoring Endurance. High-resolution seismic was concluded to provide quality imaging from the base of the reservoir (approximately 1500 m true vertical depth subsea) up through the overburden to image the entire subsurface and to demonstrate safe storage of CO2.

Cenozoic basement-involved rifting of the northern South China Sea margin

The northern South China Sea (SCS) margin developed on a thinning crust comprising basement-involved structures inherited from the post-orogenic extension of the Cathaysia Block. While, the pre-rift structural configuration and its influence on the geometry and development of the Cenozoic rift basins are ambiguous. Based on 3D high-resolution seismic reflection data in the western Pearl River Mouth Basin (PRMB) and onshore fieldwork, we delineated the intrabasement boudinage structure which is constituted by the low-angle E–W-striking Indosinian fold-thrust belts and the steeply dipping NE-striking Yanshanian thrusts. It is concluded that the pre-rift structure played a key role in controlling the architecture and two-phase early rifting of the northern SCS margin. (1) During the Early Eocene syn-rift phase, the Indosinian thrusts were rejuvenated first and controlled the formation of a series of ENE-striking grabens. (2) During the Late Eocene syn-rift phase, both the Indosinian and Yanshanian thrusts were reactivated. The Indosinian thrusts continued to maintain normal faulting and even evolved into the detachment faults. The Yanshanian thrusts were inverted into the NE-striking dextral strike-slip faults, which controlled the formation of the pull-apart basin group en échelon and even the SCS opening. The lithospheric thinning mechanism of the SCS continental margin experienced a major change from the Early Eocene pure shear to the Late Eocene simple shear.

Seismic attribute analysis of a fault zone in the Thebe field, Northwest shelf, Australia

Seismic data are one of the main ways to characterize faults in the subsurface. Faults are 3D entities and their internal structure plays a key role in controlling fluid flow in the subsurface. We have aimed to characterize a geologically sound fault volume that could be used for subsurface model conditioning. We introduce an attribute analysis of a normal fault from a high-resolution seismic data set of the Thebe field, offshore northwest Australia. We merge together a series of common attributes for fault characterization: dip, semblance, and tensor (DST), and we also introduce a new total horizontal derivative (THD) attribute to define the edges of the fault zone. We apply a robust statistical analysis of the attributes and fault damage definition through the analysis of 2D profiles along interpreted horizons. Using the THD attribute, we interpret a smaller width of the fault zone and a more straightforward definition of the boundaries than from the DST cube. Following the extraction of this fault volume, we define two seismic facies that are correlated with lithologies extracted from our conceptual model. We observe a wider fault zone at larger throws, which also corresponds to synrift sequence; hence, more complex internal fault damage. Our method provides volumes at adequate scale for reservoir modeling, and therefore could be used as a proxy for property conditioning.

Palaeo-Shoreline Configuration of the Adventure Plateau (Sicilian Channel) at the Last Glacial Maximum

The Adventure Plateau, located in the NW sector of the Sicilian Channel, experienced several episodes of exposure/erosion and subsequent drowning, with the most recent occurring after the Last Glacial Maximum (LGM). Unlike other parts of the Sicilian Channel, the Adventure Plateau is relatively tectonically stable and is therefore best suitable for reconstructing its coastal configuration before the post-LGM marine transgression. Here, we use high-resolution seismic data to identify and map the palaeo-coastline at the LGM on the basis of the internal architecture of the prograding wedges (i.e., the location of the subaqueous clinoform rollover point) and the erosional markers such as the subaerial unconformities and the wave ravinement surfaces. These data, which show an extreme variability in the palaeo-morphology of the coastal margins of the Adventure Plateau, have been complemented with vintage seismic profiles in order to entirely cover its perimeter. The mapped LGM coastline has then been compared to predictions from glacial isostatic adjustment (GIA) modeling, which considers the horizontal migration of the shorelines in response to sea level rise and to Earth’s rotational and deformational effects associated with deglaciation. The two shorelines (i.e., the coastline derived from the marine data interpretation and the one derived from the GIA model) are in good agreement at 21 kyears BP, although some discrepancies occur in the southern part of the plateau, where the seabed slope is extremely gentle, which makes the clinoform rollover points and the buried erosional unconformities difficult to detect. After 20 kyears BP, an acceleration in the rate of the sea level rise occurred. The results of this study indicate the importance of comparing experimental data with model predictions in order to refine and calibrate boundary parameters and to gain a better picture of the evolution of sea level rise over various time scales.

High resolution seismic reflection PP and PS imaging of the bedrock surface below glacial deposits in Marsta, Sweden

Multi-component high resolution seismic reflection data were acquired at the Uppsala University field test site in Marsta, Sweden, in March 2019 with the aim of obtaining an improved understanding of the subsurface structure in the area. An advantage of the site is that a number of boreholes have been drilled there for both hydrogeological and geophysical purposes, allowing surface geophysics to be compared with downhole information. The presence of a low velocity layer above the water table generates significant processing challenges due to trapped waves. A further complication with the data is that the upper part of this layer was frozen at the time of the survey, resulting in the sediments just below the surface having a significantly higher velocity than those below. By analyzing and processing both the vertical and radial component data it was possible to build a velocity model that is consistent with the observed data. A strong PS converted reflection allows the bedrock to be imaged on the radial component stacks to much higher resolution than on the vertical component stacks. Both common conversion point binning and pre-stack depth migration were used to process the radial component data. We confirmed that our processing strategy was effective by generating synthetic data that were processed in a similar manner as the real data. The PS images indicate a step in the bedrock of about 2 m, with depth to it increasing from about 15 m to 17 m close to one of the boreholes. This step is not observed on the PP stacks, due to their lower resolution.

A Case Study Using Seismic Reflection and Well Logs to Reduce and Quantify Uncertainty During a Hydrogeological Assessment

Building a representative three-dimensional conceptual model of the hydrostratigraphic units is the critical first step when undertaking a hydrogeological assessment. The construction of such conceptual model requires integrating all geological data available for reducing the uncertainty of the conceptual model. In addition, the different types of data collected do not have the same level of uncertainty, and this has to be considered during the modeling. This manuscript presents a geostatistical workflow developed to integrate high resolution seismic reflection data with geological well markers and well logs. The proposed workflow allows reducing and quantifying the uncertainty when developing the conceptual model of hydrostratigraphic units and estimating the spatial distribution of hydraulic conductivity needed to simulate groundwater flow. The study involves the field investigation and numerical modeling for understanding the hydraulic connection between a local sand and gravel esker aquifer overlying a fractured bedrock regional aquifer. Seismic reflection data recorded in 2019 and 2021 significantly modify the conceptual model originally envisioned at site by discovering unknown deep esker extensions. Seismic data also help reducing the spatial uncertainty by delimiting the lateral extents, thickness, and depth of the esker aquifer between existing boreholes. Thickness maps of major hydrostratigraphic units are computed by first interpolating the data with higher uncertainty, which are then locally deformed by successive kriging with external drift to honor data with lower uncertainty. Well logs are then combined with seismic reflection data to better represent the spatial distribution of hydrofacies within the esker aquifer, and identify subunits in the bedrock aquifer. These modifications in the conceptual model of hydrostratigraphic units and the better understanding of the vertical heterogeneity of the hydraulic conductivity significantly impact the simulation of groundwater flow, both regionally and locally at site.

Seismic Signature Analysis for Clarification of Mud Volcanoes from the New Mud Diapirs Discovered at the NE-SW Moroccan Atlantic Margin

Our aim is to clarify mud volcanoes from the new mud diapirs resulting from the seismic attributes analysis applied to the low resolution Burmah oil “1973-1974” seismic data. The latter was carried out on the Larache and Tanger-Larache offshores from the NE-SW Atlantic margin. The high resolution seismic data was essential for this evaluation. In this case, we applied seismic signature analysis to four seismic profiles from the map of the seismic data set, which includes all new mud diapirs. This helped us to classify different types of mud diapirs within the seismic profiles. And as a result, six deep mud diapirs from the Prerifaine Nappe of Morocco, a shallow mud diapir, and four seafloor-piercing mud diapirs have been observed. Furthermore, the seafloor-piercing mud diapirs show a mushroom-shaped and conical-shaped cone. As they break through the seafloor, these kinds of cones characterize mud volcanoes. In this case, we may conclude that the resultant seafloor-piercing mud diapirs are likely to be mud volcanoes. However, more geological sampling and seafloor observation are still required.

Paleolimnological reconstructions for Lake Arakhley (Central Transbaikalia, Russia) inferred from high-resolution reflection seismic data

Abstract. High-resolution seismic data (65 km profiles) have been used to study the sedimentary infill of Ivan-Arakhley lake system (Lakes Arakhley, Ivan and Shakshinskoe). It found that full limnological cycle bears only sediment cover of Lake Arakhley. Total sediment infill of Lake Arakhley is 6-7 m, and it was be divided into three sequences. The uppermost ~2.5 m of sediment cover was presented by a normal lacustrine filling. Lacustrine sediments began to form ca. 21 cal. ka BP. The lake was almost dried during the regression occurred between ca. 3 and 2.6 cal. ka BP.

A Novel Multichannel Seismic Deconvolution Method via Structure-Oriented Regularization

Seismic deconvolution is an effective approach to improve the resolution of seismic data and plays an important role in migration imaging, reservoir prediction and other fields. However, conventional deconvolution methods are usually based on sparse-type regularization (e.g., <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$L_{1}$ </tex-math></inline-formula> -norm) and adopt a trace-by-trace inversion strategy to reconstruct the subsurface reflectivity series. Although such methods can improve the resolution of seismic records to a certain extent, the lack of spatial constraint will result in poor spatial continuity in the reconstructed reflectivity. This phenomenon is particularly obvious in regions with complicated geologic structures. For the purpose of overcoming this issue, we have developed a structure-oriented regularization-based multichannel sparse spike deconvolution (SOR-based MSSD) method. This method imposes <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$L_{1}$ </tex-math></inline-formula> -norm regularization on the reflectivity to obtain the high-resolution subsurface reflectivity series and imposes structure-oriented regularization (SOR) on the expected high-resolution seismic data to improve the spatial continuity of the inversion result. First, we construct SOR term based on the local structural orientations which can be estimated from the poststack seismic data. Then, we integrate the seismic data misfit term, the <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$L_{1}$ </tex-math></inline-formula> -norm constraint term, and the SOR term to formulate the objective function. At last, we use the alternating direction method of multipliers (ADMMs) to efficiently solve the objective function. We compare the SOR-based MSSD with existing methods by using synthetic and field data. Both deconvolution examples illustrate the performance of proposed method in terms of improving the spatial continuity.

ARCHITECTURAL COMPLEXITIES AND MORPHOLOGICAL VARIATIONS OF THE SEDIMENT WAVES OF PLIO-PLEISTOCENE CHANNEL LEVEE BACKSLOPE OF THE INDUS FAN

The architecture of the turbidity current sediment waves exhibits intricate morphologies and patterns on the Indus Fan channel levee backslope. The sediment waves are present on the channel levee of Plio-Pleistocene age and are absent in the deeper sections of the study area. The architecture of channel levee backslope on the Indus Fan is poorly understood. We used seismic interpretation techniques and modelling by utilizing high-resolution seismic data to approach this problem. The morphological variations in wavelength, crest dimensions and potential wave formation patterns suggest the autogenic and allogenic processes associated with wave development. Wavelengths reach up to 1473 m with an average of 486.84 m and the height of the levee ranges between 10 m and 60 m (average 30 m). The angle of the channel levee and dimension of the sediment wave here are independent of each other. Low angle levees have accommodated high dimension sediment waves and vice versa at multiple points downslope. Characteristically, the waves have formed on the outer levee (usually left) of the channels marked by steep margins suggesting that flow overspill caused the development of the waves. Generally, the younger sediment waves followed the patterns of older sediment waves, but the varying trends are often observed in the study area. The patterns of the sediment waves towards the younger sections of the levee indicate the modified and varying architectural style of growth. Sediment waves are generated by downslope turbidity currents. However, the deformation features have also possibly triggered the development of sediment waves.

Seismic attenuation compensation with spectral-shaping regularization

Because of the viscoelasticity of the subsurface medium, seismic waves will inherently attenuate during propagation, which lowers the resolution of the acquired seismic records. Inverse-Q filtering, as a typical approach to compensating for seismic attenuation, can efficiently recover high-resolution seismic data from attenuation. Whereas most efforts are focused on compensating for high-frequency energy and improving the stability of amplitude compensation by inverse-Q filtering, low-frequency leakage may occur as the high-frequency component is boosted. In this article, we propose a compensation scheme that promotes the preservation of low-frequency energy in the seismic data. We constructed an adaptive shaping operator based on spectral-shaping regularization by tailoring the frequency spectra of the seismic data. We then performed inverse-Q filtering in an inversion scheme. This data-driven shaping operator can regularize and balance the spectral-energy distribution for the compensated records and can maintain the low-frequency ratio by constraining the overcompensation for high-frequency energy. Synthetic tests and applications on prestack common-reflection-point gathers indicated that the proposed method can preserve the relative energy of low-frequency components while fulfilling stable high-frequency compensation.

Reflection Coefficients Inversion Based on the Bidirectional Long Short-Term Memory Network

Improving the vertical resolution of seismic data to satisfy the demands for detailed characterization of reservoirs is an important part in seismic data processing. The sparse spike inversion(SSI) technique greatly enhances the seismic resolution by assuming that the reflection coefficients follow the sparse distributions. However, its characterization for the spatial structure of thin and thin interbed reservoirs is still limited. In this letter, we propose a novel approach for enhancing seismic resolution using a bidirectional long short-term memory(BiLSTM) neural network by extracting reflection coefficients directly from seismic data. By designing the network architectures, multiple seismic samples map to the specific reflection coefficient. Compared with the SSI method, the BiLSTM neural network provides higher resolution inversion results on model data, which demonstrates the effectiveness of the proposed approach for thin structure characterizations. The convergence speed of the proposed method is fast and the training process is stable. In addition, we conduct the high-resolution seismic data processing on the field data based on the transfer learning technology. High-resolution processing results illustrate the generalization ability and adaptability of the BiLSTM neural network.

Dissociation of gas hydrates by hydrocarbon migration and accumulation-derived slope failures: An example from the South China Sea

The mechanism of slope failure associated with overpressure that is caused by hydrocarbon migration and accumulation remains unclear. High-resolution seismic data and gas hydrate drilling data collected from the Shenhu gas hydrate field (site SH5) offer a valuable opportunity to study the relations between submarine slope failure and hydrocarbon accumulation and flow that is associated with a ∼2 km-diameter gas chimney developed beneath site SH5 where none gas hydrates had been recovered by drilling and sampling despite the presence of distinct bottom simulating reflectors (BSRs) and favorable gas hydrate indication. The mechanism of submarine slope failure resulted from buoyancy extrusion and seepage-derived deformation which were caused by overpressure from a ∼1100 m-high gas column in a gas chimney was studied via numerical simulation. The ∼9.55 MPa overpressure caused by hydrocarbons that migrated through the gas chimney and then accumulated beneath subsurface gas hydrate-bearing impermeable sediments. This may have resulted in a submarine slope failure, which disequilibrated the gas hydrate-bearing zone and completely decomposed the gas hydrate once precipitated at site SH5. Before the gas hydrate decomposition, the largely impermeable sediments overlying the gas chimney may have undergone a major upward deformation due to the buoyancy extrusion of the overpressure in the gas chimney, and slope failure was initiated from plastic strain of the sediments and reduced internal strength. Slope failure subsequently resulted in partial gas hydrate decomposition and sediment permeability increase. The pressurized gas in the gas chimney may have diffused into the overlying sediments controlled by seepage-derived deformation, causing an effective stress reduction at the base of the sediments and significant plastic deformation. This may have formed a new cycle of submarine slope failure and finally the total gas hydrate dissociation. The modeling results of buoyancy extrusion and seepage-derived deformation of the overpressure in the gas chimney would provide new understanding in the development of submarine slope failure and the link between slope failure and gas hydrate accumulation and dissociation.

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.

Sedimentary processes in the Discovery Gap (Central–NE Atlantic): An example of a deep marine gateway

Paleoceanographic studies of abyssal bottom currents are often complicated by low current speeds and sedimentation rates, resulting in sediment condensation or erosion. However, increased rates of erosion and deposition may occur where bottom current velocities change as they pass through deep marine gaps and gateways. Despite this, the depositional processes in these gateways and their paleoceanographic implications remain poorly understood. Based on new sedimentological, hydrological and geophysical (high resolution seismic and bathymetry) data from Discovery Gap (Azores–Gibraltar Fracture Zone) collected during the 43rd cruise of the R/V Akademik Nikolaj Strakhov in 2019, the key sedimentary processes occurring in the Late Quaternary have been determined. Two depressions with depths exceeding 5300 m in the centre and south of Discovery Gap have been identified, the latter filled with contouritic deposits. These depressions are separated by a roughly N–S trending central sill at 4860 m and a sediment filled terrace at 4720 m water depth. Elongated NE–SW trending highs and sills, are present in the north and south of the study area. Their importance in controlling the flow of water through Discovery Gap is determined by the presence of erosion at the base of these highs with adjacent sheeted or mounded contourite drifts. Pelagic, hemipelagic, reworked pelagic/hemipelagic and fine-grained contourite sedimentary facies have been identified. The sedimentary facies associations point to remarkable variability in the Antarctic Bottom Water (AABW), linked to glacial–interglacial changes, and its intermittent influence in Discovery Gap during the Quaternary. During glacial intervals (MIS 6, 4 and 2) and at their terminations there was enhanced bottom current activity coeval with higher terrigenous content, and increased carbonate dissolution. The results of this study improve our understanding of sedimentary processes in abyssal environments and highlight the value of the sedimentary record in deep marine gateways for interpreting the interaction of bottom water with abyssal morphology. Future work in other modern deep gaps is essential to shed more light on how deep gaps form and to fully reconstruct deep-water paleocirculation within oceanic basins.

Shallow seismic characteristics and distribution of gas in lacustrine sediments at Lake Erçek, Eastern Anatolia, Turkey, from high-resolution seismic data

The high-resolution analysis of single-channel, seismic reflection data from Lake Erçek (Eastern Anatolia) revealed a wide range of shallow gas anomalies consisting of enhanced reflections, seismic chimneys, acoustic blanking/acoustic turbidity, strong reflectors, and pockmarks, including both surface and buried pockmarks. The enhanced reflections are represented by the higher amplitude reflection patterns resulting from high acoustic impedance variations. They are mostly clustered in the NW-corner of the lake. Seismic chimneys are represented by vertical and thinned columnar disturbances of amplitude blanking and mostly occurred in deep basinal and faulted sections in the West and East of the lake. Some seismic chimneys, occurring together with pockmarks, represent vertical vent activations. Acoustic gas masking was represented by chaotic and diffuse seismic reflection patterns, including acoustic blanking and acoustic turbidity. As diffuse acoustic turbidity indicates gas-charged sediments, columnar disturbances showing acoustic blanking indicate degassing of the sediments. These features extend from SE to NW, coinciding with the deep basin morphology of the lake. A very local strong reflector was identified in the W-section of the lake, simulating the lake floor. This reflector is due to extended enhanced reflections, suggesting shallow free gas. Pockmarks observed in the lake are structurally classified into the two distinct types; surface (active) pockmarks found in the SE-part of the lake and buried (passive) pockmarks found in the NW. The former enlarge through deeper gas reservoir feedback, as the layering is impermeable, while the latter have resulted from a cessation of the reservoir feedback mechanism and/or permeable layering. In the lake, shallow gas distribution is controlled by faults, that provide the faulting-driven depositional control and earthquakes, that provide the seismicity-driven overpressure control. The shallow gas is then vertically–horizontally distributed and shaped by asymmetric depositional–stratigraphic factors. This study of Lake Erçek presents complementary information about a possible tectono-thermal origin of observed shallow gas.

Gas Hydrate Accumulation and Occurrence Associated with Cold Seep Systems in the Northern South China Sea: An Overview

Studying deep-water cold seep systems is of great significance to gas hydrate exploration due to their close relationship. Various cold seep systems and related gas hydrate accumulations have been discovered in the northern South China Sea in the past three decades. Based on high-resolution seismic data, subbottom profiles, in situ submergence observations, deep drilling and coring, and hydrate gas geochemical analyses, the geological and geophysical characteristics of these cold seep systems and their associated gas hydrate accumulations in the Qiongdongnan Basin, the Shenhu area, the Dongsha area, and the Taixinan Basin have been investigated. Cold seep systems are present in diverse stages of evolution and exhibit various seabed microgeomorphic, geological, and geochemical features. Active cold seep systems with a large amount of gas leakage, gas plumes, and microbial communities and inactive cold seep systems with authigenic carbonate pavements are related to the variable intensity of the gas-bearing fluid, which is usually derived from the deep strata through mud diapirs, mud volcanoes, gas chimneys, and faults. Gas hydrates are usually precipitated in cold seep vents and deeper vertical fluid migration pathways, indicating that deep gas-bearing fluid activities control the formation and accumulation of gas hydrates. The hydrocarbons collected from cold seep systems and their associated gas hydrate reservoirs are generally mixtures of biogenic gas and thermogenic gas, the origin of which is generally consistent with that of deep conventional gas. We also discuss the paragenetic relationship between the gas-bearing fluid and the seafloor morphology of cold seeps and the deep-shallow coupling of gas hydrates, cold seeps, and deep petroleum reservoirs. It is reasonable to conclude that the deep petroleum systems and gas-bearing fluid activity jointly control the development of cold seep systems and the accumulation of gas hydrates in the northern South China Sea. Therefore, the favorable areas for conventional oil and gas enrichment are also prospective areas for exploring active cold seeps and gas hydrates.

Application of Deghosting and Designature for High-Resolution Seismic Data

Application of Deghosting and Designature for High-Resolution Seismic Data