Bottom Simulating Reflector Research Articles

Identification and Characterization of Gas Hydrate using Marine Seismic and Well Data: South Makassar Strait case study

Gas hydrate has the potential as an alternative unconventional resource to current oil and gas. Identification of gas hydrate is made possible by applying the seismic method with the main indicator of the bottom simulating reflector (BSR). BSR is characterized on the seismic section by the presence of high amplitudes, reverse polarity reflections, and cutting through the stratigraphy. In this study, the presence and thickness of the gas hydrate zone have been identified using five 2D seismic tracks, namely CM02-57, SM93-06 SM93-08, SM93-09 and SM93-11, and the SULTAN-1 well. The gas hydrate zone was identified at a depth of about 2300 m. The existence of the gas hydrate zone is followed by the presence of a gas chimney indicator as a free gas migration path. The gas hydrate zones are estimated to be 15 m, 28 m, 33 m, 23 m, and 30 m thick from the respective seismic tracks CM02-57, SM93-06 SM93-08, SM93-09 and SM93-11.

Characterization of a complex sand-rich gas hydrate reservoir system in the Indian marine continental margin with downhole log and seismic data

Logging-while-drilling (LWD) and coring data were acquired in Areas A, B, C and E during the National Gas Hydrate Program Expedition 02 (NGHP-02). At Sites NGHP-02-16, −17, −20, −23 and −24 of Area B in the Indian Krishna-Godavari Basin, the gas hydrate-bearing sand-rich reservoirs directly in contact with an underlying water-saturated sediment were revealed near the seismic-inferred bottom simulating reflector (BSR) along a large anticlinal structure. Lithofacies analysis of cores show that the lower gas hydrate-bearing reservoir section at these sites is an interbedded unit with thin fine-sand layers alternating with clay-rich layers. We reanalyzed the LWD data including resistivity, compressional-wave and shear-wave acoustic velocities to assess and compare the spatial variation of sand-hosted gas hydrate and possible related free-gas accumulations. The presence of free gas-bearing sediments in direct contact with gas hydrate-bearing sediments was confirmed at Sites NGHP-02-20 and −24 along the northeastern flank of the anticline. The sand-rich reservoirs associated with the Area B anticlinal feature reveal different pore-filling constituents and variable boundary conditions, including gas hydrate/water contacts and gas hydrate/free gas contacts. We infer that the sedimentary depositional history at Sites NGHP-02-20 and −24, may have resulted in an upward shift of the base of gas hydrate stability zone, that could have led to in situ gas hydrate dissociation and the accumulation of free gas and coexistence interval below the log-inferred gas hydrate-bearing reservoir sections. This study demonstrates that reservoir heterogeneity and fluid migration were major factors affecting the distribution of gas hydrate and free gas in the identified sand-rich reservoirs.

The first record of gas flares along the Peruvian convergent margin: The case of the hydrocarbon-prolific Talara Basin

During the last few decades, streams of gas bubbles released from the seafloor have been discovered around the world with hydroacoustic systems. That being said, such a discovery had not occurred along the Peruvian convergent margin. For the first time, 75 zones of gas flares were imaged with high-resolution swath bathymetry in the well-known hydrocarbon-rich Talara Basin (4°S–5°S) of the northern Peru margin. 78% of all flares occur along the shelf, while the remaining 22% is found between the shelf break and the upper slope (up to water depths of 1700 m in the study area). Almost all flares (96%) are within the permanent oxygen minimum zone (OMZ) and above the top of the gas hydrate stability zone (GHSZ). Gas flares are not observed between the top of the GHSZ and water depths of 1300 m. Beneath the gas flare-free region, the additional 4% contains one of the most impressive gas flares of 1310 m height, which crosses the top of the GHSZ for 230 m in the water column. The projection of the largest gas flare onto seismic data shows a spatial link with bottom-simulating reflections (BSRs), landward-dipping strata, and a deep half-graben. Despite assumptions and uncertainties, we suggest that a total volume gas flux of 7–45 × 106 mol CH4.yr−1 (average of 26 × 106 mol CH4.yr−1) is being released in the form of gas bubbles in the study area. From our findings, we propose that other parts of the Peru margin are likely seeping natural gas from the seafloor, with hydrocarbon-dependent deep-sea biological communities, more so than previously thought.

Geophysical Characterization and Attenuation Correction Applied for Hydrate Bearing Sediments in Japan Sea

Estimation of rock properties from seismic data is important for exploration and production activities in the petroleum industry. Considering the compressional velocity—the speed of propagating body waves in formations—and the quality factor (Q)—a measure of the frequency-selective energy losses of waves propagating through formations—both properties are usually estimated from multichannel seismic data. Velocity is estimated during multichannel processing of seismic reflection data in either the time or depth domain. In marine seismic acquisition, Q can be estimated from the following sources: Vertical Seismic Profile (VSP) surveys, where sources are located near the sea surface and geophones are distributed at depth along a borehole; and multichannel reflection data, where sources are also located near the sea surface and receivers are distributed either at the sea surface (conventional seismic survey with streamers) or on the sea floor (use of nodes or Ocean Bottom Cables (OBC)). The aforementioned acquisition devices, VSP, conventional streamers, nodes, and OBCs are much more expensive than single-channel acquisition with one receiver per shot due to the cost of operation. There are numerous old and new datasets from academia and the oil industry that have been acquired with single-channel acquisition devices. However, there is a paucity of work addressing the estimation of velocity and Q from this type of equipment. We investigate the estimation of Q and velocity from single-channel seismic data. Using the windowed discrete Fourier transform for a single seismic trace, we calculate the peak and dominant frequency that changes with time. In the geologic environment, higher frequencies are attenuated at shallow depths (time), while lower frequencies remain at deeper positions. From the rate at which higher frequencies are attenuated with time, we estimate the effective quality factor (Qeff). However, when using Kirchhoff migration to process single-channel seismic data, events far from the vertical projection of the receiver contribute to the trace at a given time. Then, an underestimation of the effective quality factor occurs. To compensate for the effects of more distant events with lower-frequency content contaminating the shorter events, we propose a linear equation to correct the effective quality factor estimated from migrated seismic data. Effective Q and its correction are estimated in five single-channel seismic lines surveyed along the Joetsu Knoll, a SW-NE anticline structure on the eastern margin of the Sea of Japan. These results are linked to geomorphological and geological features and the velocity field. Joetsu Knoll is a known site of massive gas hydrates (GH), which occur in the first hundred metres of Neogene sediments and, together with gas chimneys, play an important role in seismic wave absorption. Qeff estimated from migrated seismic data maintains the spatial relationship between high and low Q regions. The region of low Q, which is below 124 and has an average value of 57, occurs near the anticlinal hinge and tends to coincide with the region in which the Bottom Simulating Reflector (BSR) resides. The coexistence of GH and free gas coincides with the very low P velocity gradient of 0.225 s−1. BSR occurrence, Qeff and the geometry of the Joetsu anticline testify to progressive gas hydrate depletion northward along the dome.

Seismic stratigraphy and attenuation of gas-hydrate zones within Hikurangi and Gondwana margins, eastern New Zealand

SUMMARY Gas hydrates that occur on many continental margins have received global attention. In reflection seismic imaging, the bottom-simulating reflector (BSR) is a common indicator of gas hydrates. However, it is difficult to identify gas hydrates and quantify their amounts through the BSR alone. For gas-hydrate characterization, it is therefore useful to measure seismic stratigraphic and attenuation attributes. Short-scale patterns of layering that contain information about the amount and mechanism of gas hydrates can be identified through stratigraphic and attenuation attributes. We measure the complete time-variant spectra by using sparse strongest peaks, and the spectral differences at different times through attenuation parameters Q–1 and γ. The traditional Q–1 is associated with the attenuation of the frequency-dependent part of wavefield, and the γ characterizes the frequency-independent attenuation. The measurement approach is straightforward and requires no sophisticated inverse algorithm and is applied to surface seismic data acquired over the Hikurangi and Gondwana margins, eastern New Zealand. High-quality spectral and attenuation images are obtained. Spectral attributes correlate with BSRs and large positive Q–1 and negative γ-values are below and above the BSRs, which are interpreted as being related to free-gas and gas-hydrate accumulations. These results will aid the quantification of gas hydrates and the assessment of their roles as an energy resource, as a potential geological hazard, and in climate change and ocean warming.

Advanced Data Processing as a Tool to Enhance the Vertical and Horizontal Resolution of Bottom Simulating Reflector (BSR): Implications for GAS Hydrate Exploitation

Advanced Data Processing as a Tool to Enhance the Vertical and Horizontal Resolution of Bottom Simulating Reflector (BSR): Implications for GAS Hydrate Exploitation

Dating submarine landslides using the transient response of gas hydrate stability

AbstractSubmarine landslides are prevalent on the modern-day seafloor, yet an elusive problem is constraining the timing of past slope failure. We present a novel age-dating technique based on perturbations to underlying gas hydrate stability caused by slide-impacted seafloor changes. Using three-dimensional (3-D) seismic data, we mapped an irregular bottom simulating reflection (BSR) underneath a submarine landslide in the Orca Basin, Gulf of Mexico. The irregular BSR mimics the pre-slide seafloor geometry rather than the modern bathymetry. Therefore, we suggest that the gas hydrate stability zone (GHSZ) is still adjusting to the post-slide sediment temperature. We applied transient conductive heat-flow modeling to constrain the response of the GHSZ to the slope failure, which yielded a most likely age of ca. 8 ka, demonstrating that gas hydrate can respond to landslides even on multimillennial time scales. We further provide a generalized analytical solution that can be used to remotely date submarine slides in the absence of traditional dating techniques.

3D and 2D topographic correction to estimated geothermal gradient from the base of gas hydrate stability zone in the Andaman Forearc Basin

Methane gas hydrate related bottom-simulating reflectors (BSRs) are imaged based on the in-line and cross-line multi-channel seismic (MCS) data from the Andaman Forearc Basin. The depth of the BSR depends on pressure and temperature and pore water salinity. With these assumptions, the BSR depth can be used to estimate the geothermal gradient (GTG) based on the availability of in-situ temperature measurements. This calculation is done assuming a 1D conductive model based on available in-situ temperature measurement at site NGHP-01-17 in the study area. However, in the presence of seafloor topography, the conductive temperature field in the subsurface is affected by lateral refraction of heat, which focuses heat in topographic lows and away from topographic highs. The 1D estimate of GTG in the Andaman Forearc Basin has been validated by drilling results from the NGHP-01 expedition. 2D analytic modeling to estimate the effects of topography is performed earlier along selected seismic profiles in the study area. The study extended to estimate the effect of topography in 3D using a numerical model. The corrected GTG data allow us to determine GTG values free of topographic effect. The difference between the estimated GTG and values corrected for the 3D topographic effect varies up to ∼5 °C/km. These conclude that the topographic correction is relatively small compared to other uncertainties in the 1D model and that apparent GTG determined with the 1D model captures the major features, although the correction is needed prior to interpreting subtle features of the derived GTG maps.

Quantitative evaluation of gas hydrate reservoir by AVO attributes analysis based on the Brekhovskikh equation

AVO (Amplitude variation with offset) technology is widely used in gas hydrate research. BSR (Bottom simulating reflector), caused by the huge difference in wave impedance between the hydrate reservoir and the underlying free gas reservoir, is the bottom boundary mark of the hydrate reservoir. Analyzing the AVO attributes of BSR can evaluate hydrate reservoirs. However, the Zoeppritz equation which is the theoretical basis of conventional AVO technology has inherent problems: the Zoeppritz equation does not consider the influence of thin layer thickness on reflection coefficients; the approximation of the Zoeppritz equation assumes that the difference of wave impedance between the two sides of the interface is small. These assumptions are not consistent with the occurrence characteristics of natural gas hydrate. The Brekhovskikh equation, which is more suitable for thin-layer reflection coefficient calculation, is used as the theoretical basis for AVO analysis. The reflection coefficients calculated by the Brekhovskikh equation are complex numbers with phase angles. Therefore, attributes of the reflection coefficient and its phase angle changing with offset are used to analyze the hydrate reservoir's porosity, saturation, and thickness. Finally, the random forest algorithm is used to predict the reservoir porosity, hydrate saturation, and thickness of the hydrate reservoir. In the synthetic data, the inversion results based on the four attributes of the Brekhovskikh equation are better than the conventional inversion results based on the two attributes of Zoeppritz, and the thickness can be accurately predicted. The proposed method also achieves good results in the application of Blake Ridge data. According to the method proposed in this paper, the hydrate reservoir in the area has a high porosity (more than 50%), and a medium saturation (between 10% and 20%). The thickness is mainly between 200m and 300m. It is consistent with the previous results obtained by velocity analysis.

Dissolution migration of gas, a mechanism to enrich ethane near the BSR and increase upwardly C1/C2 ratios in the hydrate-occurring zones: Insight from pore-scale experimental observation

Most experiments show that gas hydrates are often enriched in C2+ gases relative to the feeding gas source because of fractionation during hydrate crystallization directly from free gases and dissolved gases. However, sediments below and near the base of the gas hydrate stability zone (BGHSZ) in many ocean drilling program (ODP)/International Ocean Discovery Program (IODP) sites are relatively enriched in C2+ hydrocarbon gases, compared with the hydrate-occurring zone above. It is still unclear what kind of process causes the abrupt decreases in C1/C2+ ratios with the depth in headspace gas in sediments around seismic bottom-simulating reflector (BSR) and increasing upward C1/C2 ratios in the hydrate-occurring zone. To test the “dissolution/migration mechanism” and its links to the enrichment of ethane near the BSR and increasing upward C1/C2 ratios in the hydrate-occurring zone, we performed a series of pore-scale experimental observations, simulating the gas dissolution–migration–hydration processes, and investigated the effects of the composition of feeding gases and temperatures on the composition of the hydrate grown under the dissolution–migration mechanism. Hydrates are grown from aqueous fluids supplied by the migration of gases dissolved from the capillary-trapped free gas in a capillary high-pressure optical cell, with different supplying gases (90 mol% CH4 + 10 mol% C2H6, 80 mol% CH4 + 20 mol% C2H6) and a geothermal gradient (temperature from 278.15 to 293.15 K). The gas hydrate structure and composition were determined by quantitative Raman spectroscopy. Our study indicated that (1) under the dissolution–migration–hydration processes, the mole fraction of C2H6 in hydrates is depleted compared with gas sources, which confirms that the dissolution–migration of gases is a mechanism to enrich ethane near the BSR; (2) the proportion of C2H6 in structure I (sI) or structure II (sII) hydrates decreases with decreasing temperature, and decreasing temperature enlarges the difference of diffusion coefficient between methane and ethane and enhances the gas fractionation during migration, which could cause the increase upwardly C1/C2 ratios in the hydrate-occurring zone. A simplified geological model was proposed to explain the variability of hydrate composition with depth in the hydrate-occurring zone and the fractionation of gases near the BSR.

Hydrocarbon Accumulation Analysis Based on Quasi-3D Seismic Data in the Turbulent Area of the Northern South China Sea

The Mesozoic strata in the northern South China Sea have good potential for oil and gas exploration. The Dongsha Waters, where the study area is located, have complex seismic and geological characteristics; in particular, turbulence is very prominent in this area, so it is difficult to implement 3D seismic data collection. The “single-source and single-cable quasi-3D seismic survey” method integrates some key technologies in acquisition and processing, thus improving the quality of seismic imaging. Based on the interpretation of the existing research results and new data, structure B-1 has good source–reservoir–cap combination conditions. The oil–gas accumulation mode is predicted, and the drilling well B-1-1 is given. In addition, the large-scale distribution of bottom-simulating reflectors (BSRs) and the discovery of gas seepage areas in the study area suggest the presence of gas hydrate. We suggest that deep thermogenic gas from the Mesozoic strata has migrated into the overlying strata along the fault system and mixed with microbial gas to form hydrate.

Reservoir Potential Assessment of the Tortonian–Calabrian Sediments from Offshore Makran (Southwest Pakistan) through a Multidisciplinary Approach

AbstractThe Tortonian–Calabrian strata of Offshore Makran (Pakistan) is investigated for the purpose of reservoir assessment. The stratigraphy and petrophysics indicate that the Neogene sediments have tight reservoir potential with porosities in the range of 3%–9% and 40%–50% water saturation. The mineralogical cross plots indicate a mixed lithology with an abundance of dolomite and calcite, together with minor quartz content and shale. The seismic interpretation demonstrates medium–high amplitude reflection patterns, mud diapirs coupled with onlapping strata and the occurrence of bottom simulating reflectors (BSRs). The BSRs are characterized by diminished amplitude, low continuity and exhibiting acoustic blanking zones. The high formation pressure results indicate overpressure zones, suggesting the occurrence of overpressured shales in the Jal Pari‐1A. The presence of mud diapirs and gas chimneys are the result of tectonic forces acting at the junction of the Arabian, Indian and Eurasian plates, whereas, BSRs prove the existence of gas charged sediments supporting the formation of mud diapirism in the region. It is concluded that the high rates of sedimentation during the Neogene are likely to have contributed to the development of the high formation pressure. Revised mud weights, casing policies, pore pressure transients and geophysical inversion studies will help alleviate drilling risks in future exploration strategies.

Gas hydrate reservoir identification based on rock physics modelling and sensitive elastic parameters

Abstract Seismic bottom simulating reflections (BSR) analysis and seismic inversion are commonly used for gas hydrate reservoir interpretation. The relationship between gas hydrate saturation and elastic parameters can be influenced by gas hydrate occurrence state (e.g. pore-filling type gas hydrate or load-bearing type gas hydrate), and this may cause inaccurate interpretation. We first used the simplified three-phase Biot equation (STPBE) to model a formation containing two types of gas hydrate at the same time. Then the effects of occurrence state and authigenic minerals on the relationship between saturation and varied elastic parameters are analysed. Results show that bulk modulus (K), shear modulus ($\mu $), P-wave velocity (${V}_p$), S-wave velocity (${V}_s$), velocity ratio (${V}_p/{V}_s$), Poisson's ratio (v) and $\mu \rho $ increase at different rates with gas hydrate saturation, ${V}_p/{V}_s$ and v show relative higher sensitivity to occurrence state. Ratios of elastic parameter increments are further used to highlight the anomalies caused by gas hydrate. Four attributes ($\Delta K/\Delta \mu $, $\Delta {V}_p/\Delta {V}_s$, $\Delta ({V}_p/{V}_s)/\Delta \nu $ and $\Delta \lambda \rho /\Delta \mu \rho $) show good sensitivity to both gas hydrate saturation and occurrence state. $\Delta ({V}_p/{V}_s)/\Delta \nu $ and $\Delta \lambda \rho /\Delta \mu \rho $ can be used to distinguish gas hydrate with varied occurrence states from authigenic minerals (limestone, opal, pyrite and others). Two selected sensitive attributes $\Delta ({V}_p/{V}_s)/\Delta \nu $ and $\Delta \lambda \rho /\Delta \mu \rho $ are applied to well logs, four gas hydrate-bearing intervals in well 2L-38 from Mallik permafrost area in Canada and one gas hydrate-bearing interval in well A from Shenhu area in South China Sea are identified. These results are consistent with the interpreted result from the resistivity log using Archie's formula. This investigation may provide effective tools for the seismic interpretation of gas hydrate reservoirs.

Evaluation of principal component analysis for seismic attribute selection and self-organizing maps for seismic facies discrimination in the presence of gas hydrates

Mapping the lateral distribution and thickness of gas hydrates and associated free gas using even the best quality seismic surveys can be quite challenging. As the stability of gas hydrates within the Gas Hydrate Stability Zone (GHSZ) is controlled by temperature and pressure, and because their impedance can be different than those of the sedimentary matrix in which they occur, we often are able to identify a bottom simulating reflector (BSR) at the base of the GHSZ cutting across stratigraphic reflectors. However, when the stratigraphic reflectors are parallel to the ocean bottom, the BSR is easily confounded with a strong stratigraphic reflector. Seismic attributes and machine learning (ML) applied to facies analysis have been successful in quantitatively mapping patterns in seismic data. However, selecting an optimal combination of seismic attributes and training data to perform a robust seismic facies classification is highly dependent on the interpreter's skills. In this study, we apply principal component analysis (PCA) to select a suite of seismic attributes to be used as input for self-organizing maps (SOM) to identify BSRs and surrounding facies in a seismic volume located in the Blake Ridge, offshore South Carolina, USA. As PCA is based on mathematically variability unrelated to geology, we evaluate PCA as an attribute selection technique under three different training data selection strategies, finding that combining seismic attributes such as total energy, coherence, GLCM entropy, and peak magnitude as input for SOM represents an effective method to detect BSRs. Additionally, we note that using a smaller subset of samples representing the target four seismic facies when applying PCA to select input attributes for SOM offers optimal BSRs identification.

Gas Hydrate Resource Potential Of Deepwater Sabah, Malaysia: A Preliminary Assessment

Offshore NW Sabah is one of the localities identified in the United States Geological Survey (USGS) global hydrates database but not much work has been done on this potential source of energy for Malaysia and the surrounding region. The presence of gas hydrates in this area is mainly inferred from bottom-simulating reflectors (BSR) identified in seismic reflection profiles across the margin. BSRs have been mapped across almost the entire length of the deepwater fold-thrust belt in the Sabah Trough where they are commonly observed within the crests of fold-thrust anticlines. Based on an average geothermal gradient of 62.5 °C/km, the thickness of the gas hydrate stability zone is predicted to vary with water depth from zero at 640 m water depth to 300 m at 2900 m water depth. The total in-place methane resource from the Sabah gas hydrates is estimated to range from 72 to 852 trillion cu. ft. (TCF) (2.06 – 24.1 x 1012 m3) with a mean of 364 TCF (10.3 x 1012 m3 ) and a most likely (P50) value of 252 TCF (7.1 x 1012 m3). These preliminary estimates may seem large but they are comparable with those from other gas hydrate deposits in the region. More work is required to refine them in order to determine how much of the in situ volume is technically and economically recoverable.

Spatial-temporal variations of the gas hydrate stability zone and hydrate accumulation models in the Dongsha region, China

It is of great significance to study the spatial-temporal variations of the thickness of the gas hydrate stability zone (GHSZ) to understand the decomposition, migration, accumulation and dissipation of gas hydrate, the corresponding relationship between bottom-simulating reflectors (BSRs) and gas hydrate, and the distribution of heterogeneous gas hydrate. We selected the Dongsha region in the South China Sea (SCS) as the research object to calculate the spatial-temporal variation of the GHSZ since 10 Ma, analyzed the main factors affecting the thickness of the GHSZ, discussed the dynamic accumulation processes of gas hydrate, and proposed an accumulation model of gas hydrate in the Dongsha region. The results show that the thicknesses of the GHSZ in the study area were between 0 m and 100 m from 10 to 5.11 Ma, and the relatively higher bottom water temperature (BWT) was the key factor leading to the thinner thickness of the GHSZ during this period. From 5.11-0 Ma, the thickness of the GHSZ gradually increased but showed several fluctuations in thickness due to changes in the geothermal gradient, seawater depth, BWT, and other factors. The decrease in the BWT was the main factor leading to GHSZ thickening from 5.11 to 0 Ma. The thicknesses of the GHSZ are between 110 m and 415 m at present. The present spatial distribution features show the following characteristics. The GHSZ in the deep canyon area is relatively thick, with thicknesses generally between 225 m and 415 m, while the GHSZ in other areas is relatively thin, with thicknesses between 110 m and 225 m. Based on the characteristics of the GHSZ, two hydrate accumulation models are proposed: a double-BSRs model due to thinning of the GHSZ and a multilayer hydrate model due to thickness changes of the GHSZ, with single or multiple BSRs.

Shallow gas and gas hydrate accumulations influenced by magmatic complexes in the Pearl River Mouth Basin, South China Sea

We analyse occurrences of gas hydrates and free gases above a 1200 km2 complex of igneous volcanoes and sills using newly acquired three-dimensional (3D) seismic data in the Pearl River Mouth Basin of the South China Sea. In strata above volcano summits, we recognize three enhanced reflection layers at different stratigraphic levels, including discontinuous Bottom Simulating Reflectors (BSRs) and pluming BSRs. We also show that the lateral tips of eight sills, which at times appear to terminate against supra-sill faults, underlie gas chimneys linked to gas hydrate and free gas in shallower strata. These supra-sill faults and gas chimneys, as well as supra-volcano faults, provided pathways for upward fluid (gas) migration from deep sources. Given the spatial relationship between volcanoes, sills, free gas, and gas hydrates, we suggest that the igneous structures focused fluid migration into overlying faults and gas chimneys. We also explore whether higher geothermal gradient at the summits of the volcanoes caused the base of gas hydrate stability zone to be shallower, forming a pluming BSR. Our study highlights how volcanic systems can influence the formation and distribution of later gas hydrate and free gas.

Neural net detection of seismic features related to gas hydrates and free gas accumulations on the northern U.S. Atlantic margin

Bottom-simulating reflections (BSRs) that sometimes mark the base of the gas hydrate stability zone in marine sediments are often identified based on the reverse polarity reflections that cut across stratigraphic layering in seismic amplitude data. On the northern U.S. Atlantic margin (USAM) between Cape Hatteras and Hudson Canyon, legacy seismic data have revealed pronounced BSRs south of the deepwater extension of Hudson Canyon and more subtle ones from offshore Delaware south to Cape Hatteras, where the reflections sometimes follow stratigraphic layering. Using high-resolution seismic data acquired during the 2018 Mid-Atlantic Resource Imaging Experiment and a supervised neural net, we identify seismic features associated with gas hydrates and/or the top of gas between Hudson Canyon and Cape Hatteras. Using seismic attributes especially sensitive to the presence of gas, we train a neural network algorithm on seismic data from an area with strong BSRs and then apply the model to the rest of the data set. The results indicate that gas hydrate and/or shallow free gas are significantly more widespread on the northern part of the USAM than previously known. Seismic indicators of gas extend landward from the 2000 m isobath to the upper continental slope in sectors with (offshore Virginia) and, to a lesser extent, without (offshore New Jersey) pervasive upper slope methane seeps. Higher sand content and intermediate sediment thickness, factors related to the container size and gas charge in a petroleum systems framework, are associated with more robust gas indicators.

Seismic evidence of gas hydrate and seafloor fluid escape on the upper Amazon deep-sea fan, Brazilian equatorial margin

The Amazon fan contains a gas hydrate province known from a bottom-simulating reflection (BSR) that lies within an upper slope compressional belt. In this study, the extent and character of the BSR and its relation to thrust-fold structures is examined using a grid of 2D and 3D seismic data. We show the BSR to comprise a series of elongate patches up to 16 km wide that are present along 300 km of the slope in water depths of 750-2250 m and extend over a total area of 6800 km2. The elongate BSR patches show a strong spatial correspondence with the arcuate crests of thrust-fold anticlines. In profile, the BSR patches exhibit convex forms and/or locally irregular relief that rises toward the seafloor. In plan, 3D seismic horizon maps reveal columnar BSR elevations up to 1 km wide, which rise beneath seafloor mounds and depressions, up to 0.5 km wide and 30 m in relief, interpreted as small mud volcanoes and possible pockmarks. The elongate BSR patches are interpreted to record the structurally-controlled rise of warm, gas-rich fluids into the crests of thrust-folds and their leakage into the gas hydrate stability zone, and in places to seafloor, through a near-surface system of faults, hydrofractures and vents.

Analysis of seismic attributes to enhance a Bottom Simulating Reflector (BSR) in the gas hydrate area of Umitaka Spur, eastern margin of Japan Sea

This work aims to evaluate the best seismic attributes to use to identify the Bottom Simulating Reflector (BSR) of Umitaka Spur, a well-known gas hydrate area in Joetsu Basin, Japan. For this purpose, it uses 2D single-channel seismic data from cruises NT07-20 and NT08-09 provided by the Japan Agency for Marine-Earth Science and Technology (JAMSTEC). The methodology used for the qualitative analysis of six seismic attributes to highlight the BSR was done using Schlumberger’s Petrel software. These attributes were Envelope, RMS Amplitude, Amplitude Volume Technique (tecVA), Relative Acoustic Impedance, Spectral Decomposition and Instantaneous Frequency. Thus, this study was fundamental to investigate gas hydrates occurrence through the seismic assessment of some geophysical property (amplitude and frequency) and the geological attributes that highlighted the faults of the complex local geology. The application of these attributes can be used in other areas providing an effective tool to enhance the recognition of BSR all over the world.