Bottom Simulating Reflector Research Articles

Semi-automated analysis of faults with meter-scale displacements and networks of planar features using high-resolution 3D seismic to investigate gas leakage at the Vestnesa Ridge, eastern Fram Strait

We develop a semiautomated method for extracting faults and other planar features from two high-resolution (HR) seismic volumes in the eastern Fram Strait. We assess seismic data quality and resolution prior to structural interpretation to determine the appropriate processing workflow. This workflow, designed for HR data sets, identifies meter-scale (>3–4 m) faults and other planar features, picked up by subtle changes in the curvature and discontinuities of seismic reflections, to investigate their relationship with fluid migration in the shallow subsurface. Although evidence exists for Holocene (approximately <8 ka) fault-associated gas seepage, structures in both volumes are presently sealing. We assess structures based on those that have measurable throw (faults) and those that do not (planar features). Structures lacking measurable throw are significant because they appear to represent fluid flow pathways. This technique enhances the analysis of the geometries of the mapped structures, facilitating the interpretation of meter-scale fault throws and the orientations of planar features. We provide insights into the evolution of structures previously linked to episodes of fluid migration and gas leakage at the Vestnesa Ridge. We infer zones of dilation (i.e., opening) at locations where faults or networks of planar features interact and investigate how these structures influence localized uplift and shallowing of the interface between free gas and gas hydrate (i.e., the bottom-simulating reflection).

Varying free-gas zone (FGZ) and bottom-simulating reflection (BSR) response across a deep graben off Trujillo, central Peru

The complexity of marine gas hydrate systems at the Peruvian convergent margin has been linked to the post-Miocene history of vertical tectonics and subduction erosion that affected the forearc. Here, multichannel seismic data and published findings reveal that such a complexity has been further extended by the occurrence of the pre-Miocene deep Morsa Norte Graben (MNG) off Trujillo (8°–9° S) in the Central Peru margin. At water depths of 650–750 m in the upper slope, directly above the MNG depocentre (which has a 4–6 km overburden thickness), continuous bottom-simulating reflections (BSRs) and concentrated sub-BSR high-amplitude reflections are confined beneath a layered basin with a low–moderate near-seafloor heat flow (7–33 mW m −2 ). A deeper BSR modelled with a thermogenic gas composition is associated with the enhanced reflections. At a water depth of 900 m, sub-BSR reflections become less frequent in an area with a layered sediment cover defined by a moderate near-seafloor heat flow (15–33 mW m −2 ). At a water depth of 1200 m, where the MNG is relatively thick (3–4 km overburden thickness), faults connect patchy BSRs with a moderate–high near-seafloor heat flow (52–110 mW m −2 ). There, sub-BSR enhanced reflections are scarce. Immediately above the top of the gas hydrate stability zone (GHSZ), the near-seafloor heat flow reaches 81 mW m −2 . Modelling suggests a water depth-dependent transport of heat towards the seafloor with respect to the top of the GHSZ, implying that the closer the seafloor is to the top of the GHSZ, the lower the advection of heat, and vice versa. Recent seafloor-related depositional and structural features amplify such relations in agreement with near-seafloor heat-flow variability. However, towards the area outside the extent of the MNG (<3 km overburden thickness), continuous BSRs are not linked either to a deeper BSR or to sub-BSR enhanced reflections. The continuity of one of these BSRs is deflected upwards beneath a slump, suggesting an incomplete thermal re-equilibration of the GHSZ. Therefore, we conclude that the BSR responds to: (1) the confinement and thickening of the free-gas zone (FGZ) above the MNG depocentre due to the sealing effect of recent sedimentation close to the top of the GHSZ; (2) the seepage of gas-rich fluids from a thinned FGZ above the relatively thick MNG, due to the enabling effect of faults cutting the GHSZ far from the top of the GHSZ; (3) the undisturbed state of the FGZ outside the extent of the MNG; and (4) the disequilibrium state of the base of the GHSZ due to the unloading of sediments in an unstable slope environment prone to failure.

Shallow subsurface fluid dynamics in the Malvinas Basin (SW Atlantic): A geoacoustic analysis

Using a database consisting of sub-bottom profiles, 2D and 3D seismic data, a series of seep structures and acoustic anomalies associated with a plumbing system in the subsurface of the southern part of the Malvinas Basin off the Argentine coast are presented. The seep morphologies consist of mounds, pockmarks, and a carbonate mound, derived from hydrocarbons migrating from the Early Cretaceous Springhill and Lower Inoceramus formations through extensional faults and overthrusts of the Malvinas Fold-Thrust Belt and accumulating in the gas hydrate stability zone. This configuration is widely observed in the anticlines of the Malvinas Fold-Thrust Belt, which are characterized by a vertical structure highlighting a Bottom Simulating Reflector (BSR) overlain by a blanked-out zone at the seabed, interpreted as a gas hydrate stability zone. The plumbing system is influenced by active transtensional tectonics, as shown by two sets of extensional faults concentrated over some anticlines. These faults lead to displacements that, in many cases, reach the seabed. One nuance of the influence of tectonic activity on fluid escape is most evident in the Malvinas anticline, where all the pockmarks and the carbonate mound are concentrated. In the western part of the Malvinas anticline, five pockmarks are observed in an area characterized by a shallow BSR. In contrast, in the eastern part of the Malvinas Basin, one pockmark and one carbonate mound are derived from a series of extensional faults. The presented data, as well as a comparison between the western and eastern parts of the Malvinas anticline, indicate that the anticlines are the main cause of seepage in this area, as they allow the accumulation of migrating fluids. Likewise, the seepage morphologies outside the anticlines are much less pronounced, as can be observed in the mounds that overlie the bottom vents.

Sedimentation-controlled double BSRs and implications for paleo hydrate dissociation in the Shenhu area, south China sea

Double/multiple bottom-simulating reflectors (BSRs) have been discovered worldwide, but their preservation and formation mechanism remain unclear because most studies are based on seismic data, lacking direct evidence like well logging data. This study introduces a novel well log-based interpretation of double BSR, suggesting that the secondary BSR (BSR2) is the interface between little residual gas and the overlying water-saturated sediment, and the free gas is trapped by capillary seals. The BSR2's amplitude is influenced by the nearby porosity, with lower porosity resulting in stronger reflections. Seismic attributes reveal a dynamic process of hydrate cycle that hydrate dissociation occurs above BSR2, followed by gas migrate to form a new BSR1. The upshift of BSR2 is mainly controlled by sedimentation rate, which varies across different regions, leading to larger upshifts in areas with higher sedimentation rates. Moreover, BSR2's upshift is negatively related to the reflection magnitude of BSR2, because rapidly depositing sediments with higher porosity and lower percolation threshold allow lesser gas sealed beneath the BSR2. Our findings suggest that double BSRs are predominantly formed in muddy sediments under rapid geological events like rapid sedimentation. However, each BSR can be preserved as well as observed to indicate that the BSR has existed for an extended period of time during which the geologic environment has been relatively stable, e.g., low sedimentation rates. This detailed sedimentation-controlled BSR adjustment provides new insights for the interpretation of double BSR and the paleo hydrate dissociation.

MASS-WASTING INDUCED SHIFTS OF THE BASE OF THE GAS HYDRATE STABILITY ZONE: EXAMPLES FROM THE OFFSHORE NIGER DELTA BASIN, NIGERIA

Mass-wasting episodes lead to significant changes in the pressure and temperature regimes within shallow marine sediments and induce variations in the thickness of the gas hydrate stability zone. These variations are marked by vertical migrations of bottom-simulating reflections that typically mark the base of the hydrate zone. Using exploration seismic data from the Offshore Niger Delta, this study presents two examples involving the vertical migration of the base of the gas hydrate stability zones induced by mass-wasting events. The first is related to a slope failure surface along the forelimb of an active thrust-cored anticline with considerable bathymetric relief. Sediment removal has induced both a decrease in overburden pressure and an incursion of cooler temperature flux in strata in the anticline and led to a migration of the base of the gas hydrate stability zone to deeper levels. The second case is related to a recent erosional event that has led to sediment removal along a seafloor channel course. Evidence of ongoing migration of the base of the gas hydrate stability zone to deeper levels is provided by the lateral termination of a BSR along channel wall, its absence beneath the channel bottom and its apparent downward bend close to the channel fringes. Apart from unroofing hydrate systems and releasing free gas into the ocean and possibly the atmosphere, mass-wasting events on the seafloor, represent significant drivers influencing the formation of gas hydrates in shallow subsea sediments and the thickness and depths of the gas hydrate stability zone.

Preliminary analysis of the indicators and accumulation models of gas hydrates in the central Zhongjiannan Basin, western South China sea

The Zhongjiannan Basin is located in the western continental margin of the South China Sea (SCS). It was recently reported that there are geophysical indicators of gas hydrates in the northern Zhongjiannan Basin. However, due to the lack of in-depth studies, it remains unknown whether geophysical indicators of gas hydrates are also available in the central and southern Zhongjiannan Basin. Based on the latest geophysical data, we have identified the seismic anomaly features of gas hydrates and observed an obvious bottom simulating reflector (BSR) in the seismic section in the central Zhongjiannan Basin. Based on a comprehensive analysis of seismic characteristics, geological structures (e.g., faults, diapirs and pockmarks) and regional sedimentary conditions, we propose three accumulation models of gas hydrates in the central Zhongjiannan Basin. The first model is that gas develops in deep depression areas, migrates upwards from deep source rocks along vertical faults and forms reservoirs in shallow formations. The second model is that gas also forms in deep depression areas and rises along mud diapirs or gas chimneys from source rocks to shallow formations, with reservoirs formed above diapirs and/or mud volcanoes. The third model is that gas is mainly generated in slope areas where strata are relatively thin and fault structures are well developed; gas migrates upwards from source rocks along vertical faults, and reservoirs are formed beneath pockmarks. The geophysical indicators of gas hydrates are found for the first time in the central Zhongjiannan Basin of the SCS, yielding an important reference for further exploration and development.

Dynamic accumulation of a high-grade gas hydrate system: insights from the trial production gas hydrate reservoir in the Shenhu area, northern South China Sea

The ultimate enrichment level and quantity of gas hydrate resources are influenced by the dynamic process of accumulation and preservation. High-resolution 3-D seismic data, logging while drilling (LWD), pressured coring, and in situ testing were used to characterize the dynamic accumulation and preservation of the trial production high-grade gas hydrate reservoir (HGGHR) in the Shenhu area. Through seismic variance analysis and ant-tracking, we found that newly identified mud diapir-associated faults with three development stages controlled the migration and accumulation of gas hydrate and shifted the base of the gas hydrate stability zone (BGHSZ), resulting in dynamic accumulation and dissociation of gas hydrates. The recognized double bottom simulating reflectors (BSRs) were concluded to have been formed due to the shift of the BGHSZ caused by the variational equilibrium conditions. The interval between the double BSRs was inferred to be a disequilibrium zone where gas recycling occurred, contributing to the coexistence of gas hydrates and free gas and the dynamic formation of the HGGHR. Multiple gliding faults formed within the GHSZ in the late period have altered the HGGHR and control the present thickness and distribution of the gas hydrates and free gas in the hanging wall and footwall. Under the influence of geothermal fluids and the fault system associated with the mud diapir, the HGGHR experienced dynamic accumulation with three stages, including early accumulation, medium-term adjustment, and late alteration and preservation. We conclude that four factors affected the formation, distribution, and occurrence of the HGGHR: the geothermal fluids accompanying the deep mud diapir below the reservoir, the dual supply of thermogenic gas and biogenic gas, the recycling of hydrate gas beneath the BGHSZ, and the post-gas hydrate faults developed within the GHSZ. A geological model illustrating the dynamic formation of the trial production HGGHR was proposed, providing a reference for future exploration of HGGHRs with a great production potential in deepwater settings.

Slow Dynamics of Hydrate Systems Revealed by a Double BSR

AbstractDetermining how gas hydrate distribution evolved along continental margins in the past is essential to understanding its evolution in the future. Moreover, hydrate decomposition has been linked to several catastrophic events, including some of the largest submarine landslides on Earth and the massive release of greenhouse gases into the ocean. Offshore Romania, the presence of a second bottom‐simulating reflector (BSR) provides an opportunity to gain valuable insights into hydrate dynamics since the Last Glacial Period (LGP). We conducted transient modeling of hydrate thermodynamic stability by merging in‐situ observations with indirect assessments of sea‐bottom temperature, thermal conductivity, salinity, sedimentation rate, and sea‐level variations. We reveal a strong correlation between the BSRs and the base of the Gas Hydrate Stability Zone (GHSZ) during both the present and LGP periods. The gradual evolution of the GHSZ over the past 34 ka presented here supports a conceptual model that excludes catastrophic environmental scenarios.

Numerical simulation of enhanced gas recovery from methane hydrate using the heat of CO2 hydrate formation under specific geological conditions

Methane hydrate (MH) is found under the seabed in Japan's exclusive economic zone and is attracting attention as a domestic natural gas resource. One of the methods for extracting methane gas (MG) from sub-seabed MH reservoir is depressurisation. However, the production decelerates after a certain time partly due to the cooling of the reservoir caused by the heat absorption during MH dissociation. Therefore, several enhanced gas recovery (EGR) methods have been proposed, such as injecting CO2 into a sub-seabed sand layer to form CO2 hydrate (CDH) and, using the heat of CDH formation. This method is two functions: EGR and the reduction of greenhouse gas emission. In this study, a numerical method was developed to investigate the performance of the EGR from MH, using the heat of CDH formation. In our previous study, CDH formation model had been installed into a programme code for two-phase flow of CO2 and water in porous media. Based on this code, we newly implemented the state equation, enthalpy model, viscosity model, and heat conduction model of MG and the dissociation model of MH. Then, simulations were performed to form CDH in specific sand-mud alternate layers and to dissociate MH. As a result, it was suggested that increasing the CO2 injection amount potentially increases MH dissociation and the vertical distance between the mud layer capping the injected liquid CO2 and the bottom-simulating reflector of CDH is important to ensure a large CDH region to generate a large amount of heat. It was also shown that even after stopping CO2 injection, MH dissociation can continue for a certain period.

Bottom simulating reflections across the northern Gulf of Mexico slope

Bottom simulating reflections (BSRs) are widely used indicators for natural gas hydrate in reflection seismic data. BSRs, however, are typically studied on a local scale at a single site and less frequently on a large, regional scale. Herein, we map and describe BSRs over a 68,000 km2 area across the northern Gulf of Mexico slope, using publicly released industry-acquired 3D seismic data. We map 74 BSR zones that cover a total area of 2060 km2. To understand the characteristics of these BSR zones, we classify them based on their BSR characteristics in the seismic data (continuous, discontinuous, and clustered) and the geological settings in which they appear (structural, stratigraphic, and venting). The most common type of BSRs are discontinuous BSRs; such BSRs were mapped over ∼60% of the total BSR area. Discontinuous BSRs occur most commonly in structural or stratigraphic settings. Discontinuous BSRs indicate that gas and hydrate are likely concentrated in discrete layers crossing the base of the hydrate stability zone (HSZ). Continuous BSRs occur in 27% of the total BSR area and they primarily occur in venting and stratigraphic settings. At vent sites, hydrate formation likely reduces sediment permeability and focuses fluid flow laterally underneath the base of the HSZ, likely resulting in a continuous BSR along the area of lateral gas flow. Clustered BSRs, chaotic sequences of high amplitude reflections, occur in only 13% of the total BSR area and are only found within structural settings. In addition, the majority of clustered BSR zones are close to paleochannels, suggesting that clustered BSRs may be more common where coarser-grained sediments exist.

The Spatial Coupling of Fluid Pathways with Gas Hydrates and Shallow Gas Reservoirs: A Case Study in the Qiongdongnan Basin, South China Sea

Shallow gas reservoirs play a crucial role in the gas hydrate system. However, the factors influencing their distribution and their relationship with the gas hydrate system remain poorly understood. In this study, we utilize three-dimensional seismic data to show the fluid pathways and shallow gas reservoirs within the gas hydrate system in the Qiongdongnan Basin. From the deep to the shallow sections, four types of fluid pathways, including tectonic faults, polygonal faults, gas chimneys, and gas conduits, are accurately identified, indicating the strong spatial interconnection among them. The gas pipes are consistently found above the gas chimneys, which act as concentrated pathways for thermogenic gases from the deep sections to the shallow sections. Importantly, the distribution of the gas chimneys closely corresponds to the distribution of the Bottom Simulating Reflector (BSR) in the gas hydrate system. The distribution of the shallow gas reservoirs is significantly influenced by these fluid pathways, with four reservoirs located above tectonic faults and polygonal faults, while one reservoir is situated above a gas chimney. Furthermore, all four shallow gas reservoirs are situated below the BSR, and their distribution range exhibits minimal to no overlap with the distribution of the BSR. Our findings contribute to a better understanding of shallow gas reservoirs and the gas hydrate system, providing valuable insights for their future commercial development.

Methane gas flares in the forearc basin of the Andaman-Nicobar subduction zone

Gas hydrates deposits in the Andaman forearc basin are inferred from seismic data and confirmed by drilling/coring during the NGHP-01 expedition. We present new evidence of gas flares in the Andaman forearc basin, detected through water column image (WCI), subbottom profiling, and high-resolution seismic data acquired onboard RV Sindhu Sadhana (SSD-085) in November-December 2021. The gas flares are located over an elongated sedimentary ridge, featuring two prominent mounds (M1 and M2) with distinct geological features. Compressional tectonics induced by the Diligent fault (DLF) formed the ridge with varying slopes and elevations. Gas flares observed above the mound M1 in WCI and sub-bottom profiler data. Seafloor samples reveal carbonate rocks with visible pores, indicating gas/fluid migration or burrows. The regional seismic profile delineates three sedimentary sequences: folded and faulted strata, mass transport deposits, and horizontal-to-sub-horizontal sedimentary layers. Additionally, we observed a bottom simulating reflector (BSR), indicating potential subsurface gas hydrate deposits. Detailed high-resolution seismic data revealed complex fault systems near bathymetry mounds (M1 and M2), which may serve as pathways for vertical fluid/gas migration.

Gas hydrate in the North Carnarvon Basin, offshore Western Australia

Analysis of hydrate systems across basins is not common, as most studies are focused on smaller sites. Using petroleum industry well and seismic data, we identify natural gas hydrate accumulations across the entire North Carnarvon Basin, offshore Western Australia. Out of 120 wells, 52 wells have evidence for gas hydrate, though hydrate is distributed throughout the hydrate stability zone in low concentrations. In addition, we do not observe a connection between the presence of hydrate in wells and deeper thermogenic gas reservoirs. From 3D seismic data, we observe bottom simulating reflections (BSRs) are very rare. In addition, while faults are common across the basin, shallow bright spots, which indicate shallow free gas, are uncommon. Based on all these observations from well and seismic data, we argue that hydrate across the North Carnarvon Basin formed predominantly from in-situ gas that is microbial in nature.

Three-Dimensional Amplitude versus Offset Analysis for Gas Hydrate Identification at Woolsey Mound: Gulf of Mexico

The Gulf of Mexico Hydrates Research Consortium selected the Mississippi Canyon Lease Block 118 (MC118) as a multi-sensor, multi-discipline seafloor observatory for gas hydrate research with geochemical, geophysical, and biological methods. Woolsey Mound is a one-kilometer diameter hydrate complex where gas hydrates outcrop at the sea floor. The hydrate mound is connected to an underlying salt diapir through a network of shallow crestal faults. This research aims to identify the base of the hydrate stability zone without regionally extensive bottom simulating reflectors (BSRs). This study analyzes two collocated 3D seismic datasets collected four years apart. To identify the base of the hydrate stability zone in the absence of BSRs, shallow discontinuous bright spots were targeted. These bright spots may mark the base of the hydrate stability field in the study area. These bright spots are hypothesized to produce an amplitude versus offset (AVO) response due to the trapping of free gas beneath the gas hydrate. AVO analyses were conducted on pre-stacked 3D volume and decreasing amplitude values with an increasing offset, i.e., Class 4 AVO anomalies were observed. A comparison of a time-lapse analysis and the AVO analysis was conducted to investigate the changes in the strength of the AVO curve over time. The changes in the strength are correlated with the decrease in hydrate concentrations over time.

Deep Fault‐Controlled Fluid Flow Driving Shallow Stratigraphically Constrained Gas Hydrate Formation: Urutī Basin, Hikurangi Margin, New Zealand

AbstractThe Hikurangi Margin east of New Zealand's North Island hosts an extensive gas hydrate province with numerous gas hydrate accumulations related to the faulted structure of the accretionary wedge. One such hydrate feature occurs in a small perched upper‐slope basin known as Urutī Basin. We investigated this hydrate accumulation by combining a long‐offset seismic line (10‐km‐long receiver array) with a grid of high‐resolution seismic lines acquired with a 600‐m‐long hydrophone streamer. The long‐offset data enable quantitative velocity analysis, while the high‐resolution data constrain the three‐dimensional geometry of the hydrate accumulation. The sediments in Urutī Basin dip landward due to ongoing deformation of the accretionary wedge. These strata are clearly imaged in seismic data where they cross a distinct bottom simulating reflection (BSR) that dips counterintuitively in the opposite direction to the regional dip of the seafloor. BSR‐derived heat flow estimates reveal a distinct heat flow anomaly that coincides spatially with the upper extent of a landward‐verging thrust fault. We present a conceptual model of this gas hydrate system that highlights the roles of fault‐controlled fluid flow at depth merging into strata‐controlled fluid flow into the hydrate stability zone. The result is a layer‐constrained accumulation of concentrated gas hydrate in the dipping strata. Our study provides new insight into the interplay between deep faulting, fluid flow and gas hydrate formation within an active accretionary margin.

Seafloor morphology and potential gas hydrate distribution in the offshore Niger Delta

Bottom simulating reflectors (BSRs) and seismic pipe features have been used as proxies for defining the distribution of gas hydrate sediments in the offshore Niger Delta. This is the most extensive mapping of gas hydrate sediments in the Delta as of today. The seismic data merge comes from multiple surveys acquired with different parameters and seismic resolutions over the course of decades of oil and gas exploration in the region. Indicated gas hydrate distribution generally follows the structural fabric of the Niger Delta with BSRs occurring along the apexes of the thrust-related ridges that have bathymetric relief on the seafloor. The presence of swarms of seismic pipe features landwards of BSR locations suggests hydrates occur beyond BSR locations. The potential gas hydrates sediment acreage in offshore Niger Delta is 17600 sq-km, representing 20% of the area with a thickness of the gas hydrate stability zone reaching 440 m in the more outboard regions of the Delta. Total gas hydrates sediment coverage likely exceeds this value as BSRs become indistinguishable from sediment strata in regions of flat dips. The presence of double BSRs further suggests the presence of thermogenic gas hydrates in the region and allows to extend the thickness of the potential hydrate zone to 550 m in the outboard regions of the Delta.

Investigation of gas and gas hydrate accumulations along the continental margin of the Danube Delta (Romania and Bulgaria offshore) using seismic reflection data

In 2012, a comprehensive study of the Danube River’s submarine channels continental slope was conducted, employing multi-beam bathymetry and over 2300 km of high-resolution two- dimensional seismic reflection data. The investigation aimed to delve into the area's morphology, potential for gas hydrate presence, and the correlation between stratigraphic units and gas hydrates. Three distinct zones, revealed Bottom Simulating Reflectors (BSRs) indicating the base of gas hydrate accumulations in the seismic data. These BSR areas exhibited Type-1 reflections, characterized by continuous cuts across layers. Notably, five discrete levels of BSRs were detected, suggesting a consistent gas composition across them. The multiple BSR formations are attributed to higher sedimentation rates relative to gas hydrate dissolution rates. Mass transport deposits (MTDs) within the gas hydrate stability zone (6 in total) were identified; their highly consolidated nature could account for the absence of gas hydrates within them. Additionally, one MTD displayed elevated heat flow measurements, indicating a higher geothermal gradient, likely due to its relatively high thermal conductivity. This disparity in thermal properties explains the deeper-than-expected BSR in this specific region, as it forms at a lower temperature equilibrium level due to efficient heat conduction.

Slow response of the gas hydrate system to ridge erosion and sea-level rise: Insights from double BSRs on the southern Hikurangi margin (New Zealand)

We analyse reflection seismic profiles across the outer accretionary wedge at the convergent New Zealand Hikurangi margin. We identify several, in some case stacked, bottom simulating reflections (BSRs). We interpret these multiple BSRs to record changes in gas hydrate stability. With the aid of gas hydrate systems modelling, we identify two geological drivers that affect gas hydrate stability: (1.) rapid sedimentation in trough basins and (2.) uplift and erosion of thrust ridges. Rapid sedimentation in trough basins buries gas hydrates that formed above the former base of gas hydrate stability (BGHS). Locally, we observe a remnant BSR from this process, likely due to residual gas and possibly gas hydrate. The combined effects of uplift and erosion, in contrast, result in the preservation of a remnant BSR within the gas hydrate stability zone, whilst a new BSR forms locally at the present-day BGHS. However, the limited occurrence of double BSRs in seismic data and the model both suggest that the formation of a deeper BSR is limited by gas supply. Formation of significant gas hydrate at this deeper level only occurs in areas of focused gas migration. This slow formation of gas hydrate also has implications for the response to glacio-eustatic sea-level rise: gas hydrates are more likely to accumulate above the BGHS corresponding to the last glacial maximum, whereas only small amounts formed above the deeper present-day BGHS. Hence, future bottom water warming will, at least initially, not lead to significant methane release from dissociating gas hydrates in deep water.

Bottom simulating reflectors in the Manila Trench forearc and its implications on the occurrence of gas hydrates in the region

Gas hydrate occurrence and stability in active plate margins have been reported in various localities, with investigations focusing on its potential as an alternative energy resource or the threat it poses due to dissociation and methane release into the atmosphere. The Manila Trench forearc, proximal to active margins and probable methane-rich sediments, can be an analog for gas hydrate formation and geologic preconditions constrained by tectonics and sedimentation processes. The possible occurrence of gas hydrates in offshore western Luzon Island is reported for the first time based on bottom simulating reflectors (BSRs) in multi-channel seismic reflection datasets. Results indicate a prevalence of BSRs on the frontal wedge seaward of the North Luzon Trough (NLT) and in the West Luzon Trough (WLT) basin fill. Continuous, discontinuous, double, and pluming BSRs in the forearc encompass a total area of approximately 15,400 km2 at depths of 223–553 mbsf in the frontal wedge and 486 to 1428 mbsf within the basins. Enhanced amplitude reflectors (EARs) above and below the BSRs indicate possible gas hydrate and free gas accumulations, respectively. Gas chimneys associated with the upward migration of methane-rich fluids appear to be controlled by deep structural features. The accretion of continent-derived sediments along the northern segment of the frontal wedge promotes the formation of fluid migration structures, enhancing fluid migration and gas hydrate formation at shallower depths. Estimated geothermal gradient values for the frontal wedge range from 28 to 92 °C/km, consistent with previously reported in-situ measurements from offshore SW Taiwan. In contrast, arc-derived sediments within the NLT and west-ward fluid migration along the landward-tilted sequences limit gas hydrate accumulation within the basin. To the south, the preferential formation of gas hydrates within the WLT basin points to deep gas sources within the forearc basin sediments, with upward movement of methane-rich fluids along normal faults. Estimated geothermal gradient values in the basin are lower, ranging from 12 to 45 °C/km, reflecting lower thermal conditions. The absence of BSRs seaward of the WLT signifies widespread gas hydrate dissociation in the frontal wedge slope.

Identification of active faults and tectonic features through heat flow distribution in the Nankai Trough, Japan, based on high-resolution velocity-estimated bottom-simulating reflector depths

Estimates of heat flow can contribute to our understanding of geological structures in plate convergent zones that produce great earthquakes. We applied automated velocity analysis to obtain the accurate seismic profiles needed for precise heat flow estimates using six new seismic profiles acquired during R/V Kaimei KM18-10 voyage in 2018. We calculated heat flow values in the accretionary wedge of the Nankai Trough off the Kii Peninsula, Japan, from the positions of widespread bottom-simulating reflectors (BSRs) in seismic reflection profiles. Calculated conductive heat flow values from the depth of the BSR agree with previous studies where a regional trend is observed from ~ 50 mW/m2 to < 40 mW/m2 60 km landward from the deformation front. This trend is caused by thickening of accretionary sediments and the subduction of the Philippines Sea plate. Segments of profiles are marked by anomalous high heat flow values. Such anomalies represent alterations of the shallow crustal thermal structure caused either by a combination of topographic affects, surface erosion of the seafloor, or by fluid flow that transports heat by advection. We interpret heat flow anomalies (~ 100 mW/m2) as indicators of active faulting, which correspond to low seismic velocity zones along faults. Our results also showed relatively high heat flow at the landward end of several survey lines close to the Kii Peninsula, which we interpret to the possible presence of plutonic rocks that underlie the Kii Peninsula and extend offshore and may be the cause of geothermal springs, steep geothermal gradients, and high heat flow.Graphical abstract