Distribution Of Bottom Simulating Reflectors Research Articles

Seismic analysis of the geological occurrence of gas hydrate in the Colombian Caribbean offshore

The bottom-simulating reflector (BSR) recognized in seismic sections represents the division between free gas and gas hydrate. This reflector has been identified in the permafrost and several petroliferous marine basins worldwide, including the Colombia offshore basins. Using high quality three-dimensional (3D) seismic data from the Sinú offshore basin (southern Caribbean), the BSR was exceptionally well-imaged within two geomorphological settings, including the Southern Caribbean Deformed Belt and the Colombia Basin, indicating possible gas hydrate presence in the study area. The BSR occurrence was mapped and categorized following existing nomenclature developed off Taiwan into three (3) categories: Basin-type, Ridge-type and Submarine Canyon-type. The BSR occurrence was dominated by the Ridge type category that was mainly found in the accretionary prism within modern and ancient anticlinal structures, followed by the Basin-type BSR found mainly at the Colombia Basin margin and a few intraslope sub-basins. The Submarine Canyon-type was found on small segments of the main drainage routes that cut through anticlinal structures. BSR distribution and characterization was also correlated with a seabed geomorphologic map, including thrusts with bathymetric expression, revealing that the BSR typology map closely resembled the N–S trend of subsurface structures in the south, as well as their NE-SW trend in the central portion of the accretionary prism. We suggest the dimensions (width and depth) of the seafloor features does influence BSR occurrence, tending to parallel major anticlines, and wider/deeper portions of intraslope sub-basins and submarine canyons, while omitting small-scale features.

Distribution and characteristics of mud diapirs, gas chimneys, and bottom simulating reflectors associated with hydrocarbon migration and gas hydrate accumulation in the Qiongdongnan Basin, northern slope of the South China Sea

AbstractA comprehensive analysis on high‐resolution seismic data and the geological characteristics of the Qiongdongnan Basin (QDNB) in the northern South China Sea reveals multiple seismic evidence indicative of hydrocarbon migration and gas hydrate accumulation, including mud diapirs, gas chimneys, bottom simulating reflectors (BSRs), acoustic blanking, enhanced reflections, V‐AMP structures, elevated stacking velocity of sediments, and seafloor expressions. Mud diapirs and gas chimneys, which are characterized by acoustic turbidity, fuzziness, and blanking reflections on seismic profiles, with a diversity of structures and scales, are widespread and concentrated in the centres of sags and the transitional zones between sags and uplifts. Five BSR regions, with a total area of approximately 11,000 km2, have been delineated in the QDNB. They have high or medium‐high continuity amplitudes with overlying acoustic blanking on seismic profiles, indicating the occurrence of gas hydrate. Distinct fluid seepage phenomena are common below the BSRs, demonstrating that abundant free gas has migrated and accumulated beneath the gas hydrate. The distribution of BSRs and the developed zones of mud diapirs and gas chimneys presents a spatial superposition feature, and the BSRs are usually located on top of or within the structural highs of the mud diapirs and gas chimneys, indicating a close relationship between them. Mud diapirs, gas chimneys, and associated faults are efficient pathways for migrating hydrocarbons, through which the mature to highly mature thermogenic gas of the Palaeogene strata and the biogenic gas of the Neogene sediments migrate to the gas hydrate stability zones and form gas hydrate. Gas hydrate samples were recovered from the gas chimney area during the remote operated vehicle (ROV) investigation in deepwater QDNB in 2015. Based on the seismic interpretation and geochemical results, we propose two models of gas hydrate accumulation in the deepwater area of the QDNB: (a) near source biogenic gas hydrate with self‐generation and self‐accumulation, and (b) distal source thermogenic gas hydrate with lower generation and upper accumulation. We believe that the areas with a fine spatial coupling configuration between the mud diapirs, gas chimneys, and BSRs are favourable exploration targets for gas hydrate in the QDNB.

The history and future trends of ocean warming‐induced gas hydrate dissociation in the SW Barents Sea

AbstractThe Barents Sea is a major part of the Arctic where the Gulf Stream mixes with the cold Arctic waters. Late Cenozoic uplift and glacial erosion have resulted in hydrocarbon leakage from reservoirs, evolution of fluid flow systems, shallow gas accumulations, and hydrate formation throughout the Barents Sea. Here we integrate seismic data observations of gas hydrate accumulations along with gas hydrate stability modeling to analyze the impact of warming ocean waters in the recent past and future (1960–2060). Seismic observations of bottom‐simulating reflectors (BSRs) indicate significant thermogenic gas input into the hydrate stability zone throughout the SW Barents Sea. The distribution of BSR is controlled primarily by fluid flow focusing features, such as gas chimneys and faults. Warming ocean bottom temperatures over the recent past and in future (1960–2060) can result in hydrate dissociation over an area covering 0.03–38% of the SW Barents Sea.

Mapping gas hydrate and fluid flow indicators and modeling gas hydrate stability zone (GHSZ) in the Ulleung Basin, East (Japan) Sea: Potential linkage between the occurrence of mass failures and gas hydrate dissociation

The Ulleung Basin, East (Japan) Sea, is well-known for the occurrence of submarine slope failures along its entire margins and associated mass-transport deposits (MTDs). Previous studies postulated that gas hydrates which broadly exist in the basin could be related with the failure process. In this study, we identified various features of slope failures on the margins, such as landslide scars, slide/slump bodies, glide planes and MTDs, from a regional multi-channel seismic dataset. Seismic indicators of gas hydrates and associated gas/fluid flow, such as the bottom-simulating reflector (BSR), seismic chimneys, pockmarks, and reflection anomalies, were re-compiled. The gas hydrate occurrence zone (GHOZ) within the slope sediments was defined from the BSR distribution. The BSR is more pronounced along the southwestern slope. Its minimal depth is about 100 m below seafloor (mbsf) at about 300 m below sea-level (mbsl). Gas/fluid flow and seepage structures were present on the seismic data as columnar acoustic-blanking zones varying in width and height from tens to hundreds of meters. They were classified into: (a) buried seismic chimneys (BSC), (b) chimneys with a mound (SCM), and (c) chimneys with a depression/pockmark (SCD) on the seafloor. Reflection anomalies, i.e., enhanced reflections below the BSR and hyperbolic reflections which could indicate the presence of gas, together with pockmarks which are not associated with seismic chimneys, and SCDs are predominant in the western-southwestern margin, while the BSR, BSCs and SCMs are widely distributed in the southern and southwestern margins. Calculation of the present-day gas-hydrate stability zone (GHSZ) shows that the base of the GHSZ (BGHSZ) pinches out at water depths ranging between 180 and 260 mbsl. The occurrence of the uppermost landslide scars which is below about 190 mbsl is close to the range of the GHSZ pinch-out. The depths of the BSR are typically greater than the depths of the BGHSZ on the basin margins which may imply that the GHOZ is not stable. Close correlation between the spatial distribution of landslides, seismic features of free gas, gas/fluid flow and expulsion and the GHSZ may suggest that excess pore-pressure caused by gas hydrate dissociation could have had a role in slope failures.

Depositional characteristics and accumulation model of gas hydrates in northern South China Sea

The South China Sea (SCS) shows favorable conditions for gas hydrate accumulation and exploration prospects. Bottom simulating reflectors (BSRs) are widely distributed in the SCS. Using seismic and sequence stratigraphy, the spatial distribution of BSRs has been determined in three sequences deposited since the Late Miocene. The features of gas hydrate accumulations in northern SCS were systematically analyzed by an integrated analysis of gas source conditions, migration pathways, heat flow values, occurrence characteristics, and depositional conditions (including depositional facies, rates of deposition, sand content, and lithological features) as well as some depositional bodies (structural slopes, slump blocks, and sediment waves). This research shows that particular geological controls are important for the presence of BSRs in the SCS, not so much the basic thermodynamic controls such as temperature, pressure and a gas source. Based on this, a typical depositional accumulation model has been established. This model summarizes the distribution of each depositional system in the continental shelf, continental slope, and continental rise, and also shows the typical elements of gas hydrate accumulations. BSRs appear to commonly occur more in slope-break zones, deep-water gravity flows, and contourites. The gas hydrate-bearing sediments in the Shenhu drilling area mostly contain silt or clay, with a silt content of about 70%. In the continental shelf, BSRs are laterally continuous, and the key to gas hydrate formation and accumulation lies in gas transportation and migration conditions. In the continental slope, a majority of the BSRs are associated with zones of steep and rough relief with long-term alternation of uplift and subsidence. Rapid sediment unloading can provide a favorable sedimentary reservoir for gas hydrates. In the continental rise, BSRs occur in the sediments of submarine fans, turbidity currents.

シーケンス層序学手法による東部南海トラフエリアのメタンハイドレート含有海底扇状地タービダイト砂岩分布解析およびＢＳＲ分布との関連の検討

Since previous research revealed that most of the methane hydrates in the eastern Nankai Trough area occur in matrix pores of turbidite sandstones, the facies distribution of turbidite sandstones may be one of the important keys to evaluate the distributions and actual volume of methane hydrates in the eastern Nankai Trough area. This paper attempts to reconstruct depositional processes of submarine-fan turbidites, and examines the relationship between turbidite facies distributions and bottom simulating reflector (BSR) occurrence as a proxy of methane hydrate using sedimentologic and sequence stratigraphic methodology. First, 2D/3D seismic survey data and well data including cores and logs were used to identify turbidite facies, seismic facies, and depositional sequences. The targeted Plio-Pleistocene Kakegawa and Ogasa Groups can be divided into 17 depositional sequences, and include six seismic facies indicating submarine-fan elements and surrounding slope to basin-floor environments. Next, facies maps for each depositional sequence unit were created by plotting all information on seismic facies, 3D seismic geomorphology, and well facies data. The obtained facies maps reveal that 11 major submarine canyons functioned as positionally fixed sediment supply systems from main land Japan, along which submarine fans were formed in the forearc basins. Submarine-fan depositional styles changed through Plio-Pleistocene from a braided channel type, through small radial fan, trough-fill fan, and muddy sheet fan types, to a channel-levee system type. Finally, the facies maps of each depositional sequence were overlaid with the BSR distribution. The overlaid maps indicate that the BSRs occur on feeder channels, distributary channels, and proximal lobes of submarine fans, suggesting that methane hydrates selectively occur in coarser grained portions of a submarine fan. Because the lower part of the Kakegawa Group is mainly composed of braided channel-type submarine fan turbidites, the lower Kakegawa horizon serves one of the major horizons bearing methane hydrates in the eastern Nankai Trough area.

Geological controls on BSR occurrences in the incipient arc-continent collision zone off southwest Taiwan

Bottom simulating reflectors (BSRs) observed on seismic sections are often considered as indicators for the existence of free gas, delineating the base of the gas hydrate stability zone. Abundant BSRs seen on seismic sections acquired off the SW coast of Taiwan indicate the likely and prevalent existence of gas hydrates in the study area. This study aims to characterize the occurrence of BSRs off SW Taiwan and to understand their relationship to topography, tectonic activity, and possible migration paths of gas-bearing fluids in this area. The tectonic setting off SW Taiwan is during the initial stage of arc-continent collision between the Luzon arc and the northeastern continental margin of the South China Sea. A series of west-vergent, imbricated folds and emergent thrusts develop in the accretionary wedge. Each fold-and-thrust sequence corresponds to an elongated submarine ridge if its crest is not buried by flat-lying sediments. By contrast, normal faulting prevails in the northeastern margin of the South China Sea. A correlation between distribution of BSRs, topography, and tectonic features can be observed. Four major occurrences of BSR types of ridge type, basin type, submarine-canyon type, and continental slope type, are recognized on the basis of the relationship of BSRs to topographic and structural features. Main characteristics of BSRs in the study area can be described as: (1) they occur mostly beneath topographic highs; (2) a discordant relationship between surfaces of the seafloor and underlying strata where BSRs are present; (3) BSRs are prevalent especially beneath the crest and flank of the upthrusting, large and inclined slope basins; and (4) in general, a series of high-amplitude dipping reflectors beneath BSRs can be found. These features indicate that gas hydrate may accumulate preferably beneath topographic ridges especially underneath four-way-dip topographic closures. This effect may exist because the buoyancy-driven, gas-bearing fluids tend to migrate upward and laterally toward structural highs and their corresponding topographic ridges. The distribution of BSRs indicates that gas hydrates occur more commonly in the accretionary wedge than in the South China continental margin. We suggest that the more widespread occurrence of gas hydrates in the accretionary wedge is due to the existence of multiple fault zones, which may help to tap more deep-seated gas-bearing fluids, in addition to the shallow biogenic gas, in this region.

The Characteristics of BSRs and Their Derived Heat Flow on the Profile 973 in the Northeastern South China Sea

AbstractProcessing of a recently acquired seismic line in the northeastern South China Sea by Project 973 has been conducted to study the character and the distribution of gas hydrate Bottom‐Simulating Reflectors (BSRs) in the Hengchun ridge. Analysis of different‐type seismic profiles shows that the distribution of BSRs can be revealed to some extents by single‐channel profile in this area, but seismic data processing plays an important role to resolve the full distribution of BSRs in this area. BSRs in the northeastern South China Sea have the typical characteristics of BSRs on worldwide continental margins: they cross sediment bed reflections, they are generally parallel to the seafloor and the associated reflections have strong amplitude and a negative polarity. The characteristics of BSRs in this area are obvious and the BSRs indicate the occurrence of gas hydratebearing sediments in the northeastern South China Sea. The depth of the base of the gas‐hydrate stability zone was calculated using the phase stability boundary curve of methane hydrate and gas hydrate with mixture gas composition and compared with the observed BSR depth. If a single gradient geothermal curve is used for the calculation, the base of the stability zone for methane hydrate or gas hydrate with a gas mixture composition does not correspond to the depth of the BSRs observed along the whole seismic profile. The geothermal gradient therefore changes significantly along the profile. The geothermal gradient and heat flow were estimated from the BSR data and the calculations show that the geothermal gradient and heat flow decrease from west to east, with the increase of the distance from the trench and the decrease of the distance to the island arc. The calculated heat flow changes from 28 to 64 mW/m2, which is basically consistent with the measured heat flow in southwestern offshore Taiwan.

Distribution and Characters of Gas Hydrate Offshore of Southwestern Taiwan

Bottom simulating reflector (BSR) is a key indicator for the presence of gas hydrate beneath the sea floor. Widely distributed BSRs have been observed in the area offshore of southwestern Taiwan where the active accretionary complex meets with the passive China continental margin. In order to better understand the distribution and characters of the gas hydrate in the region, closely spaced (1.86-km line spacing) multichannel seismic reflection surveys have been conducted in recent years under the support of the Central Geological Survey, ROC. Over 10000 km of multichannel seismic reflection profiles have been collected that cover an area of about 10000 km^2 offshore of southwestern Taiwan. BSRs can be identified along 50% of the seismic profiles that we collected. A newly compiled BSR distribution map suggests that gas hydrates are distributed both in the passive margin of the China continental slope as well as in the submarine Taiwan accretionary wedge, from water depths of 500 to over 3000 m. Gas hydrates are most concentrated underneath anticlinal ridges in the accretionary wedge, and underneath the slope ridges of the passive continental margin that were formed due to sedimentary processes. Active fluid activities are evident from various features observed on seismic reflection and chirp sonar profiles, such as mud volcanoes, gas plumes and gas charged shallow sedimentary layers. Fluid migration model has been established from a set of pseudo 3D seismic reflection data. The predicted locations of high fluid flux correlate well with those interpreted from geochemical analyses that show very high methane concentrations and very shallow sulfate-methane interfaces (SMI). This demonstrates the importance of structural control over gas hydrate emplacement. From the observed gas hydrate distribution and characters, the area offshore of southwestern Taiwan provides an ideal place to study and compare the formation and migration of gas hydrates under different tectonic settings.

Distribution of methane hydrate BSRs and its implication for the prism growth in the Nankai Trough

Detailed mapping of a bottom simulating reflector (BSR), which marks the phase transition from the methane hydrate layer above the reflector to free gas below, was conducted in the Nankai accretionary prism off Shikoku and Tokai. BSRs are widely distributed in the prism slope from the toe region to the forearc basin. BSR positions provide us with the regional heat flow variations using pressure–temperature conditions for methane hydrate stability. Estimated heat flows generally show constant values about 50 mW/m 2 shallower than the middle slope of the prism, and gradually increase seaward in the lower prism slope. Occurrences of BSRs are regarded as accumulation of free gas beneath the base of a gas hydrate stability field (BGHS) and/or concentration of methane hydrate above the BGHS. These conditions can be accomplished by updip migration of methane gas because it is unlikely that such methane concentration is completed by in situ biogenic methanogenesis within sediments including low total organic carbon. Moreover, sedimentation, uplifting, and sediment stacking by thrust faulting cause upward migration of the BGHS and migration of methane from the dissociated hydrate to new BGHS. Such recycling of methane gas may have actively occurred in accretionary prisms. In contrast, there are five regions of no BSRs: the Nankai Trough floor, prism toe, slope basin, steep slope, and deep-sea canyon. The trough floor, the prism toe and the slope basin are characterized by young sediments with low production of methane gas and sub-horizontal strata unsuitable for migration of gases and fluids. Erosion at the steep slope and the canyon causes removal of hydrated sediments and downward movement of the BGHS. BSR distribution and thermal structure estimated from BSR positions offer information about active processes occurring in accretionary prisms.

Global and local variations of interstitial sulfate gradients in deep-water, continental margin sediments: Sensitivity to underlying methane and gas hydrates

We test a hypothesis relating large pore water sulfate gradients to upward methane flux and the presence of underlying methane gas hydrate on continental rises by examining: (1) pore water geochemical data available from the global data set of Deep Sea Drilling Project–Ocean Drilling Program (DSDP–ODP) sites; (2) sulfate data from 51 coring sites located at the Carolina Rise and Blake Ridge (offshore southeastern United States); and (3) the relationship between the distribution of bottom-simulating reflectors (BSRs) and sulfate depletion patterns at the Carolina Rise–Blake Ridge (CR–BR) area. Within continental rise sediments, large sulfate gradients are correlative with marine methane gas hydrate settings (recognized by gas hydrate recovery and the presence of BSRs). This correlation is in part due to the rapid consumption of sedimentary organic matter by sulfate reduction and early microbial production of methane during burial and early diagenesis. However, detailed interstitial geochemical evidence from sediments of the CR–BR area strongly suggests that sulfate and methane co-consumption (anaerobic methane oxidation) at the sulfate–methane interface (SMI) is an important additional process in depleting interstitial sulfate, producing steep (and perhaps linear) sulfate gradients and shallow depths to the SMI. The presence of BSRs is currently the only routine technique used to identify gas hydrate localities. However, BSRs seem to represent an abrupt interface at the base of gas hydrate stability (BGHS) where methane gas bubbles occur, rather than being a direct indicator of gas hydrates in overlying sediments. Detailed comparisons between BSR distribution and geochemical data at the CR–BR show that BSRs are patchy in their occurrence, consistent with BSRs representing accumulations of free methane gas that pool within structural and stratigraphic traps near the crest and on the flanks of the Blake Ridge. In contrast, steep sulfate gradients (and proxy indicators of gas hydrate) are pervasive components of the CR–BR area, suggesting that steep sulfate gradients may be a better general indicator of gas hydrate potential. Steep sulfate gradients apparently identify large upward fluxes of methane, indicating conditions conducive to the formation of gas hydrates, given favorable pressure and temperature conditions. Global DSDP–ODP geochemical data identify many additional deep-water marine sites with large sulfate gradients that lack BSRs, perhaps suggesting the occurrence of previously unrecognized gas hydrate localities.

Seismic stratigraphy, BSR distribution, and venting of methane-rich fluids west off Paramushir and Onekotan Islands, northern Kurils

Single-channel seismic recording, water chemistry determination, and sediment sampling were carried out off the northern Kuril Islands and the southern tip of Kamchatka. A bottom simulating reflector (BSR) was mapped in detail. The shipboard analyses of bottom water samples yielded methane concentrations up to 1000 nl/1. West off Paramushir Island, venting of methane-rich fluids from the seafloor into the water column is related to unusual trajectories of the BSR which emerges towards the seafloor. The destabilisation of gas hydrates and the origin of the vent site may correspond to anomalous high heatflow caused by recent volcanic intrusions into the sediment. No venting was found in the areas off the southern tip of Kamchatka and off Onekotan Island.