Presence Of Bottom Simulating Reflector Research Articles

Identification of gas hydrates and bottom-simulating reflectors in far-offset seismic images

The presence of marine gas hydrates is routinely inferred based on the identification of bottom simulating reflectors (BSRs) in common-depth-point (CDP) seismic images. Additional seismic studies such as amplitude variation with offset (AVO) analysis can be applied for corroboration. Although confirmation is needed by drilling and sampling, seismic analysis has proven to be a cost-effective approach to identify the presence of marine gas hydrates. Single-channel far-offset seismic images are investigated for what appears to be a more reliable and cost-effective indicator for the presence of BSRs than traditional CDP processing or AVO analysis. A nontraditional approach for processing seismic data is taken to be more relevant to imaging the gas/gas hydrate contact. Instead of applying the traditional CDP seismic processing workflows from the oil industry, we more carefully review the significant amount of information existing in the data to explore how the character of the data changes as offset angle increases. Three cases from different environments are selected for detailed analysis. These include (1) stratigraphy running parallel with the ocean bottom, (2) a potential BSR, running parallel to the ocean bottom, and cross-cutting dipping reflections, and (3) a suspected thermal intrusion without a recognizable BSR. This investigation considers recently collected multichannel seismic data from the deep waters of the central Aleutian Basin beneath the Bering Sea, the preprocessing of the data sets, and the methodology for processing and display to generate single-channel seismic images. Descriptions are provided for the single-channel near- and far-offset seismic images for the example cases. Results indicate that BSRs related to marine gas hydrates, and originating due to the presence of free gas, are more easily and uniquely identifiable from single-channel displays of far-offset seismic images than from traditional CDP displays.

The role of fluid migration in the occurrence of shallow gas and gas hydrates in the south of the Pearl River Mouth Basin, South China Sea

High-amplitude anomalies (HAAs), bottom-simulating reflectors (BSRs), vertical acoustic blanking features, polygonal faults, and mass transport complexes are observed in high-resolution 3D seismic data from the south of the Pearl River Mouth Basin, South China Sea. We have focused on the role of fluid migration in the occurrence of shallow gas and gas hydrates. HAAs are interpreted to be shallow gas accumulations or gas hydrates, depending on their polarity and the presence of BSRs. Acoustic blanking features are probably caused by the presence of gas chimneys and overlying free gas due to features including chaotic and weak seismic reflections, pull-downs, and long distances of vertical extension. These gas chimneys root into the underlying late Oligocene gas reservoir. There is a close spatial relationship among the gas reservoir, gas chimneys, shallow gas accumulations, and BSRs, suggesting a genetic link. We have developed a schematic model to explain the processes of fluid migration, which supplied gas to the shallow gas accumulations and the gas hydrate system. When the gas reservoir was critically pressured, the focused fluid migration pathways may be formed or be reactivated, and gas was transported vertically along these conduits to the shallow strata. Then, there were two destinies for the migrated gas at shallow depths. One is that gas was trapped below low-permeability seals, such as mudstone or gas hydrates, forming shallow gas accumulations or free gas zones. The other one is that gas entered the gas hydrate stability zone and formed gas hydrates. In addition, this research indicates that the shallow hydrocarbon resource in this area may not be very prolific probably due to limited fluid migration.

Constraints on seismic reflections and mode conversions at bottom simulating reflectors associated with gas hydrates

Bottom simulating reflectors (BSRs) are the primary seismic signatures of subsurface gas hydrate accumulations. Inference about the presence of gas hydrates only from P wave data hampers realist assertion and the quantification of resource estimates. Owing to uncertainty about conversion of seismic waves at the BSR interface and its identification of shear wave signature in most of the conventional seismic surveys studies of BSRs as being wave conversion boundaries are relatively sparse. In view of arriving at synthetic response and validate the assumptions that BSR works as mode conversion boundary we for the first time generate seismic reflections data and identify different conversion modes in relation gas hydrate exploration. The modeling results suggest that the hydrate and free gas interface acts as principal boundary for mode conversion from compressional (P) waves into converted shear waves (PS). It also appears that wave propagation effects in the overburden significantly influences the P-wave amplitude of BSR. To validate the modeling results we analyze seismic data from the Green Canyon, offshore USA and the Kerala-Konkan (KK) basin, India. The presence of BSRs, enhanced seismic reflection below BSR, seismic chimneys and hydrate mounds are found as the key seismic indicators for the occurrence of gas hydrates. The occurrence of double bottom simulating reflector (DBSR) over Green Canyon, Gulf of Mexico, Offshore, USA is originated with the migration of free gas through discrete layers of fractured sediments.

An Analysis on Stability and Deposition Zones of Natural Gas Hydrate in Dongsha Region, North of South China Sea

We propose several physical/chemical causes to support the seismic results which find presence of Bottom Simulating Reflector (BSR) at site 1144 and site 1148 in Dongsha Region, North of South China Sea. At site 1144, according to geothermal gradient, the bottom of stability zone of conduction mode is in agreement with BSR. At site 1148, however, the stability zone of conduction mode is smaller than the natural gas presence zone predicted by the BSR. We propose three causes, that is, mixed convection and conduction thermal flow mode, multiple composition of natural gas and overpressure in deep sediment to explain the BSR presence or gas hydrate presence. Further, our numerical simulation results suggest yet another reason for the presence of BSR at site 1144 and site 1148. Because the temperatures in deep sediment calculated from the mixed convection and conduction thermal flow mode are lower than that from the single conduction mode, the bottom of gas hydrate stability zone (GHSZ) is deeper than the bottom of gas hydrate deposition zone (GHDZ) or BSR. The result indicates that occurrence zone of natural is decided by the condition that natural gas concentrate in the zone is greater than its solubility.

Velocity and AVO analysis for the investigation of gas hydrate along a profile in the western continental margin of India

The occurrence of gas hydrate has been inferred from the presence of Bottom-Simulating Reflectors (BSRs) along the western continental margin of India. In this paper, we assess the spatial and vertical distribution of gas hydrates by analyzing the interval velocities and Amplitude Versus Offset (AVO) responses obtained from multi-channel seismics (MCSs). The hydrate cements the grains of the host sediment, thereby increasing its velocity, whereas the free gas below the base of hydrate stability zone decreases the interval velocity. Conventionally, velocities are obtained from the semblance analysis on the Common Mid-Point (CMP) gathers. Here, we used wave-equation datuming to remove the effect of the water column before the velocity analysis. We show that the interval velocities obtained in this fashion are more stable than those computed from the conventional semblance analysis. The initial velocity model thus obtained is updated using the tomographic velocity analysis to account for lateral heterogeneity. The resultant interval velocity model shows large lateral velocity variations in the hydrate layer and some low velocity zones associated with free gas at the location of structural traps. The reflection from the base of the gas layer is also visible in the stacked seismic data. Vertical variation in hydrate distribution is assessed by analyzing the AVO response at selected locations. AVO analysis is carried out after applying true amplitude processing. The average amplitudes of BSRs are almost constant with offset, suggesting a fluid expulsion model for hydrate formation. In such a model, the hydrate concentrations are gradational with maxima occurring at the base of hydrate stability zone.

Gas hydrates acting as cap rock to fluid discharge in the Makran accretionary prism?

Abstract We present a numerical model of the geothermal field of the Makran accretionary prism and of the slab being subducted below it. Calculated heat flow density values for the sea floor of the abyssal plain and the shelf slope are compared with in situ measured and bottom simulating reflector (BSR)-derived heat flow density values. The result suggests a predominance of conductive heat transport within the accretionary complex. Little evidence is found to suggest that fluid flow or frictional heat modifies the observed geothermal field to any great extent. We also studied the geothermal field associated with the decay of the potential gas hydrate layers (indicated by the presence of BSRs), as gas hydrate layers are being tectonically uplifted out of the gas hydrate stability field into shallower and warmer sea water. Theoretical considerations suggest a complete disappearance of gas hydrates at a water depth of about 750 m. The observed presence of numerous gas seeps almost exclusively at water depths of less than 800 m suggests that gas hydrate layers in the Makran accretionary prism act as a very effective cap rock to upward-directed flow of fluids containing notable amounts of dissolved gas from within the prism to the sea floor.

Inferred gas hydrates and clay diapirs near the Storegga Slide on the southern edge of the Vøring Plateau, offshore Norway

This paper presents a new data set, including single-channel airgun seismic lines, OKEAN long-range side-scan sonar data and gravity cores, acquired on the edge of the Voring Plateau and in the region of the Storrega Slide. The data acquisition was part of the 8th Training Through Research (TTR-8) expedition with R.V. Professor Logachev. The acoustic profiles clearly show two laterally separated zones characterised by the presence of bottom-simulating reflectors (BSRs) at about 350 ms TWT subbottom depth. The lower BSR zone is located on the slope of the Voring Plateau in the immediate vicinity of the northern headwall of the Storegga Slide and, in some places, below the slide deposits. This zone runs parallel to the general trend of the continental slope. The spatial distribution of the upper BSR zone, located upslope in an area where diapir-like structures are found, does not demonstrate any topographic control. Interpretation of high-backscatter patches on the OKEAN sonographs associates the observed structures with fluid escape features on the seabed. Most of them are pockmarks, but in a few places, diapirs are cropping out and form dome-like elevations. After analysing the behaviour of the BSR (sometimes crosscutting) and its acoustic characteristics (reversed polarity), and after applying seismic inversion processing to estimate the acoustic velocity change across the BSR, this reflector is interpreted to represent the base of the local gas hydrate stability field (GHSF). This information was used to derive the regional geothermal gradient. Spatial variations of the inferred geothermal field appeared to be negligible. The observation of two separated BSR zones suggests different environmental controls for the growth of the hydrates. Enhanced reflectors observed in the intermediate zone can be explained by the presence of strata-bound free gas accumulations and migration combined with overlying permeability barriers. Therefore, a model for gas hydrate formation in relation to directional fluid migration processes, fluid escape features and the presence of diapiric structures is proposed in this paper. Our model is supported by sedimentological analyses of seafloor samples, which demonstrate the presence of stiff clays with evidence of gas migration, and by paleontological studies of the cores retrieved from the pockmarks. The presence of a BSR in the sedimentary section within the slide scar area implies the repositioning of hydrates to newly established equilibrium conditions after the slide event. This observation allows us to simply estimate an upper limit of the fluid migration velocity in the order of several cm/year. Finally, the effect of hydrate dissociation on slope instability is considered. Assuming that the in situ decomposition of hydrates due to instantaneous depressurisation is slow enough to permit the excess volume of released gas and water to be re-distributed through the whole sedimentary section above the paleo-BSR (with consuming heat and increasing pore pressure, dissociation tends to shift the PT-conditions back to equilibrium values), it appears that the dissociation of hydrates due to sliding would cause only 0.2% increase of the pore pressure, which would hardly contribute to further slope instability.

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.