Petrographic evidence of the past occurrence of gas hydrates in the Tertiary Piedmont Basin (NW Italy)

Methane-derived rocks in Monferrato and the Tertiary Piedmont Basin (NW Italy) consist of seep carbonates, formed by gas seepage at the seafloor, and macroconcretions resulting from the cementation of buried sediments crossed by gas-rich fluids. These rocks are characterized by both negative δ¹³C values and a marked enrichment in δ¹⁸O. Petrographic features not commonly described and that point to enigmatic depositional and diagenetic conditions have been observed in both types of rocks: inhomogeneous distribution of cements within cavities; dolomite crystals floating within cavity-filling calcite spar; non-gravitational fabrics of internal sediments plastering cavity walls; open framework within microbial crusts. These features suggest the former presence of gas hydrates in sediments. During their dissociation, new space was formed and filled with authigenic carbonates or injected sediments. Analogous mechanisms of clathrate freeze-and-thaw processes have been inferred for the genesis of zebra and stromatactis structures and particular kinds of carbonate breccias. The term melt-seal structure is proposed for this kind of diagenetic structure. The fabrics of gas hydrates and the geochemical conditions of sediments, in turn depending on the relative rates of supply of methane-rich fluids and normal seawater, conditioned the final aspect of the rocks.     

Gas accumulations and their association with particle size distribution patterns in the Ría de Arousa seabed (Galicia, NW Spain): an application of discriminant analysis

Gassy sediments in the Ría de Arousa are preferentially distributed in areas of muddy seabed sediments. The close relationship between seabed sediment parameters and gas distribution is here studied in detail to establish better constraints on the presence of gas. Discriminant analysis was applied to the textural and compositional characteristics of 303 seabed sediment samples to classify gas-related and gas-free areas in the Ría de Arousa. The parameters considered in the classification were: particle size data (percentages of clay, silt, sand and gravel), the total inorganic carbon and the total organic carbon contents of the samples. The samples were initially classified in two groups according to the presence or absence of acoustic turbidity in the seismic profiles, shallower than 150 cm below seabed. Of the total known cases, 85.5% were correctly classified using these variables. Applying the Wilks’ lambda criterion, the most influential textural discriminating variables were the percentage of clay and the percentage of coarse fraction (gravel and sand) in the sediment sample. Discriminant analysis has achieved good differentiation between gas-related and gas-free sediments using near-seabed sediment information. The application of the discriminant method has enabled the estimation of the total area covered by gassy sediments in the Ría de Arousa. The area calculated based on the seismic data (30 km²) is a minimum estimate that is constrained by the limits of the existing seismic data. Based on the sediment information obtained from seabed samples, the statistical method estimates a total area of gassy sediments of 39 km². The new gassy areas recognized are located around the gas field at the inner ría, and the gas field west of Arousa Island, which increase in area by 8.3 and 0.4 km² respectively.     

Comparison of marine gas hydrates in sediments of an active and passive continental margin

Two sites of the Deep Sea Drilling Project in contrasting geologic settings provide a basis for comparison of the geochemical conditions associated with marine gas hydrates in continental margin sediments. Site 533 is located at 3191 m water depth on a spit-like extension of the continental rise on a passive margin in the Atlantic Ocean. Site 568, at 2031 m water depth, is in upper slope sediment of an active accretionary margin in the Pacific Ocean. Both sites are characterized by high rates of sedimentation, and the organic carbon contents of these sediments generally exceed 0.5%. Anomalous seismic reflections that transgress sedimentary structures and parallel the seafloor, suggested the presence of gas hydrates at both sites, and, during coring, small samples of gas hydrate were recovered at subbottom depths of 238m (Site 533) and 404 m (Site 568). The principal gaseous components of the gas hydrates wer methane, ethane, and CO₂. Residual methane in sediments at both sites usually exceeded 10 mll^?1 of wet sediment. Carbon isotopic compositions of methane, CO₂, and ΣCO₂ followed parallel trends with depth, suggesting that methane formed mainly as a result of biological reduction of oxidized carbon. Salinity of pore waters decreased with depth, a likely result of gas hydrate formation. These geochemical characteristics define some of the conditions associated with the occurrence of gas hydrates formed by in situ processes in continental margin sediments.     

Expulsion of Shallow Gas in the Skagerrak—Evidence from Sub-bottom Profiling, Seismic, Hydroacoustical and Geochemical Data

A combined high resolution seismic, sub-bottom profiling, and multi-beam echo-sounding survey in the Skagerrak (Danish sector of the North Sea) together with gas analyses at a station along the profile exhibit the expulsion of gas (mainly methane) and the presence of gas-charged sediments at shallow depth. The echo-soundings yield detailed insight into the distribution and shape of typical sea-floor features associated with gas seepage, such as pockmarks. The pockmarks reach dimensions of 800 m in length, 300 m in width, and 15 m in depth, with the long axis running parallel to the slope of the Norwegian Trench. Processing of the multi-channel high resolution seismic data and the digitally recorded sub-bottom profiler signals indicate an internal compressional velocity of about 1050 m s^-1 within the gas-charged sediments reaching from the sea-floor to a sub-bottom depth of about 23 m. Using the lateral distribution and thickness of the gas-charged sediments in conjunction with a mean concentration of gas of 3000 ppb, the present amount of trapped gas is estimated to be 6·45 × 10¹¹ g CH₄. The flux of methane through the sea-bed into the water column appears to be 7·2 × 10¹⁰ g CH₄ per year. To explain the small difference in size between the methane pool in near-surface sediments and the annual flux through the sea-bed, a constantly high supply of methane from leaking hydrocarbon reservoirs at greater depths has to be active.     

Geochemical and microbiological characteristics of sediments near the Malenky mud volcano (Lake Baikal, Russia), with evidence of Archaea intermediate between the marine anaerobic methanotrophs ANME-2 and ANME-3

Detailed lithological, biogeochemical and molecular biological analyses of core sediments collected in 2002–2006 from the vicinity of the Malenky mud volcano, Lake Baikal, reveal considerable spatial variations in pore water chemical composition, with total concentrations of dissolved salts varying from 0.1 to 1.8‰. Values of methane δ¹³С in the sediments suggest a biogenic origin (δ¹³С_min. ?61.3‰, δ¹³С_max. ?72.9‰). Rates of sulphate reduction varied from 0.001 to 0.7 nmol cm^?3 day^?1, of autotrophic methanogenesis from 0.01 to 2.98 nmol CH₄ cm^?3 day^?1, and of anaerobic oxidation of methane from 0 to 12.3 nmol cm^?3 day^?1. These results indicate that methanogenic processes dominate in gas hydrate-bearing sediments of Lake Baikal. Based on clone libraries of 16S rRNA genes amplified with Bacteria- and Archaea-specific primers, investigation of microbial diversity in gas hydrate-bearing sediments revealed bacterial 16S rRNA clones classified as Deltaproteobacteria, Gammaproteobacteria, Chloroflexi and OP11. Archaeal clone sequences are related to the Crenarchaeota and Euryarchaeota. Baikal sequences of Archaea form a distinct cluster occupying an intermediate position between the marine groups ANME-2 and ANME-3 of anaerobic methanotrophs.     

High-resolution acoustic mapping of shallow gas in unconsolidated sediments beneath the strait of Georgia,British Columbia

Gassy sediments are detectable acoustically in subbottom profiles of Canada's Fraser Delta slope and prodelta areas. Interstitial gas is typically represented by zones of acoustic turbidity on high-resolution seismic profiles and by gas brightening on air-gun profiles. The top of the acoustically turbid zone is generally highest within the sediment column closest to the river mouths and lies within 10 ms (two-way travel time) of the sea floor in a nearly continuous area that covers over 530 km². Most of the gas is believed to be biogenic in origin, although thermogenic gas derived from underlying Tertiary sedimentary rocks may be present in places.     

Worldwide distribution of subaquatic gas hydrates

Sediments containing natural gas hydrates occur worldwide on continental and insular slopes and rises of active and passive margins, on continental shelves of polar regions, and in deep-water (> 300 m) environments of inland lakes and seas. The potential amount of methane in natural gas hydrates is enormous, with current estimates at about 10¹⁹ g of methane carbon. Subaquatic gas hydrates have been recovered in 14 different areas of the world, and geophysical and geochemical evidence for them has been found in 33 other areas. The worldwide distribution of natural gas hydrates is updated here; their global importance to the chemical and physical properties of near-surface subaquatic sediments is affirmed.     

SW Barents Sea continental margin heat flow and Håkon Mosby Mud Volcano

The high thermal gradient and heat flow >1000?mW?m^-2 on Håkon Mosby Mud Volcano are ascribed to rapid transport of pore water, mud, and gas in a narrow, deep conduit within a 3.1-km-thick glacial sediment unit. The instability is caused by rapid loading of dense glacial sediments on less dense oozes. Changes in pressure–temperature conditions by sudden, large-scale downslope mass movement may induce structural deformation, opening transient pathways from the base of the glacial sediments to the sea floor. This model may also explain slope maxima elsewhere on the margin.     

Inferred gas hydrate on the Barents Sea shelf — a model for its formation and a volume estimate

 On the southwestern Barents Sea shelf, sediments containing gas hydrates that overlie free gas have been inferred from multichannel seismic data. The volume of suspected gas hydrate is tentatively estimated to about 1.9×10⁸ m³. The gas hydrate zone probably formed from thermogenic gas leaking from a deeper source. The hydrate zone may have thickened during the Neogene by including gas originally trapped as free gas below the hydrate following a significant downward migration of the isotherms caused by erosion and/or subsidence. Within the present oceanographic conditions, gas hydrate is suspected to be stable or slowly decomposing. Received: 20 December 1996 / Revision received: 20 August 1997     

Isotopic composition of dissolved inorganic carbon in subsurface sediments of gas hydrate-bearing mud volcanoes, Lake Baikal: implications for methane and carbonate origin

We report on the isotopic composition of dissolved inorganic carbon (DIC) in pore-water samples recovered by gravity coring from near-bottom sediments at gas hydrate-bearing mud volcanoes/gas flares (Malenky, Peschanka, Peschanka 2, Goloustnoe, and Irkutsk) in the Southern Basin of Lake Baikal. The δ¹³C values of DIC become heavier with increasing subbottom depth, and vary between ?9.5 and +21.4‰ PDB. Enrichment of DIC in ¹³C indicates active methane generation in anaerobic environments near the lake bottom. These data confirm our previous assumption that crystallization of carbonates (siderites) in subsurface sediments is a result of methane generation. Types of methanogenesis (microbial methyl-type fermentation versus CO₂-reduction) were revealed by determining the offset of δ¹³C between dissolved CH₄ and CO₂, and also by using δ¹³C and δD values of dissolved methane present in the pore waters. Results show that both mechanisms are most likely responsible for methane generation at the investigated locations.     

Comparison of high-resolution seismic profiles and the geoacoustic properties of Eckernförde Bay sediments

High-resolution seismic profiles of Eckernförde Bay and the adjacent Baltic Sea were collected, and the geoacoustic properties of sediments there were measured. Bulk densities averaged ~ 1.35 g cm^–3 and ranged from ~ 1.2 to ~ 1.7 g cm^–3. Compressional wave velocities in gas-free sediments averaged ~ 1460 m s^–1 and ranged from ~ 1425 to ~ 1555 m s^–1. In nongassy sediments, bulk density variations typically controlled changes of acoustic impedance. Impedance changes were usually too small and closely spaced to be resolved seismically, although, at certain sites, significant impedance changes are far apart enough that they correlate one-to-one with seismic reflectors. Where free gas is present, velocity decreases and wave energy is scattered, causing a prominent seismic reflector.     

Geophysical evidence of mud diapirism on the Mediterranean Ridge accretionary complex

Mud volcanoes, mud cones, and mud ridges have been identified on the inner portion of the crestal area, and possibly on the inner escarpment, of the Mediterranean Ridge accretionary complex. Four areas containing one or more mud diapirs have been investigated through bathymetric profiling, single channel seismic reflection profiling, heat flow measurements, and coring. A sequence of events is identified in the evolution of the mud diapirs: initially the expulsion on the seafloor of gasrich mud produces a seafloor depression outlined in the seismic record by downward dip of the host sediment reflectors towards the mud conduit; subsequent eruptions of fluid mud may create a flat topped mud volcano with step-like profile; finally, the intrusion of viscous mud produces a mud cone.The origin of the diapirs is deep within the Mediterranean Ridge. Although a minimum depth of about 400 m below the seafloor has been computed from the hydrostatic balance between the diapiric sediments and the host sediments, a maximum depth, suggested by geometric considerations, ranges between 5.3 and 7 km. The presence of thermogenic gas in the diapiric sediments suggests a better constrained origin depth of at least 2.2 km.The heat flow measured within the Olimpi mud diapir field and along a transect orthogonal to the diapiric field is low, ranging between 16 ± 5 and 41 ± 6 mW m^–2. Due to the presence of gas, the thermal conductivity of the diapiric sediments is lower than that of the host hemipelagic oozes (0.6–0.9 and 1.0–1.15 W m^–1 K^–1 respectively).We consider the distribution of mud diapirs to be controlled by the presence of tectonic features such as reverse faults or thrusts (inner escarpment) that develop where the thickness of the Late Miocene evaporites appears to be minimum. An upward migration through time of the position of the décollement within the stratigraphic column from the Upper Oligocene (diapiric sediments) to the Upper Miocene (present position) is identified.     

A method for the calculation of anaerobic oxidation of methane rates across regional scales: an example from the Belt Seas and The Sound (North Sea–Baltic Sea transition)

Estimating the amount of methane in the seafloor globally as well as the flux of methane from sediments toward the ocean–atmosphere system are important considerations in both geological and climate sciences. Nevertheless, global estimates of methane inventories and rates of methane production and consumption through anaerobic oxidation in marine sediments are very poorly constrained. Tools for regionally assessing methane formation and consumption rates would greatly increase our understanding of the spatial heterogeneity of the methane cycle as well as help constrain the global methane budget. In this article, an algorithm for calculating methane consumption rates in the inner shelf is applied to the gas-rich sediments of the Belt Seas and The Sound (North Sea–Baltic Sea transition). It is based on the depth of free gas determined by hydroacoustic techniques and the local methane solubility concentration. Due to the continuous nature of shipboard hydroacoustic measurements, this algorithm captures spatial heterogeneities in methane fluxes better than geochemical analyses of point sources such as observational/sampling stations. The sensibility of the algorithm with respect to the resolution of the free gas depth measurements (2 m vs. 50 cm) is proven of minor importance (a discrepancy of <10%) for a small part of the study area. The algorithm-derived anaerobic methane oxidation rates compare well with previous measured and modeling studies. Finally, regional results reveal that contemporary anaerobic methane oxidation in worldwide inner-shelf sediments may be an order of magnitude lower (ca. 0.24 Tmol year^–1) than previous estimates (4.6 Tmol year^–1). These algorithms ultimately help improve regional estimates of anaerobic oxidation of methane rates.     

Effects of ion exclusion and isotopic fractionation on pore water geochemistry during gas hydrate formation and decomposition

The effects of ion exclusion and isotopic fractionation associated with gas hydrate formation and decomposition in continental margin sediments are examined using simple mass balance calculations. In a closed system pore fluid salinity can be increased to brine levels and detectable changes in interstitial water ¹⁸O can be caused by formation of significant amounts of interstitial gas hydrate. Time- and mass-dependent models indicate that given appropriate geometries, the diffusion of dissolved salts is sufficiently rapid and their supply is large enough to establish dissolved ion gradients that can be measured in sediments obtained from piston cores or boreholes.     

A seismic study to investigate the prospect of gas hydrate in Mahanadi deep water basin,northeastern continental margin of India

The presence of gas hydrates, one of the new alternative energy resources for the future, along the Indian continental margins has been inferred mainly from bottom simulating reflectors (BSR) and the gas stability zone thickness mapping. Gas hydrate reserves in Krishna Godawari Basin have been established with the help of gas-hydrate related proxies inferred from multidisciplinary investigations. In the present study, an analysis of 3D seismic data of nearly 3,420 km² area of Mahanadi deep water basin was performed in search of seismic proxies related with the existence of natural gas hydrate in the region. Analysis depicts the presence of BSR-like features over a large areal extent of nearly 250 km² in the central western part of the basin, which exhibit all characteristics of a classical BSR associated with gas hydrate accumulation in a region. The observed BSR is present in a specific area restricted to a structural low at the Neogene level. The coherency inversion of pre-stack time migration (PSTM) gathers shows definite inversion of interval velocity across the BSR interface which indicates hydrate bearing sediments overlying the free gas bearing sediments. The amplitude versus offset analysis of PSTM gathers shows increase of amplitude with offset, a common trend as observed in BSR associated with gas hydrate accumulation. Results suggest the possibility of gas hydrate accumulation in the central part of the basin specifically in the area of structural low at the Neogene level. These results would serve as preliminary information for selecting prospective gas hydrate accumulation areas for further integrated or individual study from geophysical, geological, geochemical and microbiological perspectives for confirmation of gas hydrate reserves in the area. Further, on the basis of these results it is envisaged that biogenic gas might have been generated in the region which under suitable temperature and pressure conditions might have been transformed into the gas hydrates, and therefore, an integrated study comprising geophysical, geological, geochemical and microbiological data is suggested to establish the gas hydrate reserves in Mahanadi deep water basin.     

Stable carbon isotope ratios of CH4-rich gas inclusions in shale-hosted fracture-fill mineralization: A tool for tracing hydrocarbon generation and migration in shale plays for oil and gas

Fluid inclusion gases in minerals from shale hosted fracture-fill mineralization have been analyzed for stable carbon isotopic ratios of CH₄ using a crushing device interfaced to an isotope ratio mass spectrometer (IRMS). The samples of Paleozoic strata under study originate from outcrops and wells in the Rhenish Massif and Campine Basin, Harz Mountains, and the upper slope of the Southern Permian Basin. Fracture-fill mineralization hosted by Mesozoic strata was sampled from drill cores in the Lower Saxony Basin. Some studied sites are candidates for shale gas exploration in Germany. Samples of Mesozoic strata are characterized by abundant calcite-filled horizontal fractures which preferentially occur in TOC-rich sections of the drilled sediments. Only rarely are vertical fractures filled with carbonates and/or quartz in drill cores from Mesozoic strata but in Paleozoic shale they occur frequently. The δ¹³C(CH₄) values of fluid inclusions in calcite from horizontal fractures hosted by Mesozoic strata suggest that gaseous hydrocarbons were generated during the oil/early gas window and that the formation of horizontal fractures seems to be related to hydraulic expulsion fracturing. The calculated maturity of the source rocks at the time of gas generation lies below the maturity derived from measured vitrinite reflectance. Thus, the formation of horizontal fractures and trapping of gas that was generated in the oil and/or early gas window obviously occurred prior to maximal burial. Rapidly increasing vitrinite reflectance data seen locally can be explained by hydrothermal alteration, as indicated by increasing δ¹³C (CH₄–CO₂) values in fluid inclusions. The formation of vertical fractures in studied Mesozoic sediments is related to stages of post-burial inversion; gas-rich inclusions in fracture filling minerals recorded the migration of gas that had probably been generated instantaneously, rather than cumulatively, from high to overmature source rocks. Since no evidence is given for the presence of early generated gas in studied Paleozoic shale, it appears likely that major gas loss from shales occurred due to deformation and uplift of these sediments in response to the Variscan Orogeny.     

Authigenic carbonates from active methane seeps offshore southwest Africa

The southwest African continental margin is well known for occurrences of active methane-rich fluid seeps associated with seafloor pockmarks at water depths ranging broadly from the shelf to the deep basins, as well as with high gas flares in the water column, gas hydrate accumulations, diagenetic carbonate crusts and highly diverse benthic faunal communities. During the M76/3a expedition of R/V METEOR in 2008, gravity cores recovered abundant authigenic carbonate concretions from three known pockmark sites—Hydrate Hole, Worm Hole, the Regab pockmark—and two sites newly discovered during that cruise, the so-called Deep Hole and Baboon Cluster. The carbonate concretions were commonly associated with seep-benthic macrofauna and occurred within sediments bearing shallow gas hydrates. This study presents selected results from a comprehensive analysis of the mineralogy and isotope geochemistry of diagenetic carbonates sampled at these five pockmark sites. The oxygen isotope stratigraphy obtained from three cores of 2–5?m length indicates a maximum age of about 60,000–80,000?years for these sediments. The authigenic carbonates comprise mostly magnesian calcite and aragonite, associated occasionally with dolomite. Their very low carbon isotopic compositions (–61.0?<?δ¹³C ‰ V-PDB?<?–40.1) suggest anaerobic oxidation of methane (AOM) as the main process controlling carbonate precipitation. The oxygen isotopic signatures (+2.4?<?δ¹⁸O ‰ V-PDB?<?+6.2) lie within the range in equilibrium under present-day/interglacial to glacial conditions of bottom seawater; alternatively, the most positive δ¹⁸O values might reflect the contribution of ¹⁸O-rich water from gas hydrate decomposition. The frequent occurrence of diagenetic gypsum crystals suggests that reduced sulphur (hydrogen sulphide, pyrite) from sub-seafloor sediments has been oxidized by oxygenated bottom water. The acidity released during this process can potentially induce the dissolution of carbonate, thereby providing enough Ca²⁺ ions for pore solutions to reach gypsum saturation; this is thought to be promoted by the bio-irrigation and burrowing activity of benthic fauna. The δ¹⁸O–δ¹³C patterns identified in the authigenic carbonates are interpreted to reflect variations in the rate of AOM during the last glacial–interglacial cycle, in turn controlled by variably strong methane fluxes through the pockmarks. These results complement the conclusions of Kasten et al. in this special issue, based on authigenic barite trends at the Hydrate Hole and Worm Hole pockmarks which were interpreted to reflect spatiotemporal variations in AOM related to subsurface gas hydrate formation–decomposition.     

Acoustic imagery evidence for methane hydrates in the Ulleung Basin

Approximately 12,000 km² of acoustic backscatter imagery (sidescan) data and swath bathymetry data were collected jointly by Republic of Korea (ROK) Navy, the Naval Oceanographic Office (NAVOCEANO), Hawaii Mapping Research Group (HMRG) and the Naval Research Laboratory (NRL) in the East Sea (Sea of Japan) in 1995. Preliminary analysis of these data have revealed a large network of canyons with well-developed fan deposits and slumps which were not previously mapped. Also identified is a 1400 km² area occupied by more than 300 circular, low-backscatter features ca. 50–1000 m in diameter which are interpreted to be pockmarks or mounds created by escaping methane gas, methane-rich porewater and mud.Indirect evidence for the probable existence of methane gas hydrate include the five following observations: (1) Core samples in the region contain high levels of organic carbon (>7%), degassing cracks caused by gas expansion, and emit a strong H₂S odor. (2) Extensive canyon formation and slumping may have occurred as the result of the destabilization of sediments due to gas accumulation. (3) Several of the high backscatter objects occur at the crest of a bathymetric high under which gas could be accumulating and periodically releasing in a manner similar to that documented on the Vestnesa Ridge in the Norwegian-Greenland Sea. (4) Pockmark-like features have been identified in 3.5 kHz records on the northern edge of the Ulleung Basin. (5) Drill core samples from the morphologically similar Yamato Basin, which is adjacent to the Ulleung Basin, have positively identified methane and numerous gas voids in unconsolidated sediments. No bottom simulating reflector (BSR) has been identified in seismic reflection profiles collected across the slope in Ulleung Basin.     

Seismic quality factors across a bottom simulating reflector in the Makran Accretionary Prism, Arabian Sea

The hydrate-bearing sediments above the bottom simulating reflector (BSR) are associated with low attenuation or high quality factor (Q), whereas underlying gas-bearing sediments exhibit high attenuation. Hence, estimation of Q can be important for qualifying whether a BSR is related to gas hydrates and free-gas. This property is also useful for identifying gas hydrates where detection of BSR is dubious. Here, we calculate the interval Q for three submarine sedimentary layers bounded by seafloor, BSR, one reflector above and another reflector below the BSR at three locations with moderate, strong and no BSR along a seismic line in the Makran accretionary prism, Arabian Sea for studying attenuation (Q⁻¹) characteristics of sediments. Interval Q for hydrate-bearing sediments (layer B) above the BSR are estimated as 191 ± 11, 223 ± 12, and 117 ± 5, whereas interval Q for the underlying gas-bearing sediments (layer C) are calculated as 112 ± 7, 107 ± 8 and 124 ± 11 at moderate, strong and no BSR locations, respectively. The large variation in Q is observed at strong BSR. Thus Q can be used for ascertaining whether the observed BSR is due to gas hydrates, and for identifying gas hydrates at places where detection of BSR is rather doubtful. Interval Q of 98 ± 4, 108 ± 5, and 102 ± 5, respectively, at moderate, strong and no BSR locations for the layer immediately beneath the seafloor (layer A) show almost uniform attenuation.     

Evidence of slumping/sliding in Krishna–Godavari offshore basin due to gas/fluid movements

The Krishna–Godavari (KG) offshore basin is one of the promising petroliferous basins of the eastern continental margin of India. Drilling in this basin proved the presence of gas hydrate deposits in the shallow marine sediments beyond 750 m water depths, and provided lithologic and stratigraphic information. We obtained multibeam swath bathymetry covering an area of about 4500 km² in water depths of 280–1800 m and about 1260 line km of high resolution seismic (HRS) records. The general lithology of midslope deposits is comprised of nannofossil-rich clay, nannofossil-bearing clay and foraminifera-bearing clay. The HRS records and bathymetry reveal evidence of slumping and sliding of the upper and midslope sediments, which result in mass transport deposits (MTD) in the northwestern part of the study area. These deposits exhibit 3–9.5 km widths and extend 10–13 km offshore. The boundaries of the MTDs are often demarcated by sharp truncation of finely layered sediments (FLS) and the MTDs are characterized by acoustically transparent zones in the HRS data. Average thickness of recent MTDs varies with depth, i.e., in the upper slope, the thickness is about 45 m, while in the lower slope it is about 60 m, and in deeper offshore locations they attain a maximum thickness of about 90 m. A direct indication for slumping and mass transportation of deposits is provided by the age reversal in ¹⁴C AMS dates observed in a sediment core located in the midslope region. Seismic profiling signatures provide indications of fluid/gas movement. We propose that the presence of steep topographic gradients, high sedimentation rates, a regional fault system, diapirism, fluid/gas movement, and neotectonic activity may have facilitated the slumping/sliding of the upper slope sediments in the KG offshore basin.