Kerala-Konkan Basin Research Articles

Crustal structure along the Kerala-Konkan Basin, southwest continental margin of India, using multi-channel seismic and gravity modelling: Implications on India-Madagascar rifting and basin evolution

Crustal structure along the Kerala-Konkan Basin, southwest continental margin of India, using multi-channel seismic and gravity modelling: Implications on India-Madagascar rifting and basin evolution

Raman Spectroscopic Technique to Distinguish Constituents of Hydrocarbon-Bearing Fluid Inclusions of Kerala-Konkan Basin, Western offshore, India

Fluid inclusion studies have a great diversity of applications in exploration geology and are necessary tools in the determination of palaeotemperature and nature of fluids associated with the rocks in a basin. Using various fluid inclusion techniques such as petrography, microthermometry and Laser Raman Spectroscopy of fluid inclusions with Hydrocarbon fluid inclusions (HCFIs) help us to understand the generation potential of the basin. The representative micron sized fluid inclusions that intruded into the different geological formations of the KK-4C-A1well drilled by Oil and Natural Gas Corporation in Kerala-Konkan Basin, India has been selected for this study. Petrographic analyses confirm the presence of HCFIs with the help of Ultraviolet (UV) light. Raman spectra of HCFIs identified in different formations were examined. The temperature of homogenization (Th) obtained through microthermometric analysis of the fluid inclusions indicate the palaeotemperature of the sedimentary rock units. Coeval-aqueous inclusions associated to HCFIs show Th within the oil window range 60-140oC, indicating a temperature favourable for oil generation in Kerala-Konkan Basin (K-K Basin).Characterisation of hydrocarbon bearing fluid inclusions were carried out using Raman spectroscopy. HCFIs were observed in the annealed micro-cracks of Cannanore (Early Miocene), Calicut (Early Oligocene) formation (Type I) and Kasaragod (Palaeocene to Early Eocene) Formation (Type II), might get trapped along the micro-fracture by re-healing process. Laser Raman study could decipher hydrocarbon species such as Alkanes, SO2, COS, H2S, etc. Keywords: Hydrocarbon Fluid Inclusions, Oil Window Temperature, Raman Spectroscopy, Kerala-Konkan Basin

Hydrocarbon fluid inclusions and source rock parameters: A comparison from two dry wells in the western offshore, India

Fluid inclusions represent the direct evidence of paleofluids and can provide valuable information on the evolution of sedimentary basins and oil-bearing strata. Hydrocarbon fluid inclusion(s) (HCFIs) are the vestiges of oil from the geological formations. The paper delineates the paleotemperature (Th)/ oil window, the oil quality of HCFIs and Raman peaks corresponding to hydrocarbon species of HCFIs using fluid inclusion techniques, and source rock potential of hydrocarbon generation, thermal maturity, the quantity of organic matter, and the kerogen types obtained through Rock-Eval pyrolysis data from two dry wells RV-1 well of Mumbai offshore and KKD-1A well of Kerala-Konkan Basin. The present study compares the fluid inclusion parameters as well as the source rock geochemical characteristics of these two dry wells to address the scientific problem of the wells going dry. Further, evaluated whether the results agree with an earlier finding from a case study of two wells named KK4C-Al (Kerala-Konkan basin) and RV-1 well where only a few parameters such as temperature of homogenization (Th) & API gravity were utilised, and the chances of getting oil in the nearby areas of these two wells were reported. In the present study, the fluid inclusion parameters such as the palaeotemperature (Th), API Gravity and Raman spectra were obtained from micron sized fluid inclusions at different depths for a quick assessment of nature of oil inclusions within the two dry wells. Along with fluid inclusion parameters, different source rock parameters obtained from Rock-Eval Pyrolysis analysis (secondary data) such as S1, S2, S3, Tmax, Hydrogen Index (HI), Oxygen Index (OI), Potential Yield (PY), Production Index (PI) and Total Organic Carbon Content (TOC) were also considered for a detailed source-rock evaluation of two wells (RV-1 and KKD-1A) and the results act as the supporting evidence to address the reason for the wells gone dry.Temperature of homogenisation (Th) of hydrocarbon Fluid Inclusion Assemblages (FIAs) from both the wells fall in the oil window (60–150 °C) range indicating that there was a conducive thermal condition favourable for oil generation in these two basins. API gravity of oils in RV-1 well of Mumbai offshore (48–53) was lighter when compared to those in KKD-1A (18–22) of Kerala-Konkan basin. Raman spectra of HCFI samples could decipher important hydrocarbon species from RV-1 well samples. Raman spectra of KKD-1A well show less prominent peaks (broad) only. Pyrolysis data shows that Paleocene–Early Eocene source rocks of Panna formation of RV1 well are mature enough to generate hydrocarbons. On the other hand, Paleocene aged source rocks of Kasargod formation of KKD-1A well are immature. Source rock maturity therefore could be considered as crucial in hydrocarbon generation in these two wells even if oil-window was achieved. This study reports that, in RV-1 well, even though it is a dry well in a proven basin, the oil window, API gravity of oils and constituents from HCFIs of RV-1 well and the source-rock maturity opens up a demand for detailed exploration in nearby areas of RV-1 in the Mumbai offshore basin hopeful of finding a high-value prospect for oil, whereas the fluid inclusion studies in the HCFIs of KKD-1A well of Kerala-Konkan basin is showing only a minimal chance of oil generation that too of a heavy nature and the source rock immature characteristics suggesting only minimal generation of hydrocarbons. Due to the heaviness of the available oil in the KKD-1A well impedes migration. Our study suggests that there is no potential for finding oil in the nearby areas of KKD-1A well of Kerala-Konkan basin.

Sr isotope numerical depositional age of Miocene marine strata (Quilon Formation), Kerala–Konkan Basin, India

Sr isotope numerical depositional age of Miocene marine strata (Quilon Formation), Kerala–Konkan Basin, India

Physio-Chemical and Mineralogical Characteristics of Gas Hydrate-Bearing Sediments of the Kerala-Konkan, Krishna-Godavari, and Mahanadi Basins

The characteristics of the hydrate-bearing sediments affect the formation and dissociation of gas hydrate in sediments. The mineral composition, their dispersion, and chemical composition of hydrate-bearing sediment samples plays a dominant role in the hydrate stability condition and its economic development. In this paper, the physical properties of hydrate-bearing sediment of India are compared with each other. The sediment samples are taken from the Krishan-Godavari basin (Depth—127.5 and 203.2 mbsf), Mahanadi basin (Depth—217.4 mbsf), and Kerala-Konkan basin (Depth—217.4 mbsf). The saturation of the gas hydrate observed at these sites is between 3 and 50%. Particle size is an important parameter of the sediments because it provides information on the transportation and deposition of sediment and the deposition history. In the present study, we investigated the mineralogy of hydrate-bearing sediments by chemical analysis and X-ray Diffraction. XRD, FTIR, and Raman Spectroscopy distinguished the mineralogical behavior of sediments. Quartz is the main mineral (66.8% approx.) observed in the gas hydrate-bearing sediments. The specific surface area was higher for the sediment sample from the Mahanadi basin, representing the sediments’ dissipation degree. This characterization will give important information for the possible recovery of gas from Indian hydrate reservoirs by controlling the behavior of host sediment. SEM analysis shows the morphology of the sediments, which can affect the mechanical properties of the hydrate-bearing sediments. These properties can become the main parameters to consider for the design of suitable and economic dissociation techniques for gas hydrates formed in sediments.

Seismic imaging of Deccan-related lava flows at the K-T boundary, deepwater west India

Continuous improvements in geophysical technology have enabled seismic imaging of Mesozoic strata under the K-T transition boundary — a thick lava blanket spread along the continental margin of India's west coast. The new images reveal features of the subbasalt geology of this large offshore region. The relationship of the offshore lava flows with the equally vast lava flows of the onshore Deccan Traps adds a parameter of comparison to the whole Mesozoic stratigraphy of the south, west, and northwest coasts of India. The lava flows are part of the deepwater margin that embraces several basins, the largest being the southernmost underexplored Kerala-Konkan Basin. Newly acquired and processed regional seismic data sets were integrated with new data sets of potential field data to better uncover the Mesozoic stratigraphy. The 3D seismic data, acquired with long spreads and broadband processing through time and depth migration, enabled imaging of the complex structure of layers under basalt. India detached from Madagascar during Cretaceous time; the northward path to its present position was affected by Reunion hotspot activity that melted the west coast near the K-T transition time. The volcanic lithology observed at the K-T boundary is related to episodic emplacement of intrusive dykes and extrusive sills. A possible cause-effect event in that span of time is the Chicxulub meteorite impact that may have increased the volume of Deccan lava flows. The deepwater lava volumes estimated from interpretation of seismic data from the offshore region at the end of Cretaceous time are comparable to, or perhaps much greater than, the volumes estimated for the precisely age-dated Deccan Traps basalts exposed onshore. This suggests an enormous outpouring of flood basalts (onshore and offshore) at K-T time that was comparable to other major flood basalt events, such as the Siberian traps, and associated with mass extinctions.

Evaluation of Organic Matter, Hydrocarbon Source, and Depositional Environment of Onshore Warkalli Sedimentary Sequence from Kerala-Konkan Basin, South India

Multidisciplinary analysis of the carbonaceous sediments of Warkalli Formation (Mio-Pliocene) from the Warkalli cliff section has been done to assess the source of organic matter, palaeodepositional settings and the hydrocarbon potential. The n-alkane distribution from n-C12 to n-C33 along with bimodal distribution indicates significant organic matter contribution from microbial activity and higher plants. The contribution from angiosperm source vegetation is indicated by the oleanane type of triterpenoids. The hopanes distribution indicates the immature stage of the organic matter, which is in agreement with the Tmax (av. 401 °C) and huminite reflectance (av. 0.28% Rr) values. The total organic carbon (TOC) contents vary between 0.8 and 6.72 wt. % in the studied sediments. Hydrogen index and oxygen index values range from 16 to106 mg HC/g TOC, and 113 to 344 mg CO2/g TOC, respectively. The maceral content is low, being dominated by the detrohuminite submaceral and the mineral matter accounts for 68 to 77% of the total composition. The phytoclast group (63–87%) is dominant with subordinate amorphous organic matter (4–35%). The study shows that the sediments were deposited in a marginal suboxic basin with intermittent variations. All the parameters unequivocally suggest that the studied sequence holds the potential to generate gaseous hydrocarbons.

National Gas Hydrate Program Expedition 01 offshore India; gas hydrate systems as revealed by hydrocarbon gas geochemistry

Abstract The National Gas Hydrate Program Expedition 01 (NGHP-01) targeted gas hydrate accumulations offshore of the Indian Peninsula and along the Andaman convergent margin. The primary objectives of coring were to understand the geologic and geochemical controls on the accumulation of methane hydrate and their linkages to underlying petroleum systems. Four areas were investigated: 1) the Kerala-Konkan Basin in the eastern Arabian Sea, 2) the Mahanadi and 3) Krishna-Godavari Basins in the western Bay of Bengal, and 4) the Andaman forearc Basin in the Andaman Sea. Upward flux of methane at three of the four of the sites cored during NGHP-01 is apparent from the presence of seafloor mounds, seismic evidence for upward gas migration, shallow sub-seafloor geochemical evidence of methane oxidation, and near-seafloor gas composition that resembles gas from depth. The Kerala-Konkan Basin well contained only CO2 with no detectable hydrocarbons suggesting there is no gas hydrate system here. Gas and gas hydrate from the Krishna-Godavari Basin is mainly microbial methane with δ13C values ranging from −58.9 to −78.9‰, with small contributions from microbial ethane (−52.1‰) and CO2. Gas from the Mahanadi Basin was mainly methane with lower concentrations of C2-C5 hydrocarbons (C1/C2 ratios typically >1000) and CO2. Carbon isotopic compositions that ranged from −70.7 to −86.6‰ for methane and −62.9 to −63.7‰ for ethane are consistent with a microbial gas source; however deeper cores contained higher molecular weight hydrocarbon gases suggesting a small contribution from a thermogenic gas source. Gas composition in the Andaman Basin was mainly methane with lower concentrations of ethane to isopentane and CO2, C1/C2 ratios were mainly >1000 although deeper samples were

Crustal structure and sedimentation history over the Alleppey platform, southwest continental margin of India: Constraints from multichannel seismic and gravity data

The Alleppey Platform is an important morphological feature located in the Kerala-Konkan basin off the southwest coast of India. In the present study, seismic reflection data available in the basin were used to understand the sedimentation history and also to carry out integrated gravity interpretation. Detailed seismic reflection data in the basin reveals that: (1) the Alleppey Platform is associated with a basement high in the west of its present-day geometry (as observed in the time-structure map of the Trap Top (K/T boundary)), (2) the platform subsequently started developing during the Eocene period and attained the present geometry by the Miocene and, (3) both the Alleppey platform and the Vishnu fracture zone have had significant impact on the sedimentation patterns (as shown by the time-structure and the isochron maps of the major sedimentary horizons in the region). The 3-D sediment gravity effect computed from the sedimentary layer geometry was used to construct the crustal Bouguer anomaly map of the region. The 3-D gravity inversion of crustal Bouguer anomaly exhibits a Moho depression below the western border of the platform and a minor rise towards the east which then deepens again below the Indian shield. The 2-D gravity modelling across the Alleppey platform reveals the geometry of crustal extension, in which there are patches of thin and thick crust. The Vishnu Fracture Zone appears as a crustal-scale feature at the western boundary of the Alleppey platform. Based on the gravity model and the seismic reflection data, we suggest that the basement high to the west of the present day Alleppey platform remained as a piece of continental block very close to the mainland with the intervening depression filling up with sediments during the rifting. In order to place the Alleppey platform in the overall perspective of tectonic evolution of the Kerala-Konkan basin, we propose its candidature as a continental fragment.

Microbial iron reduction and methane oxidation in subsurface sediments of the Arabian Sea

Arabian Sea is one of the most productive regions of the world's ocean with seasonal upwelling and a characteristic oxygen minimum zone. It receives a continuous input of windborne iron-rich dust which possibly stimulates phytoplankton productivity. A sediment core from this area, which is on the western continental margin of India in the Kerala-Konkan basin was studied from the surface to 63 m below seafloor, to establish the co-occurrence of microbial iron reducing activity and methane oxidation. Total bacterial abundance (TC), viable aerobic counts accounting for aerobic and facultative anaerobes (TVCa), viable anaerobic counts (TVCan), methane, sulfate, ferrous [Fe(II], hydroxylamine extractable iron and HCl extractable iron concentrations were measured. Average TC was 52.6 ± 29.8 × 105 cells g−1 sediment while TVCa and TVCan were an order less. Methane and sulfate concentrations were 1.3 ppm and 23.2 mM, respectively. Average Fe(II) concentration, hydroxylamine and HCl extractable Fe was 0.32 ± 0.26, 0.86 ± 1.44 and 1.90 ± 2.47 mmol g−1 sediment, respectively. Interestingly, Fe(II) significantly correlated with TVCan (r = 0.66, p ≤ 0.001).In the experimental tubes, the build-up of Fe(II) was accompanied by the disappearance of methane. Average methane concentration and Fe(II) concentration in the experimental sediment slurries was 0.56 ± 0.76 ppb and 0.50 ± 0.27 mmol g−1 sediment respectively. The microcosm experiment showed that Fe(II) determined was significantly higher (ANOVA, F = 6.74, p ≤ 0.014) after 5 days of incubation, thus implying microbial iron reducing activity. A strong negative linear correlation (p ≤ 0.001) of methane concentration with Fe(II) suggests that increasing activity of microbial iron reduction caused suppression in methane production. This is also indicative of the co-occurrence of iron reduction and methane oxidation in these sediments.

Comparison of the physical and geotechnical properties of gas-hydrate-bearing sediments from offshore India and other gas-hydrate-reservoir systems

The sediment characteristics of hydrate-bearing reservoirs profoundly affect the formation, distribution, and morphology of gas hydrate. The presence and type of gas, porewater chemistry, fluid migration, and subbottom temperature may govern the hydrate formation process, but it is the host sediment that commonly dictates final hydrate habit, and whether hydrate may be economically developed.In this paper, the physical properties of hydrate-bearing regions offshore eastern India (Krishna-Godavari and Mahanadi Basins) and the Andaman Islands, determined from Expedition NGHP-01 cores, are compared to each other, well logs, and published results of other hydrate reservoirs. Properties from the hydrate-free Kerala-Konkan basin off the west coast of India are also presented. Coarser-grained reservoirs (permafrost-related and marine) may contain high gas-hydrate-pore saturations, while finer-grained reservoirs may contain low-saturation disseminated or more complex gas-hydrates, including nodules, layers, and high-angle planar and rotational veins. However, even in these fine-grained sediments, gas hydrate preferentially forms in coarser sediment or fractures, when present. The presence of hydrate in conjunction with other geologic processes may be responsible for sediment porosity being nearly uniform for almost 500 m off the Andaman Islands.Properties of individual NGHP-01 wells and regional trends are discussed in detail. However, comparison of marine and permafrost-related Arctic reservoirs provides insight into the inter-relationships and common traits between physical properties and the morphology of gas-hydrate reservoirs regardless of location. Extrapolation of properties from one location to another also enhances our understanding of gas-hydrate reservoir systems. Grain size and porosity effects on permeability are critical, both locally to trap gas and regionally to provide fluid flow to hydrate reservoirs. Index properties corroborate more advanced consolidation and triaxial strength test results and can be used for predicting behavior in other NGHP-01 regions. Pseudo-overconsolidation is present near the seafloor and is underlain by underconsolidation at depth at some NGHP-01 locations.

Geologic implications of gas hydrates in the offshore of India: Krishna–Godavari Basin, Mahanadi Basin, Andaman Sea, Kerala–Konkan Basin

Gas hydrate resource assessments that indicate enormous global volumes of gas present within hydrate accumulations have been one of the primary driving forces behind the growing interest in gas hydrates. Gas hydrate volumetric estimates in recent years have focused on documenting the geologic parameters in the “gas hydrate petroleum system” that control the occurrence of gas hydrates in nature. The primary goals of this report are to review our present understanding of the geologic controls on the occurrence of gas hydrate in the offshore of India and to document the application of the petroleum system approach to the study of gas hydrates.National Gas Hydrate Program of India executed the National Gas Hydrate Program Expedition 01 (NGHP-01) in 2006 in four areas located on the eastern and western margins of the Indian Peninsula and in the Andaman Sea. These areas have experienced very different tectonic and depositional histories. The peninsular margins are passive continental margins resulting from a series of rifting episodes during the breakup and dispersion of Gondwanaland to form the present Indian Ocean. The Andaman Sea is bounded on its western side by a convergent margin where the Indian plate lithosphere is being subducted beneath southeast Asia.NGHP-01 drilled, logged, and/or cored 15 sites (31 holes) in the Krishna–Godavari Basin, 4 sites (5 holes) in the Mahanadi Basin, 1 site (2 holes) in the Andaman Sea, and 1 site (1 hole) in the Kerala–Konkan Basin. Holes were drilled using standard drilling methods for the purpose of logging-while-drilling and dedicated wireline logging; as well as through the use of a variety of standard coring systems and specialized pressure coring systems.NGHP-01 yielded evidence of gas hydrate from downhole log and core data obtained from all the sites in the Krishna–Godavari Basin, the Mahanadi Basin, and in the Andaman Sea. The site drilled in the Kerala–Konkan Basin during NGHP-01 did not yield any evidence of gas hydrate. Most of the downhole log-inferred gas hydrate and core-recovered gas hydrate were characterized as either fracture-filling in clay-dominated sediments or as pore-filling or grain-displacement particles disseminated in both fine- and coarse-grained sediments. Geochemical analyses of gases obtained from sediment cores recovered during NGHP-01 indicated that the gas in most all of the hydrates in the offshore of India is derived from microbial sources; only one site in the Andaman Sea exhibited limited evidence of a thermogenic gas source. The gas hydrate petroleum system concept has been used to effectively characterize the geologic controls on the occurrence of gas hydrates in the offshore of India.

Drilling hazard assessment for hydrate bearing sediments including drilling through the bottom-simulating reflectors

To achieve the scientific goals of the NGHP Expedition 01 project, it was necessary as a preliminary step to examine the available marine 2-D seismic data covering then-proposed investigation drilling sites for potential drilling geohazards. A total of 19 potential drilling sites, located in offshore areas of the northern Indian Ocean: the Krishna–Godavari Basin, the Kerala–Konkan Basin, the Mahanadi Basin, and the Andaman Islands, were investigated. The NGHP Expedition 01 staff identified several potential drilling hazards to be investigated at each site, including, but not limited to, the common drilling hazards of seafloor slopes and failure conditions, complex seafloor topography and anomalous amplitude response, shallow gas hydrates, shallow free gas, shallow water-flow, and faults/fractures. Specific proposed drilling sites were investigated over a depth range beginning at the seafloor and extending to 350–700 m below the seafloor, depending upon the specific planned depth at each site. These investigations resulted in some sites being interpreted to be clear of potential drilling geohazards, while other sites were determined to have potentials for specific geohazards significant enough for reconsideration. These reconsiderations consisted of moving specific sites to more favorable locations interpreted to be relatively unsusceptible or less susceptible to a specific identified geohazard, and/or reducing the planned depth of drilling, or removing a potential site at least temporarily from consideration. This work contributed to favorable results from the NGHP Expedition 01 in that no drilling geohazards were reportedly encountered.

Velocity-Porosity and Velocity-Density Relationship for Shallow Sediments in the Kerala-Konkan Basin of Western Indian Margin

Abstract: During the expedition 01 of the National Gas Hydrate Program (NGHP), drilling and coring were carried out at one site in the Kerala-Konkan basin on the west cost of India to validate gas hydrate. Drilling/coring results show a homogenous sequence of oozes and explains that the reflector, which was identified as a bottom-simulating reflector (BSR) on seismic section, is mainly due to changes in formation density because of less clay content in carbonate-rich sediment. Downhole logs collected at this site are of good quality and have been used to establish empirical relationship between the P-wave velocity (VP), S-wave velocity (VS), density (ρ) and porosity (ϕ). The established relations show very good fit with high R2 value (>0.73), and can be used for further studies in this region. Well known existing empirical formulas between VP, VS, ρ and ϕ deviate significantly from our established relations.

Long-timescale variation in bulk and clay mineral composition of Indian continental margin sediments in the Bay of Bengal, Arabian Sea, and Andaman Sea

This study documents X-ray diffraction results from bulk powders and oriented clay-size aggregates using samples from sites drilled and cored during the Indian National Gas Hydrate Expedition 01 (NGHP01). These sites are located in the Krishna–Godavari Basin, Mahanadi Basin, and Andaman accretionary wedge of the Bay of Bengal, and the Kerala-Konkan Basin of the Arabian Sea. Calcite is more abundant at the pelagic sites of the Andaman Sea and Kerala-Konkan Basin, which is consistent with previous studies of biological productivity and dilution by lithogenous influx. Hemipelagic sediments in the Krishna–Godavari Basin and Mahanadi Basin are comprised primarily of smectite-rich and illite-rich clay mineral assemblages, respectively. We attribute those contrasts to differences in detrital sources between the Deccan basalts (smectite sources) and Precambrian rocks of the Eastern Ghats Belt; those sources remained consistent over the entire history of sedimentation (0–9 Ma). Higher quartz content in the Mahanadi Basin and higher feldspar content at the Krishna–Godavari Basin reinforce these interpretations of detrital provenance. Smectite is the most abundant clay mineral in the Andaman Sea sediments likely due to weathering of volcanic sources along the Sunda Arc. Strata from the Kerala-Konkan Basin show a shift at 23 Ma from a smectite–kaolinite clay mineral assemblage to an increasingly illite-rich assemblage. We also see steady decreases in kaolinite and increases in chlorite and quartz over the 30-Myr record, which indicates increasing influences of material derived from physical weathering. The higher abundance of fully hydrated smectite in the Krishna–Godavari Basin may play a minor role in gas hydrate formation by sustaining higher permeabilities at any given value of mudstone porosity or void ratio.

Mangrove habitat dynamics in response to Holocene sea level and climate changes along southwest coast of India

Pollen signatures, palaeobotanical data and offshore peat indicated that the west coast of India harbored an excellent cover of mangroves until the Neogene. However, the mangrove area has shrunken considerably since then, and it is now confined largely to river mouths and tidal creeks of Kerala–Konkan basin. Sea levels and geomorphologic modifications along with climate variability have brought significant changes in the hydrological regimes which in turn have altered the habitat of the mangroves considerably. Evolution of the coastal landforms, especially the modification of the wetlands and loss of sheltered bays, has affected the mangrove habitats to a greater extent that eventually led to displacement of certain core mangrove taxa. The palynological and chronological data revealed that the mangroves of southern west coast are essentially of Middle Holocene age (7220–3880 yrs BP). Such mangrove development seems to be of global significance, as Mid-Holocene establishment of mangroves has been well recorded elsewhere when sea level stabilized. Stability of mangrove development coincides with the Holocene climate optimum (HCO) when Monsoon Asia witnessed heavy precipitation. However, the mangroves responded to environmental changes due to freshwater influx controlled by monsoon rainfall and sea level changes, leading to change in species composition as revealed by their pollen signatures. The decline of mangroves since 3500 yrs BP and further degradation except at a few sites have been attributed to the prevailing arid climate and weakening of monsoon until 1500 yrs BP. Mangrove development and sustenance in fringe areas of West Kallada along the Kerala coast may be due to local hydrological regime, with steady flow of fresh water and gradual development of deltaic conditions. As such, this area has excellent potential for the rehabilitation of mangroves. Human activity resulted in probable environmental stress for mangrove cover along the North Konkan coast, while the positive trend in emergence of mangroves in the least disturbed areas of South Konkan may be attributed to strengthening of summer monsoon in the recent past.

Linking Methane Seepage to Fluid Flow Mechanisms: Evidence from AVO Characteristics of Bottom Simulating Reflectors

The presence of gas hydrates over continental margins may be inferred by various seismic indicators, including the bottom simulating reflector (BSR). Recently, the occurrence of two BSRs have been reported from many regions of the world. In this study we estimate the uncertainty in amplitude versus offset (AVO) behaviour of the single BSR and double bottom simulating reflector (DBSR) observed over two geological provinces; the Kerala-Konkan Basin, offshore India and Green Canyon, offshore USA, and attempt to infer a mechanism for the observed anomalies from the AVO patterns. Anomalous behaviour of seismic velocities within the gas hydrate stability zone (GHSZ) associated with the occurrence of DBSRs, low amplitude seismic chimneys and bright spots, indicates increased hydrate concentration and fluid venting structures underneath the DBSR locations. Such structures, if extended upward into the regional GHSZ through discrete fracture networks, may act as a passage for methane escape into the ocean. Our analysis indicates that the variability in AVO signatures for gas hydrate saturated sediments is potentially linked to the discrete zones of steeply inclined fractures that are responsible for the migration of deep gas and its escape through the seabed. Key Words : Methane hydrate; Gas dynamics; Seismic reflections; Plumbing system; Amplitude versus offset (AVO); Bottom simulating reflector

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.

Seismic velocities and quantification of gas-hydrates from AVA modeling in the western continental margin of India

The most commonly used marker for the investigation of gas-hydrates is the bottom simulating reflector (BSR), which is caused by gas-hydrate laden sediment underlain by either brine or gas-saturated sediment. A BSR has been identified by seismic experiment in the Kerala-Konkan Basin of the western continental margin of India. Here we perform AVA modeling of seismic reflection data from a BSR to investigate the seismic velocities for quantitative assessment of gas-hydrates and to understand the origin of the BSR. The result reveals a P-wave velocity of 2.245 km/s and an S-wave velocity of 0.895 km/s for the sediments above the BSR. This corresponds to a Poisson ratio of 0.406 and hydrates saturation of ∼30% in the study area. The comparison of estimated P-wave velocity (1.77 km/s) above the hydrates-bearing sediment to that (1.78 km/s) below the BSR implies that the origin of the BSR is mainly due to gas-hydrates, as the presence (even in small quantities) of free-gas reduces the P-wave velocity considerably.

Stratigraphical and palynological appraisal of the Late Quaternary mangrove deposits of the west coast of India

The organic deposits derived from the mangrove swamps form reliable stratigraphic markers within the Late Quaternary sequence of Kerala–Konkan Basin. Three generations of such deposits have been identified. The older one is dated to around 43,000–40,000 14C yr B.P., with a few dates beyond the range of radiocarbon. The younger ones date from the Middle Holocene to latest Pleistocene (10,760–4540 14C yr B.P.) and the Late Holocene (<4000 14C yr B.P.). Pollen analyses confirm that the deposits are mostly derived from the mangrove vegetation. Peat accumulation during the period 40,000–28,000 14C yr B.P. can be correlated with the excess rainfall, 40–100% greater than modern values, of the Asian summer monsoon. The low occurrence of mangrove between 22,000 and 18,000 14C yr B.P. can be attributed to the prevailing aridity and/or reduced precipitation associated worldwide with Last Glacial Maximum, because exposure surfaces and ferruginous layers are commonly found in intervals representing this period. The high rainfall of 11,000–4000 14C yr B.P. is found to be the most significant as the mangrove reached an optimum growth around 11,000 14C yr B.P. but with periods of punctuated weaker monsoons. From the present and previous studies, it has been observed that after about 5000 or 4000 14C yr B.P., the monsoons became gradually reduced leading to drying up of many of the marginal marine mangrove ecosystems. A case study of Hadi profile provided an insight to the relevance of magnetic susceptibility (χ) to record the ecological shift in Late Holocene.