Krishna Godavari Research Articles

Assessment of Groundwater Quality Index, Suitability for Irrigation and Domestic Purposes in Krishna-Godavari Delta Region, Southern India

The water quality index (WQI) is a significant indicator for evaluating the quality of drinking water for end-users. The present work is aimed to assess the groundwater quality of the Krishna Godavari delta region in Andhra Pradesh State, for knowing its suitability for domestic, irrigation, and drinking purposes by calculating the WQI parameters and comparing it with WHO and Indian Standards. A total of 105 groundwater samples each for both Pre-monsoon and Post-monsoon periods have been collected during June 2017 and January 2018. The following 11 parameters were taken into account when calculating the WQI: pH, Electrical Conductivity (EC), Total Hardness (TH), Calcium (Ca2+), Magnesium (mg2+), Bicarbonate, Chloride(Cl-), Nitrate (Na+), Sulphate, Potassium (K+), and Total Dissolved Solids (TDS). Based on the WQI result, the samples were divided into five categories: Excellent, Good, Poor, Very Poor, and unfit for drinking purposes. In Pre-monsoon and Post monsoon season, the water quality index is 9%, 25%, 30%, 22%, 15% respectively 16%, 41%, 31%, 7%, and 5%. It can be concluded that during pre-monsoon and post-monsoon season 67% and 43% of water is not suitable for drinking purposes respectively and hence they require treatment before usage.

Organic petrology and geochemistry of Early Permian shales from the Krishna‐Godavari Basin, India: Implications for Gondwana palaeoenvironment and climate

Lower Gondwana Kommugudem Formation of the Krishna‐Godavari (KG) Basin in the eastern coast of India records the deglaciation sequences of the Early Permian and is characterized by well‐preserved sandstone—shale—coal cyclothems. Kommugudem shales from six deep borewells of the KG Basin were studied for the organic petrological and geochemical compositions. The objective was to elucidate its source rock properties and depositional settings, which provide significant insights on the shifts in the Gondwanan palaeoenvironment and its climatic controls during the Early Permian. Maceral identification reveals the abundance of vitrinite that is distinctive to organic‐rich Permian Gondwana sediments. SiO2 enrichment with the dominance of quartz and clay minerals suggests moderate to strong weathering with palaeoclimate shift from cold, arid to hot, and semi‐arid during the Early Permian. The occurrence of higher short and mid‐chain alkanes with predominant n‐C18 and important biomarker ratios suggest a transitional fluvio‐marine environment of deposition, which is also supported by the enrichment of redox‐sensitive trace elements. Prevailing anoxia enhanced the preservation of organic matter, despite reduced primary productivity, as indicated by low P/Ti ratios. A depleted δ13Corg (mean = −21.55‰) in the shales, distinct to the interglacial period, reflects the changes in pCO2 of the Early Permian atmosphere. Sedimentary biomarkers, carbon isotopes, and trace elements explicate the palaeoceanographic conditions and sea‐level changes, indicative of marine influence in the dominantly fluvial regime. The deposition and preservation of organic‐rich sediments under anoxic (possibly euxinic) water‐column took place, forming the prolific Pre‐trappean Lower Gondwana Kommugudem source rock in the KG Basin.

Benthic Foraminiferal Response to the Millennial-Scale Variations in Monsoon-Driven Productivity and Deep-Water Oxygenation in the Western Bay of Bengal During the Last 45 ka

In this study, we presented a high-resolution benthic foraminiferal assemblage record from the western Bay of Bengal (BoB) (off Krishna–Godavari Basin) showing millennial-scale variations during the last 45 ka. We studied temporal variations in benthic foraminiferal assemblages (relative abundances of ecologically sensitive groups/species, microhabitat categories, and morphogroups) to infer past changes in sea bottom environment and to understand how monsoon induced primary productivity-driven organic matter export flux and externally sourced deep-water masses impacted the deep-sea environment at the core site. Our records reveal a strong coupling between surface productivity and benthic environment on glacial/interglacial and millennial scale in concert with Northern Hemisphere climate events. Faunal data suggest a relatively oxic environment when the organic matter flux to the sea floor was low due to low primary production during intensified summer monsoon attributing surface water stratification and less nutrient availability in the mixed layer. Furthermore, records of oxygen-sensitive benthic taxa (low-oxygen vs. high-oxygen benthics) indicate that changes in deep-water circulation combined with the primary productivity-driven organic matter flux modulated the sea bottom oxygen condition over the last 45 ka. We suggest that the bottom water at the core site was well-ventilated during the Holocene (except for the period since 3 ka) compared with the late glacial period. At the millennial timescale, our faunal proxy records suggest relatively oxygen-poor condition at the sea floor during the intervals corresponding to the cold stadials and North Atlantic Heinrich events (H1, H2, H3, and H4) compared with the Dansgaard/Oeschger (D-O) warm interstadials. The study further reveals oxygen-poor bottom waters during the last glacial maximum (LGM, 19–22 ka) which is more pronounced during 21–22 ka. A major shift in sea bottom condition from an oxygenated bottom water during the warm Bølling–Allerød (B/A) (between 13 and 15 ka) to the oxygen-depleted condition during the cold Younger Dryas (YD) period (between 10.5 and 13 ka) is noticed. It is likely that the enhanced inflow of North Atlantic Deep Water (NADW) to BoB would have ventilated bottom waters at the core site during the Holocene, B/A event, and probably during the D-O interstadials of marine isotope stage (MIS) 3.

Impact of sedimentary environment on pore parameters of thermally mature Permian shale: A study from Kommugudem Formation of Krishna Godavari Basin, India

The pores are significant factors to assess the fracture system of a shale reservoir. The present study evaluates organic matter-associated parameters, redox condition, paleoclimate and paleosalinity and their role on the pore structure of thermally mature Kommugudem shale of Krishna Godavari Basin. The XRD analysis supported by FE-SEM mainly indicates silica-dominated lithotype and clay-rich siliceous mudstone in the Kommugudem Formation. Intercrystalline and intracrystalline pores were observed in association with mineral and organic matter that provide storage sites for hydrocarbon. Excellent TOC content and genesis of gas prone, type III kerogen dominantly observed in admixture with type II or type IV kerogen. The clay, TOC and Tmax have a positive correlation with micropore volume indicating a good contribution of clay micropore and associated organic matter. Fractal dimensions were estimated using the fractal FHH method with an average value of 2.65 suggesting that the pore surfaces are rough and exhibit complex and heterogeneous pore structure. The Kommugudem Formation constitutes detrital sediments sourced from felsic volcanic rocks in humid and warm climate along the passive tectonic continental margin. The redox and paleosaline investigations reveal the fluvio-marine transitional environment preserved under suboxic to oxic conditions. The REEs and trace elements are positively correlated with pore volume indicating their possible role in the pore genesis of silicate minerals and corroborating the positive influence of the environment. The redox proxies are strongly correlated with TOC and pore volume supports the probable development of organopores. The investigation reveals the significant role of multiproxy paleoenvironmental indices to understand the complex mechanism of pore characteristics.

Comprehensive Rock Physics, Petrophysics workflow to correct Sonic Logs for improved seismic QI: Study using wells of Krishna Godavari basin

Rock physics analysis uses well log data as direct measurement of physical properties of the subsurface formations. Hence, in seismic reservoir characterisation they do not suffer the same level of scrutiny as seismic. Mud-filtrate invasion during drilling contaminates porous formation affecting log responses in non-intuitive ways. Ignoring invasion effect on well logs prior to quantitative petrophysical interpretations may led to erroneous reserve estimates as well as well-to-seismic misties. In addition, sonic logs are prone to ambiguity incorporated by weak waveform generation and non-unique coherence picking. This gives rise to non-unique compressional and shear slowness data. To correct the invasion affected logs, we have designed a workflow that effectively improves the recorded data. This workflow uses modified Gassmann (1951) fluid replacement modelling (FRM) method for sonic data correction. In addition, following an improved coherence picking we get a further updated sonic slowness. Using the corrected data in synthetic seismic generation enables us to achieve substantial improvements in synthetic-seismic correlation values. Furthermore, use of updated data results in a better three way matching with vertical seismic profiling (VSP) data.

Natural Gas Hydrates in the Krishna-Godavari Basin Sediments under Marine Reservoir Conditions: Thermodynamics and Dissociation Kinetics using Thermal Stimulation

The gas hydrate deposits of the Krishna-Godavari (KG) basin in India have a huge potential of fulfilling the energy needs of the country for several years to come. We present a detailed thermodynamic and kinetic study of methane hydrate formation and dissociation in the presence of an actual Krishna-Godavari (KG) basin marine sediment sample at varying concentrations, i.e., 10, 20, and 35 wt %, in an aqueous solution. This study revealed the possibility of dual induction during methane hydrate formation in the presence of pure water and 10 wt % KG basin sediment solution. As the sediment concentration increased, the inhibition effect during the methane hydrate formation became more pronounced. The heat of dissociation values were found to be decreasing with the increase in sediment concentration. The kinetic experiments depicted the stochastic nature during the hydrate formation. The methane gas consumption and the rate of methane hydrate formation were found to be high in the case of pure water, followed by 10, 20, and 35 wt % sediment solutions. However, the initial rate of methane hydrate formation among the sediment solutions was observed to be high in the case of the 20 wt % sediment solution. The water-to-hydrate (W-H) conversion and gas-to-hydrate (G-H) conversion were found to be reducing with an increase in sediment concentration due to the confining effect of the water–gas system. During kinetic dissociation, the stable temperature range before the hydrates actually started dissociating was found to be increasing with sediment concentration, and the maximum temperature up to which the hydrates remained stable was ∼6 °C. The rate of hydrate dissociation was high in the case of the pure water system and slightly decreased in the 10 wt % sediment solution; however, it was found to be almost similar in 20 and 35 wt % sediment solutions. This study will certainly add value toward a better understanding of the KG basin reservoir for future exploration of methane gas from the hydrate deposits.

Gas hydrate/free gas migration pathways in submarine slope failures: East Indian Margin

Gas hydrates from the continental margin settings are mostly confined to continental slopes. The free gas occurring below the gas hydrate laden sediments play a crucial role in the submarine slope failures and can potentially trigger the submarine landslides. The present study is carried out to investigate such a scenario in the shale dominated Krishna–Godavari (KG) basin having proven gas hydrate deposits. Shale reservoirs are less permeable compared to sandstone reservoirs and as such, the free gas that is locked in the pore spaces of shales would eventually create hydro fractures or faults to escape from the pore spaces. This overpressure from the gas zone will find a pathway to reach the shallower depths. In the present study, two faults are identified of which, one is acting as a potential pathway for overpressures to travel through it, while the other is found to be critically pressurized due to the gas zone below it. The calculated gas column height is ~51 m and any further increase in the gas column height would create a fault slip and cause the mechanical failure of the sediments, creating another potential pathway. We infer that these faults are acting as a migration pathway in our study area and the advection of fluid/gas from the gas zone via the pathway might have played a major role in creating a slope break above the fault.

Genome sequencing and analysis of a psychrotrophic methanogen Methanosarcina sp. nov. MSH10X1 cultured from methane hydrate deposits of Krishna Godavari Basin of India.

Methanosarcina sp. strain MSH10X1, a psychrotrophic methanogen, was isolated from sub-seafloor methane hydrate deposits of Krishna Godavari Basin on India's east coast. The strain could grow from 5 to 40°C following all three i.e. methylotrophic, acetoclastic, and hydrogenotrophic modes of methanogenesis utilizing different substrates like methanol, trimethylamine, H2/CO2 (80/20), acetate, valerate, isobutyrate, isopropanol, and isobutanol. The genome sequencing and analysis of this strain revealed a circular chromosome of 3,557,383bp length having 42.47mol%G+C content, which consisted of 3110 coding genes, 58 tRNA genes, and 3 rRNA operons. The KEGG analysis highlighted the presence of genes responsible for all three modes of methanogenesis. The presence of genes like mtaB, mtaC, and mttB in the genome provided evidence for possible adaptation of strain MSH10X1 in the deep sea's low-temperature conditions.

Mineralogical and structural characterization of the sediments of Krishna Godavari and Mahanadi Basin and their influences on hydrate formation kinetics

Structural and mineralogical characterization of sediment samples of Krishna Godavari (KG) and Mahanadi (MH) basin was performed with the aim of providing in-depth analysis of the effect of mineralogy, grain size and other properties on the hydrate formation. Bulk mineralogy and clay mineralogy of the sediments analyzed through different characterization techniques like X-Ray Diffraction (XRD) and Field Emission Scanning Electron Microscopy (FESEM) that was associated with Energy Dispersive X-Ray (EDX) analysis. Quartz was the main component observed in the sediment samples. A relation between particle size and pore size was also established through particle size (DLS) and pore size characterization (BET) techniques. Finer clay materials were leading in both KG and MH basin samples having slight variation in their quartz content. Illite and smectite were the dominating clay minerals found in the sediments with calcite and albite being the other contributing minerals. Framboids as the pyrites were also observed in sediments and were clearly visible in FESEM observations. Hydrate formation studies were also performed in the sediments to observe the effect of their sedimentology and geology on the occurrence of hydrates in oceanic sediments. Hydrate formation in sediments occurred in less time as compared to pure water system with an accelerated rate of nucleation as per the observations. Kinetic studies were also performed that showed the higher initial rate of hydrate formation in sediments that could be due to the high surface area and additional nucleation sites provided by the sediments as confirmed by BET analysis. Rate constants and other kinetic parameters were also calculated by assuming the first order reaction kinetics of hydrate formation.

Multi-attribute and Artificial Neural Network Analysis of Seismic Inferred Chimney-like Features in Marine Sediments: A Study from KG Basin, India

Abstract In marine seismic sections, features like “chimneys”, characterized by low similarity chaotic zones or vertical disordered characteristics arise due to migration of gas. Numerous interpretation tools have been developed for the identification of chimney features from seismic data. Here, a study over a 2D seismic reflection profile, acquired by CSIR-National Geophysical Research Institute (NGRI), Hyderabad in the Krishna-Godavari (KG) basin in 2010 was carried out. The data is optimally conditioned or post-processed before extracting the attributes such as the similarity, dip variance, energy, and frequency washout, to capture the chimneys on seismic data. Finally, the attributes are combined and trained over interpreter’s acquaintances through a non-linear multi-layer perceptron (MLP) neural network to compute a hybrid attribute, defined as the chimney meta-attribute. The neural training results into an overall normalized root mean squared (RMS) error of 0.6 to 0.7 between the train and test data sets. Three chimneys of ~1.5, ~0.9, ~1.3 km heights are observed over the seismic line. As chimneys represent gas migration paths, the computation of chimney attribute helps in understanding the genesis of gas-hydrates not only in the KG basin, studied here, but also in any other prospective basin.

Subsurface porosity estimation: A case study from the Krishna Godavari offshore basin, eastern Indian margin

Porosity is one of the fundamental petrophysical parameters essential for appraisal of hydrocarbon reserve and planning production operations. Several interpretation techniques have evolved over time that allows geologists to meticulously estimate reservoir properties by integrating seismic and borehole data. The present study investigates the use of Multi-Attribute Transform Method (MATM) and Artificial Neural Networks (ANN) for the delineation of lateral variation of porosity along a 2D seismic reflection dataset in the gas hydrate inferred area of Krishna–Godavari basin, eastern Indian margin. The best set of attributes that can predict the porosity are derived using multi-attribute regression analysis. These attributes are applied in MATM and are also used to train the neural networks. This is achieved using different networks of neural computation like Probabilistic Neural Network (PNN), Multi-Layer Feedforward Network (MLFN) and Radial Basis Function (RBFN) with an aim to decipher an optimized approach best suits to predict porosity of the study area. The correlation between observed and predicted porosity logs are tested both by cascading and non-cascading the networks with the MATM. The predictive power is found improved as we progress from MATM to neural network cascaded with the trend of MATM. Finally, it is observed that PNN cascaded with the results of MATM shows the least prediction error than the other networks.

Controls of mass transport deposit and magnetic mineral diagenesis on the sediment magnetic record from the Bay of Bengal

We conducted rock magnetic, mineralogical, sedimentological and geochemical analyses on a sediment core (MD161/Stn-11) retrieved from a complex marine sedimentary system of Krishna-Godavari (K-G) basin to delineate the control of mass transport deposits (MTD's) and methane-induced diagenesis on the sediment magnetic record. Four sediment magnetic zones (Z-I, Z-II, Z-III, Z-IV) were defined based on rock magnetic signatures. The sediment magnetic signal is mainly carried by complex magnetic mineral assemblages of detrital (titanomagnetite, titanohematite) and diagenetic (pyrite) minerals. Changes in rock magnetic properties are mainly controlled by fluctuations in supply of detrital magnetic particles, onset of MTD's and differential rate of methane-influenced magnetic minerals diagenesis in the studied sediment core. Downcore reduction in magnetic susceptibility followed by subsequent precipitation of iron sulfides within sediment magnetic zone (Z-I) representing the period of normal sedimentation can be attributed to diagenetic dissolution caused by anaerobic oxidation of methane coupled to sulfate reduction. Decline in magnetic susceptibility and increase in sediment grain size within MTD-rich sediment intervals (Z-II, Z-III, Z-IV) is linked to loss of finer magnetic grains due to diagenetic dissolution and dilution caused by increase in concentration of diamagnetic minerals. Lower values of magnetic grain size diagnostic (ARM/SIRM) parameter indicate loss of finer and selective retention of coarser magnetic particles due to diagenetic dissolution beyond 12 mbsf. Elevated content of total organic carbon (TOC) content in Z-III and Z-IV can be attributed to the efficient preservation of labile organic matter due to rapid sediment deposition. A conceptual model is presented to explain the control of mass transport deposit and magnetic mineral diagenesis on the sediment magnetic record.

India‐Elan Bank‐East Antarctica Breakup, Crustal Architecture, and Margin Evolution: Results From Constrained Potential Field and Process‐Oriented Gravity Modeling of Conjugate Margin Segments

AbstractWe present here the results of a seismically well‐constrained potential field modeling along with process‐oriented gravity modeling across the Eastern Continental Margin of India (ECMI) and its conjugate East Antarctica Continental Margin (EACM). The study revealed both along‐strike variations in crustal architecture and effective elastic thickness (Te), showing a locally asymmetric pattern. Crustal stretching factors (β) estimated at these margins range between 1.5 and 4.0 with slightly lower values varying over the narrow zone in the Cauvery basin‐western Enderby basin and relatively higher values in other margin segments. The models reveal typical magma‐poor margin characteristics in the Krishna‐Godavari (K‐G) and Cauvery basin segments of ECMI. The ocean‐continent transition (OCT) in the K‐G basin and its conjugate (central Enderby basin) indicates asymmetric rifting marked by the development of a detachment fault separating the combined India‐Elan Bank margin and the EACM. The higher β values and magmatic underplating at OCT in the Mahanadi basin segment of ECMI may be related to the post‐breakup magmatic emplacement associated with the proximal activity of the Kerguelen plume after the separation of Elan Bank from India by a ridge jump during M2/M0 (124–120 Ma). The pre‐rift reconstruction model through the palinspastic restoration of margins suggests that the central part of ECMI (16–20°) is the ideal pre‐rift location for the Elan bank.

An ANN-based identification of geological features using multi-attributes: a case study from Krishna-Godavari basin, India

Artificial neural network (ANN)–based workflow has been applied to the multi-channel seismic reflection data from the Krishna-Godavari (K-G) basin on the Eastern continental margin of India for delineation of prominent structural features such as chimneys and faults. To eliminate the noise and enhance the data quality, the seismic data is conditioned and filtered by data-conditioning techniques such as dip-steered and structural filters. A single attribute is not sufficient for the delineation of structures such as faults and chimneys; hence, an amalgamation of multiple attributes is necessary for the enhancement of laterally continuous seismic events. Hence, multi-attributes such as dip variance, curvature, coherency, energy, similarity, instantaneous frequency, and frequency washout ratio are extracted from the conditioned data which were combined and fed as inputs for the ANN for the chimney and fault detection. The neural network is trained at the identified chimney/non-chimney and fault/non-fault locations, which generates a final output, namely chimney probability attribute and a fault probability attribute, revealing an improved visibility of faults and chimneys in the seismic data. This multi-attribute approach shows more reliability in comparison with individual attribute responses, which helps in better structural interpretation. The study presents a workflow for better visualization of the subsurface structural features which thereby helps in detailed structural interpretation of the seismic data.

Angiosperms from the Early Cretaceous sediments of India

This study presents the first report of angiosperm macrofossil assemblage from the Early Cretaceous sediments of India, containing a fruit, a spike, a petal, leaves, and an axis from the Krishna Godavari Basin. This assemblage provides clues to angiosperm evolution and ecology during the Early Cretaceous of India. The described enigmatic forms are comparable to fruits of Trapa, and palm leaves and spikes of Potamogeton. The fossil material also includes ribbon-like leaves with a small axis; fossil remains suggest affinity with the monocotyledon group and support recent morphological and molecular phylogenetic studies to establish the divergence of this group from dicotyledons, during the early Cretaceous. Fossils described in the present study suggest an affinity for an aquatic environment that appears to be ideal for some early angiosperms.

Synthesized luminescence ages of palaeo-beach ridges in Krishna–Godavari twin delta plain, east coast of India

Abstract Luminescence ages of eight sandy beach-ridge samples from the Krishna–Godavari delta region on the east coast of India were obtained by synthesizing ages based on three luminescence signals, i.e., optically stimulated luminescence (OSL) from quartz, infrared stimulated luminescence (IRSL), and post-infrared (pIR) IRSL290°C from feldspar. Standard statistical procedures and hierarchical Bayesian modeling (ChronoModel) were used to synthesize the ages that are consistent within an uncertainty of ±10% and conforming to most of the published radiocarbon (AMS 14C) ages from this region. This approach, therefore, provides for the conjunctive use of luminescence signals of quartz and feldspar for a single and possibly more accurate age. Additionally, our study showed the potential that the reworked sediment could be identified, and pristine beach ridge sand grains could be isolated using single grain quartz luminescence measurements using appropriate statistical models. Further methodological development may improve the precision of these ages, if not the accuracy.

Amount of gas hydrate estimated from rock physics analysis based on morphology and intrinsic anisotropy in area B, Krishna Godavari offshore basin, expedition NGHP-02

India's National Gas Hydrate Program (NGHP) Expedition-02 was conducted in 2015 to investigate the prospective locations of sand-rich gas hydrate reservoirs along the eastern offshore, India. We select Area-B among four areas A, B, C and E of expedition NGHP-02 as it was completely unexplored during NGHP-01. Logging while Drilling (LWD) data were collected at twelve sites in area B in Krishna Godavari (KG) offshore basin, where a huge amount of gas hydrate (up to 90% of the pore volume) is present as pore-fill, fracture-fill and both pore- and fracture-filled (mixed) morphology. The resistivity log generally overestimates gas hydrate saturation because of the increase of resistivity of fresh water near gas hydrate accumulation and also in the presence of fractured-filled gas hydrate. Though gas hydrate saturation obtained from pressure cores are assumed to be ground truth but they are discrete. Hence, the application of an advanced rock physics model is required to obtain continuous quantification of gas hydrate throughout the depth considering the gas hydrate morphology and microstructure of sediment. We apply a combination of self-consistent approximation (SCA) and differential effective medium (DEM) theories to model biconnected marine gas hydrate reservoir that can deal with inherent anisotropy due to the presence of clay platelets and pore-filling, fracture-filling or mixture of both morphologies of gas hydrate. First, we determine the distribution pattern of clay platelets from the observed background velocity (water-saturated sediment) and then estimate the gas hydrate saturation at four holes of area B, where pressure core measurements are available. We predict maximum gas hydrate saturation as 50–90% of the pore space above the BSR in this area. Comparison with other rock physics model shows that prediction from the combined SCA-DEM theory matches better with the pressure core measurements.

Widespread occurrence of methane seeps in deep-water regions of Krishna-Godavari basin, Bay of Bengal

Drilling/coring activities during NGHP-Expeditions 01 and 02 discovered significant gas hydrate deposits in the Krishna-Godavari (KG) basin, a proven petroliferous basin located along the eastern continental margin of India. Active and paleo-cold seeps are also reported from deep waters (>1000 m), indicating methane venting episodes in the KG basin. In the present study, we analyzed geophysical data acquired onboard RV Sindhu Sadhana to study the spatial distribution of gas flares in the KG offshore basin. Twenty-two gas flares were observed in water-column images at seafloor depths ranging from 638 m to 1960 m. All flares except one are detected within the gas hydrate stability zone (GHSZ), which occurs beyond 700–720 m water depths. Most gas flares seem to terminate at these depths, indicating that the bubbles might be coated with a thin methane hydrate skin. Seabed sampling of flare regions shows the presence of chemosynthetic communities and shallow gas hydrates within 2–4 mbsf.The spatial distribution of gas flares is not random and tends to align with the toe-thrust zones and shale diapiric mounds. These compressive structures formed due to shale tectonism in the KG basin have provided an environment conducive for the formation of gas hydrate deposits and gas flares/cold seeps. The analysis of high-resolution seismic data across the flares shows the presence of seismic chimneys and faults, which facilitated migration of deep-seated fluid/gas through the GHSZ. Focused fluid flow along the fault zone and the perturbation of methane + seawater phase curve due to salinity increase owing to the continuous formation of methane hydrate are the most likely mechanisms to explain methane gas migration.

Post-breakup deformations in the Bay of Bengal: Response of crustal strata to the sediment load

Passive continental margins are tectonically inactive, but a few of them including the East India Passive Margin (EIPM) show evidences for post-breakup deformations. This unusual process prompted us to investigate the post-rift deformations on EIPM and adjacent deep-water region for understanding the possible mechanisms. Seismic reflection images of the Bay of Bengal reveal a post-rift deformation with a manifestation of extensional faults in Krishna–Godavari (K–G) basin and on flanks of the 85°E Ridge. In K–G basin, one of the rift-related faults reactivated during the Early Miocene time (~16 Ma), while on flanks of the 85°E Ridge new normal faults originated at about 6.8 Ma. From detailed analysis of fault throws, it is observed that the fault in K–G basin recorded a cumulative throw of about 900 m between the basement and Early Miocene horizon (~16 Ma), later the fault was reactivated at 6.8 Ma and continued the activity progressively until 0.3 Ma before cessation. The fault system on the margin spatially extends for about 300 km between offshore extensions of the Pranahita–Godavari graben and Nagavali–Vamshadhara shear zone. The faults on 85°E Ridge, initiated at 6.8 Ma and continued until 0.8 Ma, have cumulative throws of about 60 and 110 m on western and eastern flanks of the ridge, respectively. Back-stripping analysis of the fault from the K–G basin discloses two distinct phases of subsidence history: (i) during the first phase (120–23 Ma) the basement subsided at a rate of 46–18 m/Myr due to thermal cooling of the lithosphere, (ii) during the second phase (23 Ma–Present) rapid subsidence rate (69.56 m/Myr) of basement is noticed as a consequence of deposition of copious amounts of Bengal Fan sediments. The thick sedimentary strata exerted vertical load on underlying heterogeneous crust that led to build excessive internal stress and release through weak zones (lying at intersecting planes of heterogeneous crustal blocks). The stress, thus released through fault planes has caused the deformation of crust as well as overlying sedimentary strata.

Effective medium theory and multiple linear regression‐based velocity estimation in syn‐rift clastic sequence of the Krishna–Godavari basin, India

ABSTRACTAn inclusion model, based on the Kuster–Toksöz effective medium theory along with Gassmann theory, is tested to forward model velocities for fluid‐saturated rocks. A simulated annealing algorithm, along with the inclusion model, effectively inverts measured compressional velocity (VP) to achieve an effective pore aspect ratio at each depth in a depth variant manner, continuously along with depth. Early Cretaceous syn‐rift clastic sediments at two different depth intervals from two wells [well A (2160–2274 m) and well B (5222–5303 m)], in the Krishna–Godavari basin, India, are used for this study. Shear velocity (VS) estimated using modelled pore aspect ratio offers a high correlation coefficient (>0.95 for both the wells) with measured data. The modelled pore aspect ratio distribution suggests the decrease in pore aspect ratio for the deeper interval, mainly due to increased effective vertical stress. The pore aspect ratio analysis in relation to total porosity and volume of clay reveals that the clay volume has insignificant influence in shaping the pore geometry in the studied intervals. An approach based on multiple linear regression method effectively predicts velocity as a linear function of total porosity, the volume of clay and the modelled pore‐space aspect ratio of the rock. We achieved a significant match between measured and predicted velocities. The correlation coefficients between measured and modelled velocities are considerably high (approximately 0.85 and 0.8, for VP and VS, respectively). This process indicates the possible influence of pore geometry along with total porosity and volume of clay on velocity.