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Post-stack inversion of seismic data is routinely carried out to derive acoustic impedance (AI) and, hence petrophysical properties in an area. We have been introducing here an uncommon methodology of inverting post-stack seismic data into porosity from porosity log. The post-stack inversion for estimation of direct porosity is performed by utilizing an estimated porosity wavelet, low frequency model and model based inversion. This methodology is implemented on two types of 2D post-stack seismic dataset; (a) deep water Mahanadi (MN) offshore containing gas hydrate sediments and (b) clay rich, shaly sediments in Krishna–Godavari (K–G) shallow offshore. The total porosity (φ) estimated from density log for the depth interval of 1725–2032 m ranging from 49 to 75% has been used as input for porosity inversion from the seismic data of unconsolidated sediments at 1701 m bathymetry in MN basin. The total porosity for the depth interval of 410–1494 m ranging from 5 to 45% has been used as input for porosity inversion from the 2D post-stack seismic data of shallow offshore sediments at 31 m bathymetry in K–G basin. This prediction is applied to dataset having good (for MN) and poor correlation (for K–G) between AI and porosity. In MN basin, the porosity along 2D multichannel seismic ranges from 53 to 65% in the gas hydrated zone with maximum value of 70% at the free gas filled unconsolidated sediment below the bottom simulating reflector (BSR). The water bearing silt/clay sediments indicates porosity of about 60–65% below the seafloor. In K–G basin, the porosity in Raghavapuram shale varies from 13 to 30% with maximum value of 52% is observed in Paleocene sediments. [ABSTRACT FROM AUTHOR]

The bottom-simulating reflector (BSR) is weak and patchy on the seismic section in the Krishna–Godavari basin, eastern Indian margin, where massive gas-hydrates have been recovered at Site 10 of the Indian National Gas-hydrates Program Expedition 01 (NGHP-01). The depth of the BSR near this site is around 160 m below the sea floor (mbsf). The average reflection coefficient from the BSR is −0.06, significantly smaller than the common global values of −0.1 to −0.2. The BSR shows a strong lateral variation in amplitudes along the seismic line due to the presence of faults. The methane solubility is modeled using a theoretical model of the gas-hydrates system, and methane concentrations from the pressure core show that the distribution of free gas below BSR is not uniform. A combination of synthetic seismogram analysis and rock physics modeling leads to the conclusion that weak and patchy BSRs are primarily caused by lateral discontinuities induced by the gas-filled fractures below BSRs. The free gas zone is thin and it shows segmented characteristics on the seismic section and acoustic impedance profile that we inverted. Fault zones increase the permeability and therefore trap gas in associated fractures that can scatter seismic energy and create low velocity zones. [ABSTRACT FROM AUTHOR]

Abstract: A reservoir of gas hydrates filling fractures was discovered in the Krishna–Godavari (KG) Basin during the Indian National Gas Hydrate Program (NGHP) Expedition 01 at site NGHP01-10. The existing methods for estimating gas hydrate saturation from seismic data rely on establishing an empirical relation between acoustic impedance and density porosity from well logs assuming isotropic pore-filling gas hydrate. This method, however, yields a misleading saturation for fractured clay-dominated sediments. Here we present a methodology to estimate gas hydrate saturation from seismic data based on geostatistical inversion. It integrates the verticals detail of well log data with the lateral details from seismic data to produce highly detailed estimates of gas hydrate saturation. First, gas hydrate saturation is calculated from P-wave velocities assuming anisotropic distribution at the well site. Then, probability density functions (PDFs) between acoustic impedance and the calculated gas hydrate saturation at the well site are analyzed. A Markov Chain Monte Carlo method is employed to integrate well logs with the seismic data to produce acoustic impedance. Finally, crossplots and histograms at the well site are used to estimate gas hydrate saturations along the seismic line from inverted acoustic impedance. The spatial distribution of gas hydrate varies both laterally and vertically along the line with an average saturation of 22.5%. The estimate matches reasonably with the value at the wells. [Copyright &y& Elsevier]

Gas hydrate is considered as a potential major energy resource for India. The presence of gas hydrate is inferred mainly by identifying an anomalous reflector, known as the bottom simulating reflector (BSR) on seismic sections. Understanding the properties of gas hydrate/free gas-bearing sediments and studying the amplitude-versus-offset (AVO) or amplitude versus incidence angle (AVA) characteristic from a BSR are important. The objective of the present work is to study the behaviour of reflection coefficient with angle of incidence for gas sand/shale and gas hydrate/free gas models. The amplitudes of reflected seismic waves are directly proportional to seismic reflection coefficients at an interface between different geological strata. The variation of AVO or AVA can be used in arriving at petrophysical/lithological models. The study shows variation in the reflection coefficient obtained by elastic impedance method and from Zoeppritz equations. Different gas hydrate saturations show a positive, negative or no AVO anomalies in BSR. [ABSTRACT FROM AUTHOR]

Multi-channel seismic recording was carried out along the western continental margin of India in the early nineties for the exploration of hydrocarbons. Analysis of seismic data demonstrated a characteristic reflector, which usually coincides with the predicted base of methane hydrates stability field and mimics the seafloor, known as the bottom simulating reflection (BSR) on marine seismic reflection data. Existence of reflections which mimic the seafloor, reverse polarity, seismic blank zone, strong diffraction patterns around the mud diapirs, weak amplitude blocks and pockmarks suggests that gas hydrates are present in deepwater regions. Five characteristic seismic facies associated with bottom simulating reflectors and mud diapirs were identified. In this study, we present the results of seismic surveys which indicate the existence of natural gas hydrates in the western margin of India. These results will be applied to select areas for coring (or drilling) and detailed exploration such as 2D seismic survey with long offset or 3D seismic survey in the future. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The Indian National Gas Hydrate Program Expedition 01 (NGHP-01) is designed to study the occurrence of gas hydrate along the passive continental margin of the Indian Peninsula and in the Andaman convergent margin, with special emphasis on understanding the geologic and geochemical controls on the occurrence of gas hydrate in these two diverse settings. The NGHP-01 expedition established the presence of gas hydrates in the Krishna–Godavari and Mahanadi Basins, and the Andaman Sea. The expedition discovered in the Krishna–Godavari Basin one of the thickest gas hydrate accumulations ever documented, in the Andaman Sea one of the thickest and deepest gas hydrate stability zones in the world, and established the existence of a fully developed gas hydrate petroleum system in all three basins. The primary goal of NGHP-01 was to conduct scientific ocean drilling/coring, logging, and analytical activities to assess the geologic occurrence, regional context, and characteristics of gas hydrate deposits along the continental margins of India. This was done in order to meet the long-term goal of exploiting gas hydrate as a potential energy resource in a cost effective and safe manner. During its 113.5-day voyage, the D/V JOIDES Resolution cored and/or drilled 39 holes at 21 sites (1 site in Kerala–Konkan, 15 sites in Krishna–Godavari, 4 sites in Mahanadi, and 1 site in the Andaman deep offshore area), penetrated more than 9250 m of sedimentary section, and recovered nearly 2850 m of core. Twelve holes were logged with logging-while-drilling (LWD) tools and an additional 13 holes were wireline logged. The science team utilized extensive on-board laboratory facilities to examine and prepare preliminary reports on the physical properties, geochemistry, and sedimentology of all the data collected prior to the end of the expedition. Samples were also analyzed in additional post-expedition shore-based studies conducted in leading laboratories around the world. One of the specific objectives of this expedition was to test gas hydrate formation models and constrain model parameters, especially those that account for the formation of concentrated gas hydrate accumulations. The necessary data for characterizing the occurrence of in situ gas hydrate, such as interstitial water chlorinities, core-derived gas chemistry, physical and sedimentological properties, thermal images of the recovered cores, and downhole measured logging data (LWD and/or conventional wireline log data), were obtained from most of the drill sites established during NGHP-01. Almost all of the drill sites yielded evidence for the occurrence of gas hydrate; however, the inferred in situ concentration of gas hydrate varied substantially from site to site. For the most part, the interpretation of downhole logging data, core thermal images, interstitial water analyses, and pressure core images from the sites drilled during NGHP-01 indicate that the occurrence of concentrated gas hydrate is mostly associated with the presence of fractures in the sediments, and in some limited cases, by coarser grained (mostly sand-rich) sediments. [ABSTRACT FROM AUTHOR]

Abstract: Several approaches exist for the estimation of gas hydrate and free gas depending on the nature and distribution of gas hydrate into the sediments. Here, we apply various rock physics models to the P-wave sonic velocity logs that were collected during the Indian National Gas Hydrate Program (NGHP) Expedition 01 at sites NGHP-01-05 and NGHP-01-07 for the resource estimate of gas hydrate. Using the weighted equation, it is found that maximum up to13% and 12% volumes of the pores of regional sediments above bottom simulating reflector (BSR) of sites NGHP-01-05 and NGHP-01-07 respectively are occupied by gas hydrates. Alternatively, gas hydrate saturations computed for these sites using effective medium models are found maximum up to 16% and 22%. We have further used the velocity porosity transforms to estimate the free gas saturations below the BSRs for both the sites. The saturation of free gas in the sediments at both the sites is found in the range of 0.5–2.2%. [Copyright &y& Elsevier]

Abstract: Supplies of conventional natural gas and oil are declining fast worldwide, and therefore new, unconventional forms of energy resources are needed to meet the ever-increasing demand. Amongst the many different unconventional natural resources are gas hydrates, a solid, ice-like crystalline compound of methane and water formed under specific low temperature and high pressure conditions. Gas hydrates are believed to exist in large quantities worldwide in oceanic regions of continental margins, as well as associated with permafrost regions in the Arctic. Some studies to estimate the global abundance of gas hydrate suggest that the total volume of natural gas locked up in form of gas hydrates may exceed all known conventional natural gas reserves, although large uncertainties exist in these assessments. Gas hydrates have been intensively studied in the last two decades also due to connections between climate forcing (natural and/or anthropogenic) and the potential large volumes of methane trapped in gas hydrate accumulations. The presence of gas hydrate within unconsolidated sediments of the upper few hundred meters below seafloor may also pose a geo-hazard to conventional oil and gas production. Additionally, climate variability and associated changes in pressure-temperature regimes and thus shifts in the gas hydrate stability zone may cause the occurrence of submarine slope failures. Several large-scale national gas hydrate programs exist especially in countries such as Japan, Korea, Taiwan, China, India, and New Zealand, where large demands of energy cannot be met by domestic supplies from natural resources. The past five years have seen several dedicated deep drilling expeditions and other scientific studies conducted throughout Asia and Oceania to understand gas hydrates off India, China, and Korea. This thematic set of publications is dedicated to summarize the most recent findings and results of geo-scientific studies of gas hydrates in the marginal seas and continental margin of the Asia, and Oceania region. [Copyright &y& Elsevier]

• Coefficient of best fit curve from velocity-effective stress plot in the wells. • Post-stack inversion for obtaining acoustic impendence, velocity and density section. • Estimation of vertical stress and effective stress from inverted seismic data. • Pore pressure prediction from seismic in the gas hydrate sediments, Mahanadi basin. Estimation of pore pressure and in-situ vertical stress magnitude is essential for understanding the geomechanical behavior of the gas hydrate sediments in deep offshore of the Mahanadi basin. The basin located at northern side of eastern continental margin of India (ECMI) contains gas hydrate in clay/silt sediments. The pore pressure and vertical stress are mapped on two 2D-multi channel seismic data of lines (MH-38A and MH-38B) with aid of information of three wells (namely NGHP-01-19, NGHP-01-09 and NGHP-01-08). Initially, the coefficients of best fit curve have been computed from velocity-effective stress plot for the individual wells and applied on the seismic velocity to transform into the effective stress. The vertical stress has been computed from both the seismic and log data. Therefore, the pore pressure is predicted by subtracting the effective stress from the vertical stress. The pore pressure have been mapped in gas hydrate stability zone (GHSZ) and sediments below bottom simulating reflector (BSR). The pore pressure and vertical stress gradient are 10.11 MPa/km and 10.67 MPa/km, respectively. The pore pressure and vertical stress from seismic data are closely matched at well location with excellent goodness of fit (R2) varying from 0.82 to 0.95. Normal pressure is observed in the gas hydrate bearing sediments but slightly high pressures are noticed below the BSR indicating presence of free gas. The pore pressure from seismic data will guide drillers for choosing the mud weight during well drilling and casing optimizing in other part of deep offshore in the Mahanadi basin. [ABSTRACT FROM AUTHOR]