0016-7622/2017-90-3-261/$ 1.00 © GEOL. SOC. INDIA | DOI: 10.1007/s12594-017-0713-9 JOURNAL GEOLOGICAL SOCIETY OF INDIA Vol.90, September 2017, pp.261-266 Seismic Attenuation for Characterization of Gas Hydrate Reservoir in Krishna-Godavari Basin, Eastern Indian Margin Veligeti Jyothi 1 , Kalachand Sain 1 *, Vivekanand Pandey 1 and Ajoy K. Bhaumik 2 1 CSIR-National Geophysical Research Institute, Uppal Road, Hyderabad - 500 007, India 2 Department of Applied Geology, IIT (ISM), Dhanbad - 826 004, India *E-mail: kalachandsain@yahoo.com ABSTRACT Gas hydrates have received global attention as a possible alternative non-conventional energy resource. Hence, the detection, characterization and quantification of gas hydrates are very important for evaluating the resource potential. Presence of gas hydrates in sediments above the bottom simulating reflector or BSR is associated with low attenuation or high quality factor (Q), whereas, free gas bearing sediments below the BSR exhibit high attenuation or low seismic Q. Here the logarithm spectral ratio (LSR) method is applied to marine seismic reflection data along two cross lines (18 and 46) in the Krishna-Godavari (KG) basin in eastern Indian margin, where gas hydrates have already been established by drilling/coring. The interval Qs is calculated for three sedimentary layers (A, B, and C) bounded by the seafloor, BSR, one reflector above and another reflector below the BSR at some common depth points (CDPs) to study the attenuation characteristics of sediments across the BSR. The estimated average interval Q (160) for the hydrate bearing sediments (layer B) is much higher than the average interval Q (80) for both the loose clayey sediments (Layer A) and underlying free gas saturated sediments (layer C). This demonstrates that estimation of seismic quality factor Q can be used for characterization of gas hydrate reservoir. INTRODUCTION Gas hydrates are ice-like crystalline substance containing gases of low molecular weight (mainly methane) in a lattice of water molecules. Globally, they are found in shallow sediments of outer continental margins and permafrost regions. They are formed at high pressure and moderately low temperature when methane concentration exceeds the solubility limit. Globally, gas hydrates have been recognized mainly by seismic experiment based on an anomalous reflector, known as the bottom simulating reflector (BSR), which is the physical boundary between gas hydrates bearing sediments above and free gas saturated sediments below, and is often associated with the base of gas hydrates stability zone (GHSZ). The BSR mimics the shape of seafloor topography, exhibits opposite polarity with respect to sea floor reflection, and crosscuts underlying dipping sedimentary strata. Sediments above the BSR is also characterized by high velocity and amplitude blanking, and the sediments below the BSR shows high reflection strength and frequency shadow (Sain et al., 2000; Satyavani et al., 2008; Ojha and Sain, 2009; Satyavani and Sain, 2015). Another important characteristic property for identifying and characterizing gas hydrates bearing zone is high seismic quality factor (Q) or low attenuation, which is the frictional energy loss per cycle (Aki and Richards, 1980). Laboratory and field measurements show that Q correlates with rock property, fluid type, and degree of fluid saturation (Winkler and Nur, 1982; Sheriff and Geldert, 1995). Therefore, seismic Q is considered as a diagnostic tool for reservoir characterization, and identification of gas hydrates and underlying free gas (Toksöz et al., 1979; Petersen et al., 2007; Sain et al., 2009; Sain and Singh, 2011; Dewangan et al., 2014). There are three reasons to estimate interval Qs from seismic data (Castagna et al., 2003; Silin et al., 2006). Firstly, one can design an inverse-Q filter to improve the resolution of seismic data (Hirsche et al., 1984; Pramanik et al., 2000) allowing the interpretation of stratigraphic features in detail (Chopra and Marfurt, 2007). Secondly, attenuation forms the basis of attribute classification together with time, amplitude and frequency (Brown, 2004), and Q can be used as a direct hydrocarbon indicator (Dvorkin and Mavko, 2006; Odebeatu et al., 2006). Thirdly, estimated Q can be used for quantifying gas hydrates by establishing a relation between seismic Q and saturation of gas hydrates like the relation between seismic velocity and saturation, followed by rock physical modeling. All these have quested for a reliable method of estimating Q from seismic data. In practice, determination of seismic Q is much harder because the wave amplitude is highly sensitive to noise, scattering, receiver coupling, and interference from fine-layered boundaries. It is necessary to take extra care in preparing the data for the estimation of Q. The offshore extension of the Krishna-Godavari (KG) basin along the eastern coast of India has gained significant attention after the discovery of large quantities of gas hydrates by drilling/coring during the Expedition-01 (Collett et al., 2008, 2014; Kumar et al., 2014) and Expedition-02 (2015) of Indian National Gas Hydrates Program (NGHP). The thickness of GHSZ in deep water region of the KG basin under study is calculated as < 300 m (Sain et al., 2011), and most of the inferred BSRs occur between 200 to 300 m below the sea floor in the KG offshore (Ramana and Ramprasad, 2010). Widespread occurrences of gas hydrates have been reported from the analysis of seismic data in the KG basin (Sain et al., 2012; Sain and Gupta, 2012; Singha et al., 2014; Chatterjee et al., 2016). Here seismic Q is calculated at some common depth point (CDP) locations along two cross lines KG- 18 and KG-46 in KG basin (Fig.1), from where massive gas hydrates were recovered during the Expedition-01 by NGHP, with a view to understand the attenuation characteristics of gas hydrate- and free gas- bearing sediments. This type of study is very useful in identifying gas hydrates without any well-identified BSR on seismic section, and to ascertain whether an identified BSR is related to gas hydrates and underlying free-gas or something else. DETERMINATION OF Q IN LAYER MEDIA Attenuation of a medium is described by absorption coefficient α, or by the seismic quality factor Q. If a volume of material is cycled in a stress at a frequency ω, the Q can be expressed as: Q = 2πE (1) ΔE Where, E is the peak energy in the volume and ΔE is the energy loss in each cycle. This loss caused by the imperfect elasticity of heterogeneous materials.