Seismic Characterization Of Gas Hydrates Bearing Sediments Makran Accretionary Prism Offshore Pakistan

Natural gas hydrates are composed of water and hydrocarbon gases(methane, ethane, propane) molecules that are stable at very low temperature(about 0 °C) and very high pressure. These hydrates are solid like ice and normally occur in shallow sediments of continental slope, deep inland seas and permafrost areas. The amount of methane potentially trapped in gas hydrates may be significant from 1015 to 1017 cubic meters which make it major interest to explore as an alternative energy source. These large reservoirs of methane hydrate are considered to be vital future energy source and are known to present in the marine sediments lying beneath the world’s oceans. Pakistan is facing now a day’s energy crises and it is a great need to explore unconventional energy resources to overcome these energy crises. Makran offshore area is most feasible for the exploration of gas hydrate reserves in Pakistan.The aim of this thesis is to make an integrated study, related to rock physics modeling; Seismic attributes analysis and seismic attenuation to quantify gas hydrate bearing sediments in Makran offshore area Pakistan. It is noticed that a significant amount of gas hydrates is interpreted through seismic reflection data in the form of bottom simulating reflector(BSR) present in the sediments of the convergent continental margin of PakistanHowever, the seismic character of these hydrate-bearing unconsolidated sediments is not properly investigated. Seismic velocities of the gas hydrate-bearing sediments in the study area are estimated by using the effective medium theory and the fluid substitution modeling. The results show that the presence of gas hydrates increases the stiffness of the unconsolidated sediments; whereas the presence of free gas decreases the stiffness of these sediments. It is noted that seismic velocities and density of hydrate-bearing sediments are highly affected by saturation and distribution pattern of gas hydrates. As pure gas-hydrates have much higher seismic velocities than those of host sediments, presence of gas-hydrate increases the seismic velocities, whereas free-gas below the hydrate-bearing sediments decreases the velocities. Seismic reflection from the BSR exhibits a wide range of amplitude variation with offset characteristics, which depend upon the saturation and distribution of hydrates above and free gas below the BSR. I have also demonstrated that some attributes like acoustic and shear impedances, and AVO can be used as important proxies to detect gas hydrate saturation in study area.Moreover detail AVO analysis is performed to elaborate the effect of gas hydrates saturation on seismic amplitude and it is clearly observed that seismic amplitudes negatively increased with increase in gas hydrate saturation. I applied different pairs of AVO-derived attributes to differentiate free gas and gas hydrates saturated sediments. Based on my analysis, it is concluded that low and high saturation of gas hydrates can be deduced from seismic amplitude anomalies. It is also observed low and high saturation of gas hydrates can be distinguishable from background trend. I found that the fluid factor, pore space modulus and Poisson reflectivity are more sensitive attributes for discrimination of gas hydrates saturation.Since Seismic attenuation and velocity dispersion are important phenomenon to identify gas hydrates reserves, I applied wave induced fluid flow technique to compute attenuation and velocity dispersion response for gas hydrate and free gas saturated sediments, especially when gas hydrates were part of fluid. I considered Gassmann Wood, and Gassmann Hill approach to depict velocity dispersion and attenuation at low and high frequency regime. It is elaborated that seismic attenuation is inter-related to difference of seismic and elastic properties response at low and high frequency regime, and it will help us to delineate medially saturated gas hydrates reserves and low gas saturated zone.The major emphasis in this study is to elaborate gas hydrates occurrence pattern with rock physics modeling approaches for Makran offshore area Pakistan. After analyzing theoretical models, I overlaid theoretical model compressional wave velocity response with well log velocity for required target zone. I analyzed that my rock physics model velocity response, when gas hydrates are part of fluid shows good coupling with the well log velocity response. As my results show good validation that gas hydrates are parts of fluid in this area, so I relied my further forward modeling related to AVO analysis and seismic attenuation on it. My work is an integration of theoretical and practical work within constrain of data limitation as some data were unavailable due to strict policies of companies and countries.