Appraisal of gas hydrate resources based on a P- and S-impedance reflectivity template: Case study from the deep sea sediments in Iran

Abstract

The occurrence of a bottom simulating reflector (BSR) in the 2D seismic data from Makran's accretionary prism reveals the presence of gas hydrate and free gas several hundred meters below the seafloor of Iran's deep sea. According to the global distribution of marine hydrates, they are widely present in deep sea sediments, where high operational costs and hazards cause a lack of well log information. Therefore, developing a method to quantify the hydrate resources with seismic data is an ultimate goal for unexplored regions. In this study, the so-called reflectivity templates (RTs) are introduced for quantification of the hydrate and free gas near the BSR. These RTs are intuitive crossplots of P-impedance and S-impedance contrasts across the BSR. They are calculated theoretically based on the effective medium theory for different hydrate distribution modes with some assumptions on porosity and mineralogical composition of unconsolidated sediments. This technique suggests the possibility of using the amplitude variation versus offset (AVO) analysis of the BSR for a quantitative interpretation when well log data are not available. By superimposing the AVO-derived P-impedance and S-impedance contrasts across the BSR on these RTs, the saturations of the hydrate and free gas near the BSR could be estimated. Validation of this approach by synthetic data showed that a reliable quantification could be achieved if the model parameters were rearranged to a form in which the AVO inversion was independent of the S-wave to P-wave velocity-ratio assumption. Based on this approach applied on the 2D marine pre-stack time migrated seismic line in offshore Iran, 4% to 28% of the gas hydrate and 1% to 2% of the free gas are expected to be accumulated near the thrusted-ridge and thrusted-footwall types of BSRs. © 2013 Sinopec Geophysical Research Institute.