Seismic characteristics of two deep-water drilling hazards: Shallow-water flow sands and gas hydrate

A 3D simultaneous Amplitude Variation with Angle (AVA) Inversion is applied to three common-angle aperture prestack time migrated 3D data volumes from the Garden Banks area of the Gulf of Mexico, to obtain a seismic compressional to shear velocity (V_p/V_s) ratio volume. With constraints and calibrations from two wells, high V_p/V_s anomalies (>9) inverted from conventional seismic data are used to identify potential shallow water flow (SWF) sands away from the wells. The results show that V _p/V_s values are more diagnostic than seismic amplitudes alone for SWF sand detection. Elastic seismic amplitude variation with angle data provide unreliable indicators where SWF sands are thin (where interference occurs between reflections from the layer tops and bottoms) or if viscoelasticity is significant.; A new approach based on empirical relations provides cost-effective estimates of the saturation, concentration and distribution of gas hydrate and free-gas away from the control wells. The water-filled porosity is calibrated to acoustic impedance twice; one calibration is used where gas hydrate is present, and the other, where free-gas is present. The water-filled porosity is then used to estimate gas hydrate or free-gas saturations. The method is first developed and evaluated using well logs from Ocean Drilling Program (ODP) Leg 164 from the Blake Ridge area off the east coast of North America, and then applied to a single channel seismic line from that area. The results match well with other independent estimations.; An elastic impedance inversion is applied to four time-migrated common-angle aperture (CAA) data sets. Two new algorithms are developed and evaluated using well-log data from ODP Leg 164 to estimate P-impedance and S-impedance from the elastic impedance. The V_s/V_p ratio, Poisson's ratio, and Lamé parameter terms λρ and λ/μ are estimated from the P-impedance and S-impedance. The results show that the hydrated sediments have high elastic impedance, high P-impedance, high S-impedance, high λρ, slightly higher V_s/V_p ratio, slightly lower Poisson's ratio and slightly lower λ/μ values compared to those of the surrounding unhydrated sediments. The sediments containing free-gas have low elastic impedance, low P-impedance, non-anomalous background S-impedance, high V_s/V_p ratio, low Poisson's ratio, low λρ, and low λ/μ values.