Pore pressure prediction in gas-hydrate bearing sediments of Krishna–Godavari basin, India

Abstract

Predrill estimate of pore pressure (PP) is important for well design especially in areas containing gas hydrate bearing sediments. Here we report the pressure distribution in the gas hydrate bearing sediments in Krishna–Godavari (K–G) basin predicted from the seismic data along two lines passing through the site 10 of Expedition-01 (Exp-01) conducted by the Indian National Gas Hydrate Programme (NGHP). A series of elastic parameters namely P-wave velocity (Vp), S-wave velocity (Vs), density (ρ), Vp/Vs, P impedance (Zp) and S impedance (Zs) were derived from seismic data using pre-stack inversion. First the PP estimated from a sonic transit time log was calibrated with pressure core measurements in the depth interval of 1060–1280 m. Its magnitude is found to be ranging from 10.5 to 12.9 MPa where the vertical stress varies from 10.7 to 13.4 MPa within the same depth interval. This estimated PP was next treated as a target log (desired output) and seismic attributes resulting from pre-stack seismic inversion were used as input parameters in the training of a multi-layered feedforward network (MLFN). Finally we use the trained network to generate a subsurface PP distribution map along two 2D multichannel seismic profiles (line-X and line-Y) passing through NGHP01-10 within a time interval of 1420–1620 ms corresponding to a depth interval of 1060–1280 m. This corresponds to sedimentary column from 22 to 242 mbsf. Our results demonstrate that the pressure remains hydrostatic within the gas hydrate bearing sediments and is mostly above-hydrostatic below the gas hydrate bearing zone.