Gas generation and overpressure: Effects on seismic attributes

Drilling of deep gas resources is hampered by high risk associated with une xpected overpressure zones. Knowledge of pore pressure using seismic data, as for instance from seismic-while-drilling techniques, will help producers plan the drilling process in real time to control potentially dangerous ab normal pressures. We assume a simple basin-evolution model with a constant sedimentation rate and a constant geothermal gradient. Oil/gas conversion starts at a given d epth in a reservoir volume sealed with faults whose permeability is suffici ently low so that the increase in pressure caused by gas generation greatly exceeds the dissipation of pressure by flow. Assuming a first-order kineti c reaction, with a reaction rate satisfying the Arrhenius equation, the oil /gas conversion fraction is calculated. Balancing mass and volume fractions in the pore space yields the excess pore pressure and the fluid saturation s. This excess pore pressure determines the effective pressure, which in tu rn determines the skeleton bulk moduli. If the generated gas goes into solu tion in the oil, this effect does not greatly change the depth and oil/gas conversion fraction for which the hydrostatic pressure approaches the litho static pressure. The seismic velocities versus pore pressure and differential pressure are c omputed by using a model for wave propagation in a porous medium saturated with oil and gas. Moreover, the velocities and attenuation factors versus f requency are obtained by including rock-frame/fluid viscoelastic effects to match ultrasonic experimental velocities. For the basin-evolution model us ed here, pore pressure is seismically visible when the effective pressure i s less than about 15 MPa and the oil/gas conversion is about 2.5% percent.