Process and program for characterising evolution of an oil reservoir over time

A process for characterising sub-surface evolution of a hydrocarbon reservoir in the process of producing by co-analyzing changes in the propagation times and seismic amplitudes of a seismic wavelet along propagation paths in the ground, the process comprising:

• providing a base seismic survey conducted on the reservoir at a first time T and comprising a set of seismic traces at the first time T associated to a first velocity Vb, the seismic traces representing an image of the sub-surface at the first time T;
• providing a monitor seismic survey conducted on the reservoir at a second time T+ΔT after a period of production of hydrocarbons from the reservoir and comprising a set of seismic traces at the second time T+ΔT associated to the same positions as in the base survey and associated to a second velocity Vm, the seismic traces representing an image of the sub-surface at the second time T+ΔT;
• for a set of points in the base survey, computing a sum S over the points of the set of a norm of the difference between
• an amplitude bi of a seismic trace in the base survey at a point i and
• a sum of an amplitude mi′ of a seismic trace at a time-corresponding point i′ in the monitor survey and an amplitude due to reflectivity change local to the time-corresponding point i′ induced by relative change in the velocity of the earth in and around the reservoir between the first time T and the second time T+ΔT;
• wherein the time-corresponding point i′ is shifted in time by a time-shift derived from velocity changes along a propagation path from a surface to the time-corresponding point i′;
• characterising the evolution of the hydrocarbon reservoir between the first time T and the second time T+ΔT by minimizing the sum S, wherein the evolution represents changes in the state of the reservoir consequent upon production of hydrocarbons from the reservoir between the first time T and the second time T+ΔT.