An Inverse Approach for Relating Seismic Velocity and Overpressure to Permeability and Sedimentation History in Deep Basins

Moreno, Hector M.¹; Minshull, Tim A.¹; Edwards, Rosemary A.¹
(1)Marine Geoscience, National Oceanography Centre, Southampton, United Kingdom.

Pore pressures exceeding hydrostatic (“overpressures”) can play an important role in driving fluid flow in the subsurface. Sediments become overpressure mainly by three mechanisms: disequilibrium compaction, because pore fluids cannot be expelled rapidly enough during ongoing sedimentation and compaction, chemical reactions, including diagenesis reactions which release water (smectite converting to illite) and oil and gas formation from kerogen which implies a volumetric increase of the products and thermal expansion of water. In deep basins and thickly sedimented continental margins, extreme overpressures can develop and can play a decisive part in basin evolution and hydrocarbon migration. A range of analytical and numerical techniques have been developed to model patterns of overpressure. Overpressures can also be estimated from remote geophysical observations such as P and S wave velocities, but the resulting estimates can have large uncertainties. These two separate approaches have rarely been brought together. Here, we bring them together by developing a method for estimating overpressure from a disequilibrium compaction mechanism in deep sedimentary basins that satisfies both the geophysical observations and the constraints imposed by physical principles and sedimentation history. The 1D inverse algorithm minimizes the misfit between the observed and the predicted P wave velocity. We have developed a 1D forward numerical method for the calculation of pore pressure in a column of sediment using Athy’s law for the porosity evolution with depth and an empirical law for the permeability as a function of porosity. Model output has been used to estimate the P wave velocity in the sediment column by applying a rock physics model. Then, considering a trial P wave profile and the calculated P wave velocity profile, the proposed inverse method has been applied. The methology has been implemented in a series of synthetic problems allowing a good correlation between the permeability and sedimentation rate inputs in the forward method and the outputs from the inverse method. This inverse approach will be applied to data from the Eastern Black Sea Basin where the P and S wave velocity structure are well known from wide-angle seismic work and the lithology and sedimentation history are constrained by nearby exploration boreholes. The inversion algorithm is a step forward towards a more generalized 2D implementation including other overpressure mechanisms.

AAPG Search and Discovery Article #90135©2011 AAPG International Conference and Exhibition, Milan, Italy, 23-26 October 2011.