Inverse modelling and seismic data constraints on overpressure generation by disequilibrium compaction and aquathermal pressuring: application to the Eastern Black Sea Basin

Abstract

SUMMARY Pore pressure above the hydrostatic (overpressure) is common in deep basins. It plays an important role in pore fluid migration, represent a significant drilling hazard, and is one of the factors controlling slope stability and deformation in seismically active areas. Here, we present an inverse model to calculate overpressure due to disequilibrium compaction and aquathermal pressuring. We minimize a function that contains the misfits between estimates from our forward model and observed values using a non-linear least squares approach. The inverse model allows the introduction of observed seismic and geological constraints such as P-wave velocity (Vp) and density data, and depth of the layer boundaries, for a better pore-pressure prediction. The model output also provides estimates of: (1) surface porosity, (2) compaction factor, (3) intrinsic permeability at surface conditions, (4) a parameter controlling the evolution of the intrinsic permeability with porosity, (5) the ratio between horizontal and vertical permeability and (6) uncompacted thickness (so sedimentation rate assuming known time intervals), for each sedimentary layer. We apply our inverse approach to the centre of the Eastern Black Sea Basin (EBSB) where the Vp structure has been inferred from wide-angle seismic data. First, we present results from a 1-D inverse model and an uncertainty analysis based on the Monte Carlo error propagation technique. To represent the observed rapid change from low Vp to normal Vp below the Maikop formation, we impose a zero overpressure bottom boundary, and subdivide the layer below the Maikop formation into two sublayers: an upper layer where the rapid change is located and a lower layer where the Vp is normal. Secondly, we present the results from a 2-D inverse model for the same layers using two alternative bottom boundary conditions, zero overpressure and zero flow. We are able to simulatetheobserved Vp,suggestingthatthelowvelocityzone(LVZ)at ∼3500–6500mdepth below the seabed (mbsf) can be explained by overpressure generated due to disequilibrium compaction (>90 per cent) and to aquathermal pressuring (<10 per cent). Our results suggest thattheuppersublayer,belowtheMaikopformation,behavesasasealduetoitslowpermeability ∼0.3–2 × 10 −14 ms –1 . This seal layer does not allow the fluids to escape downwards, and hence overpressure develops in the Maikop formation and not in the layers below. This overpressure was mainly generated by the relatively high sedimentation rate of ∼0.29 m ka –1 of the Maikop formation at 33.9–20.5Ma and an even higher sedimentation rate of ∼0.93 m ka –1 at 13–11 Ma. We estimate a maximum ratio of overpressure to vertical effective stress in hydrostatic conditions (λ ∗ )o f∼0.62 at ∼5200 mbsf associated with an overpressure