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.