Kenneth Duffaut

Bayesian lithology and fluid prediction from seismic prestack data

A fast Bayesian inversion method for 3D lithology and fluid prediction from prestack seismic data, and a corresponding feasibility analysis were developed and tested on a real data set. The objective of the inversion is to find the probabilities for different lithology-fluid classes from seismic data and geologic knowledge. The method combines stochastic rock physics relations between the elastic parameters and the different lithology-fluid classes with the results from a fast Bayesian seismic simultaneous inversion from seismic data to elastic parameters. A method for feasibility analysis predicts the expected modification of the prior probabilities to posterior probabilities for the different lithology-fluid classes. The feasibility analysis can be carried out before the seismic data are analyzed. Both the feasibility method and the seismic lithology-fluid probability inversion were applied to a prospect offshore Norway. The analysis improves the probability for gas sand from 0.1 to about 0.2–0.4 with s...

Can hydrocarbon source rocks be identified on seismic data

Hydrocarbon source rocks contain signifi cant volumes of organic matter, are capable of expelling petroleum when heated, and have produced most of the world’s known oil volumes. Recently, source rocks have also become recognized as unconventional economic reservoirs. Here we present a new way of identifying, characterizing, and mapping spatial distributions and variations of thick source rocks (>20 m) that is based on seismic data only. This has a signifi cant impact on the prospect risk assessment of petroleum plays. Rock property studies of organic-rich claystones show that the acoustic impedance (AI), which is the product of compressional velocity and density, decreases nonlinearly with increasing total organic carbon (TOC) percent. Claystones mixed with low-density organic matter (TOC > 3%‐4%) have signifi cant lower AI and higher intrinsic anisotropy than otherwise similar nonorganic claystones. This gives the top and base source rock refl ections characteristic negative and positive high amplitudes, respectively, which dim with increasing refl ection angle. In addition, the TOC profi le, which is a smoothed TOC percent curve, infl uences the top and base amplitude responses. An upward-increasing TOC profi le has the highest amplitude at the top, while the opposite asymmetry is observed for downward-increasing TOC profi les. By using seismic data, we therefore can map lateral distribution, thickness, variation in TOC profi les, and, with local well calibration, convert AI data to TOC percent. This approach to mapping source rocks may change the way petroleum systems are evaluated.

Discrimination between pressure and fluid saturation changes from marine multicomponent time‐lapse seismic data

Discrimination between pressure and fluid saturation changes from marine multicomponent time-lapse seismic data

Vp ∕ Vs ratio versus differential stress and rock consolidation — A comparison between rock models and time-lapse AVO data

The compressional to shear wave velocity ratio ( Vp ∕ Vs ) is an important parameter in seismic amplitude versus offset (AVO) analysis, and this parameter plays a key role especially for lithology and fluid prediction methods. The P-wave velocity is a key parameter in traditional pressure prediction methods, because overpressure often results in a velocity reduction. However, for AVO-based pore pressure prediction methods, one expects that the Vp ∕ Vs ratio also is a key parameter. The Hertz-Mindlin geomechanical model predicts a constant Vp ∕ Vs ratio as the differential stress changes in a dry package of identical spheres. Ultrasonic core measurements show increased Vp ∕ Vs ratios as the differential stress decreases, especially for unconsolidated wet sands. Thus, one is likely to assume that the Vp ∕ Vs ratio is dependent on rock consolidation. By combining the Hertz-Mindlin model with the Gassmann model, we show how to obtain a sim-ple rock-physics framework including both the differential stress and ...

Shear-wave elastic impedance

It is well known that if we are able to estimate acoustic impedance (AI) and a parameter related to shear-wave velocity from seismic data, our ability to discriminate between different lithologies and fluid phases will increase. Prestack inversion on individual CDP gathers and inverting directly for VP , VS , and density have been tested in several ways, but the estimated parameters are often poorly determined. A more robust approach is to apply poststack inversion on partial stacks. For inversion of the near-offset stack, AI can be calculated directly from well logs. However, for the far-offset stack, we need to derive an equivalent of the acoustic impedance that can be used to calibrate the non-zero-offset seismic reflectivity. Connolly (1998, 1999) derived such an equivalent—elastic impedance (EI)—and demonstrated how, by using elastic impedance logs (which requires shear-wave logs), he was able to perform well calibration and inversion of far-offset data. This article describes a new function—shear-wave elastic impedance (SEI)—for linking converted-wave stacks to wells using a linearization of the Zoeppritz equations. SEI is similar to EI but adapted to P-S converted seismic data. It can be computed from acoustic log data ( P - and S -wave velocities and density) and used for well calibration, wavelet estimation, and inversion of P-S reflectivity data leading to improved interpretability of converted wave data. (Equation 4 in Appendix 1 is the key mathematical formula in this approach to SEI). To test the SEI technique, we used a 3-D four-component (4-C) ocean-bottom cable (OBC) survey acquired in 1997 that covers approximately 10 km2 of the Statfjord Field (Rogno et al., 1999). Both P -wave and converted-wave data were 3-D prestack time-migrated. The hydrophone ( P ) and the vertical geophone ( Z ) component were summed to attenuate receiver side water-layer reverberations and both data sets ( PZ …

Using Mindlin theory to model friction-dependent shear modulus in granular media

An explicit expression for the effective shear modulus of a random packing of identical spheres is derived as a function of Mindlin’s tangential stiffness with interparticle contact friction. The motivation behind the approach is to incorporate the effect of intergrain friction to predict velocities in unconsolidated sands. The Mindlin friction term, allowing partial slip across the contact area between pairs of spheres, can be viewed as a parameter accounting for the growing macroscopic intergrain friction in sands as burial progresses. Hence, both moduli and velocities will gradually increase as the compressional- to shear-wave velocity ratioVP /VS or Poisson’s ratio decreases. An estimate of effective elastic constants in particular shear modulus can be obtained for a spherical grain pack with an arbitrary frictional behavior ranging between two special contact boundary conditions representing infinite friction and zero friction. The proposed model predicts a nonlinear transition between the two special grain-contact conditions when compared to previously published linear relationships. Comparison of elastic properties, i.e., dynamic shear-modulus predictions assuming zero contact friction with experimental data on loose glass bead and sand samples undergoing hydrostatic compression, appears to match reasonably well at low confining stressless than 5 MPa but deviates gradually as stress increases. It is advocated that the increasing effective internal frictional resistance of the experimental core samples control both the frictional attenuation mechanism in loose grain packs under low confining stress for strain amplitudes typical of seismic wavesless than 106 and the higher stress-velocity sensitivity. Circumstantial evidence of this is found in publications describing both laboratory attenuation analysis and consolidation experiments on granular materials with different degrees of competence or static shear strength.

V-p/V-s ratio versus differential stress and rock consolidation - A comparison between rock models and time-lapse AVO data

The compressional to shear wave velocity ratio ( Vp ∕ Vs ) is an important parameter in seismic amplitude versus offset (AVO) analysis, and this parameter plays a key role especially for lithology and fluid prediction methods. The P-wave velocity is a key parameter in traditional pressure prediction methods, because overpressure often results in a velocity reduction. However, for AVO-based pore pressure prediction methods, one expects that the Vp ∕ Vs ratio also is a key parameter. The Hertz-Mindlin geomechanical model predicts a constant Vp ∕ Vs ratio as the differential stress changes in a dry package of identical spheres. Ultrasonic core measurements show increased Vp ∕ Vs ratios as the differential stress decreases, especially for unconsolidated wet sands. Thus, one is likely to assume that the Vp ∕ Vs ratio is dependent on rock consolidation. By combining the Hertz-Mindlin model with the Gassmann model, we show how to obtain a sim-ple rock-physics framework including both the differential stress and ...

Geophysical basin modeling: Methodology and application in deepwater Gulf of Mexico

AbstractA truly integrated velocity model building method has been developed and applied for seismic imaging. Geophysical basin modeling is designed to mitigate seismic data limitations and constrains the velocity model building by taking advantage of information provided by geologic and geophysical input. The information from geologic concepts and understanding is quantified using basin model simulations to model primary control fields for rock properties, temperature, and effective stress. Transformation of the basin model fields to velocity is made by universally calibrated rock models. Applications show that high-quality seismic images are produced in areas of geologic complexity, where it is challenging to define these properties from seismic data alone. This multidisciplinary operation is of high value in exploration because it offers a significant reduction in the time and effort required to build a velocity model, while also improving the resulting image quality.

Stress and fluid sensitivity in two North Sea oil fields—comparing rock physics models with seismic observations

During 4D seismic reservoir characterization, it is important to have reliable rock physics models for both static (e.g., mineralogy, porosity, cement volume) and dynamic (e.g., saturation, pressure, temperature) reservoir parameters. Without a good understanding of reservoir geology and associated static rock physics properties, it is impossible to interpret time-variant changes in pore pressure and saturation (Andersen et al., 2009). The dry rock properties of the reservoir can be obtained from well-log data combined with geological information about mineral composition and rock texture, and Gassmann theory to estimate the effect of pore fluid changes. Normally, core measurements are undertaken to quantify stress sensitivity, but these are often affected by induced fractures caused by the coring acquisition that will enhance the stress sensitivity of the rock (Holt et al., 2005). Duffaut and Landro (2007) showed how calibrated Hertz-Mindlin contact theory could be applied to estimate stress sensitivity ...

Low-frequency Layer-induced Anisotropy

We develop a low-frequency dynamic effective medium for a horizontally layered transversely isotropic medium with vertical symmetry axis (VTI), and approximate this effective medium by a homogeneous VTI medium. This results in frequency-dependent anisotropy parameters valid for low frequencies and quasi-vertical propagation.