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Bergen, Norway

Avseth P.,Odin Petroleum | Mukerji T.,Stanford University | Mavko G.,Stanford University | Dvorkin J.,Stanford University
Geophysics | Year: 2010

Rock physics has evolved to become a key tool of reservoir geophysics and an integral part of quantitative seismic interpretation. Rock-physics models adapted to site-specific deposition and compaction help extrapolate rock properties away from existing wells and, by so doing, facilitate early exploration and appraisal. Many rock-physics models are available, each having benefits and limitations. During early exploration or in frontier areas, direct use of empirical site-specific models may not help because such models have been created for areas with possibly different geologic settings. At the same time, more advanced physics-based models can be too uncertain because of poor constraints on the input parameters without well or laboratory data to adjust these parameters. A hybrid modeling approach has been applied to siliciclastic unconsolidated to moderately consolidated sediments. Specifically in sandstones, a physical-contact theory (such as the Hertz-Mindlin model) combined with theoretical elastic bounds (such as the Hashin-Shtrikman bounds) mimics the elastic signatures of porosity reduction associated with depositional sorting and diagenesis, including mechanical and chemical compaction. For soft shales, the seismic properties are quantified as a function of pore shape and occurrence of cracklike porosity with low aspect ratios. A work flow for upscaling interbedded sands and shales using Backus averaging follows the hybrid modeling of individual homogenous sand and shale layers. Different models can be included in site-specific rock-physics templates and used for quantitative interpretation of lithology, porosity, and pore fluids from well-log and seismic data. © 2010 Society of Exploration Geophysicists. Source

Avseth P.,Odin Petroleum | DraGe A.,Statoil
SEG Technical Program Expanded Abstracts | Year: 2011

We present a rock physics study of well log data in the Troll East area, North Sea, where we focus on shallow-marine and deltaic pre- and syn-rift late Jurassic reservoir rocks. These reservoir sandstones are poorly consolidated, and presently buried at around 1100-1200m. However, the rifting phase implied fault-block rotation and differential compaction and burial in the Troll East area. The eastern section of the fault-block was down-tilted relative to the western section. In the post-rift phase, the fault-block subsided, the environment changed to a deep marine setting, and the reservoir sandstones were buried by Cretaceous and Tertiary shales, marls and occasionally sandstones. During Tertiary, there was tectonic inversion, gradual uplift in the east, and regional erosion, before quarternary shallow marine and glaciar sediments deposited. In this study, we investigate the effect of the burial history on the present day rock physics and seismic properties. We demonstrate how important it is to understand not only the present day situation when interpreting rock physics properties, but also the burial history of the rocks. The rocks have "memory" of the stress and temperature history from deposition, via mechanical and chemical compaction, to uplift and present day burial. Therefore we occasionally observe well cemented and high velocity rocks not corresponding with todays temperatures and depths. Finally, we show how we can use porosity logs to derive maximum diagenetic temperature for two wells by combining rock physics and geochemical models. © 2011 Society of Exploration Geophysicists. Source

Carcione J.M.,National Institute of Oceanography and Applied Geophysics - OGS | Helle H.B.,Odin Petroleum | Avseth P.,Odin Petroleum | Avseth P.,Norwegian University of Science and Technology
Geophysics | Year: 2011

Source rocks are described by a porous transversely isotropic medium composed of illite and organic matter (kerogen, oil, and gas). The bulk modulus of the oil/gas mixture is calculated by using a model of patchy saturation. Then, the moduli of the kerogen/fluid mixture are obtained with the Kuster and Toksöz model, assuming that oil is the inclusion in a kerogen matrix. To obtain the seismic velocities of the shale, we used Backus averaging and Gassmann equations generalized to the anisotropic case with a solid-pore infill. In the latter case, the dry-rock elastic constants are calculated with a generalization of Krief equations to the anisotropic case. We considered 11 samples of the Bakken-shale data set, with a kerogen pore infill. The Backus model provides lower and upper bounds of the velocities, whereas the Krief/Gassmann model provides a good match to the data. Alternatively, we obtain the dry-rock elastic moduli by using the inverse Gassmann equation, instead of using Krief equations. Four cases out of 11 yielded physically unstable results. We also considered samples of the North Sea Kimmeridge shale. In this case, Backus performed as well as the Krief/Gassmann model. If there is gas and oil in the shale, we found that the wave velocities are relatively constant when the amount of kerogen is kept constant. Varying kerogen content implies significant velocity changes versus fluid (oil) saturation. © 2011 Society of Exploration Geophysicists. Source

Golikov P.,Norwegian University of Science and Technology | Avseth P.,Odin Petroleum | Stovas A.,Norwegian University of Science and Technology | Bachrach R.,Tel Aviv University
Geophysical Prospecting | Year: 2013

In this paper, we create rock physics templates for heterogeneous and anisotropic thin-bedded sand-shale intervals as a function of group angle, net-to-gross, saturation and porosity as variable parameters. These templates are basically cross-plots of acoustic impedance versus P- to S-velocity ratio. We apply these templates to interpret well log data from a vertical and a deviated well, respectively, in a North Sea turbidite system. We are able to infer the shale anisotropic elastic moduli and Thomsen parameters by comparing the measured velocities in a deviated well with the velocities in an adjacent vertical well. Our modeling captures the observed trends in the data as we go from a vertical well to a deviated well through a heterogeneous reservoir saturated with light oil and water. We can clearly see how the reservoir properties changes due to the presence of anisotropy. We also perform an AVO sensitivity study as a function of heterogeneity and hydrocarbon saturation. © 2012 European Association of Geoscientists & Engineers. Source

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