Present day rock physics properties and associated seismic signatures are controlled by burial hi... more Present day rock physics properties and associated seismic signatures are controlled by burial history, tectonic events and temperature changes, diagenetic alterations and pressure modifications. Hence, in order to fully understand the seismic signatures of a prospect, we should not only relate rock properties to present day geologic factors (porosity, clay content, mineralogy, in situ pressure, etc.), but honour the geologic processes through time (Avseth and Drge, 2011). There are very few studies documenting the effect of complex tectonic and uplift on rock physics and seismic properties (e.g., Brevik et al., 2011). Taking into account diagenesis and tectonic events, we can predict compaction trends and associated seismic velocities in areas with more complex burial history involving both mechanical and chemical compaction, as well as uplift episodes and corresponding erosion. The resulting rock physics trends help us to better constrain AVO inversion and classification under such circumstances. We propose a methodology for statistical classification of fluids and facies in uplifted areas. The robustness of the method was validated by comparing the results to the information obtained from newly drilled well situated near the Loppa High in Barents Sea.
Summary We have developed a method for joint inversion of PP and PS data with the goal of unique ... more Summary We have developed a method for joint inversion of PP and PS data with the goal of unique and accurate determination of important layer parameters, with particular focus on Vp/Vs ratios across reflection interfaces. These can be used in quantitative seismic interpretation for improved lithology and fluid discrimination. Our results show that by exploring the full Zoeppritz equation jointly for PP and PS reflectivities, instead of approximative equations, we obtain more accurate and unique estimates of Vp/Vs ratios. Moreover, the method gives reliable estimates of density ratio, a parameter that can be directly linked to hydrocarbon saturations.
We have demonstrated how burial history and modeling of compaction and diagenesis can be combined... more We have demonstrated how burial history and modeling of compaction and diagenesis can be combined with rock physics modeling in order to predict expected AVO signatures for a given reservoir. This method can be used in a forward modeling way to predict seismic signatures given a known burial history. We show examples from the Barents Sea, offshore Norway, and demonstrate that the proposed methodology can be useful to screen for hydrocarbons from seismic AVO data if knowledge/information about net erosion (uplift) is available.
C004 Seismic Reservoir Mapping Using Rock Physics Templates: Example from a North Sea Turbidite S... more C004 Seismic Reservoir Mapping Using Rock Physics Templates: Example from a North Sea Turbidite System Abstract 1 Rock physics templates (RPTs) are locally calibrated rock physics models that can be used for interpretation of well log and seismic data. Odegaard and Avseth (2003) showed how to construct the templates and presented examples of how to apply RPTs for well log and seismic data analysis. In this case study we present an example of the full workflow of RPT analysis starting with the selection of the most appropriate RPT using well log cross-plot analysis followed by rock physics interpretation of elastic
We explore some new rock physics attributes that can be used to screen for reservoir sandstones a... more We explore some new rock physics attributes that can be used to screen for reservoir sandstones and hydrocarbon pore-fill from seismic inversion data. We honor the physical properties of the rocks by defining attributes, in general referred to as fluid- and rock-impedances, that comply with calibrated rock physics models. Two different approaches are presented, both of which attempt to separate out the effects of fluids and rock stiffness. The first set of attributes include the fluid saturation sensitive CPEI (Curved pseudo-elastic impedance for saturation), and the rock stiffness/lithology attribute PEIL (Pseudo-elastic impedance for lithology), which are near orthogonal to each other in a Vp/Vs versus acoustic impedance (AI) domain. The second set of attributes includes a porosity attribute derived from shear impedance, and the fluid deviation attribute from a background trend in acoustic impedance. The latter set of attributes shows almost perfect orthogonality for any burial depth. We demonstrate the use of these attributes on well log and seismic inversion data from the Norwegian Sea, and successfully screen out reservoir rocks filled with hydrocarbons.
In this paper the effect of several factors influencing the porosity determination from seismic i... more In this paper the effect of several factors influencing the porosity determination from seismic inversion as well as their interplay have been discussed. The main findings may be summarized in three points: 1) Porosity cannot be uniquely determined from acoustic impedance without having information about what fluid saturates the pore space. As the AI is decreasing, the range of the possible porosity values is increasing in function of effective fluid density. 2) Upscaled (Backus averaged) and the well data show the same (quadratic) trend, i.e. the scale does not affect the relationship between AI and porosity. However, high and low porosity tails are lost as a consequence of averaging a finite thickness into a single point, thus a high resolution information is lost. 3) Upscaled well log data with noise is showing a different trend in high porosity part when compared with quadratic trend from well log data and upscaled well log data.
Purely physics-driven inversion scheme free of empirical relationships has been developed with th... more Purely physics-driven inversion scheme free of empirical relationships has been developed with the goal of inverting the noise-free PP-gathers for the model parameters, namely the exact Ď - AI - - quadruplet entering Zoeppritz formula. In order to achieve this goal, three scaler-independent angles were observed, two of which for the following conditions hold: 1) RC(θ) = 0, 2) RC(θ) = min. The third angle is the critical angle. Moreover, assuming that the scaler does not depend on the incident angle of the plane wave, it has been shown that its value can be accurately reconstructed.
As a result of continuous improvement in seismic data quality the lithology and fluid prediction ... more As a result of continuous improvement in seismic data quality the lithology and fluid prediction from seismic data has become more reliable during the last decade. This is also a requirement for more complex and less robust exploration targets, as well for increased hydrocarbon recovery in production. Milestones on the road for improved LFP have been Elastic Inversion and Simultaneous Inversion.
ABSTRACTRockâphysics properties of sands and sandstones are strongly affected by geological proce... more ABSTRACTRockâphysics properties of sands and sandstones are strongly affected by geological processes of the past, including deposition, compaction and exhumation. By honouring these geological processes, the rockâphysics modelling will be more predictive in areas with limited well control. This study performs rockâphysics modelling constrained by a given geological history, starting from deposition to mechanical and chemical compaction. Different geological factors, including sorting, grain size and clay coating, will affect the quartz cementation and rock stiffening of quartzose reservoir sandstones. By combining compaction models with rockâphysics contact theory, we can model the rockâphysics properties of sands/sandstones as a function of geologic time. We have demonstrated the approach on well log data from three selected wells on the Norwegian Continental Shelf, where the burial histories of the target reservoir sandstones are significantly different. We conclude that rockâphysics modelling constrained by burial history can be used to predict elastic properties quite accurately in these wells. The integrated approach presented in this study allows for more realistic rockâphysics depth trends in areas with complex burial history that can be used in AVO studies or to estimate net erosion associated with tectonic uplift.
Turbidite reservoirs, deepwater clastic systems often characterized by complex sand distribution,... more Turbidite reservoirs, deepwater clastic systems often characterized by complex sand distribution, stretch the limits of conventional seismic and geostatistical modeling and analysis. Nevertheless, such reservoirs must produce at a high rate to cover large drilling and production costs. Hence, reservoir heterogeneity must be accurately quantified and the associated uncertainty measured to determine the investment risk.
ABSTRACTWe have demonstrated an approach for dataâdriven rock physics analysis, where we first do... more ABSTRACTWe have demonstrated an approach for dataâdriven rock physics analysis, where we first do facies classification using elastic well log data from several wells, followed by faciesâconstrained regression analysis to establish local rock physics relations for the prediction of VP and VS from geological input parameters. We have applied this approach to a multiâwell log data set (53 wells, 40 of which had reliable/useful data) from the greater Alvheim area. We show how we can derive very robust local empirical rock physics relations for the prediction of Pâwave and Sâwave velocities as well as densities, for given combinations of porosity and clay volume. These locally derived empirical relations are recommended instead of universal rock physics models, even when the latter are locally calibrated. Using elastic facies with geological characteristics (cemented versus unconsolidated; normally compacted versus injectites; homogeneous versus heterogeneous) helps to improve the predictability of the regression models. The local rock physics relations that we obtain can furthermore be used to create training data for AVO classification.
We introduce a methodology for probabilistic calculation of the Vp/Vs ratios in layers 1 and 2 so... more We introduce a methodology for probabilistic calculation of the Vp/Vs ratios in layers 1 and 2 solely from calibrated PP seismic gathers without postulating Vp/Vs1 ratio to be known. We utilize the Zoeppritz equation for calculation of PP-reflection coefficients at an interface of two homogeneous, isotropic, elastic layers and invert it to estimate Vp/Vs1 and Vp/Vs2. This way we obtain all Vp/Vs1-Vp/Vs2 pairs that satisfy intercept-RC(angle) at each point of interest. Moreover, we calculate statistical parameters such as standard deviation at each point. By doing so, we create uncertainty maps which can yield useful information about quality of data and depict zones of unreliable data. We test the methodology in the Barents Sea gas reservoir. It serves as an alternative to reflectivity-based AVO screening during early exploration, opening up for more quantitative interpretations without going to more cumbersome simultaneous AVO inversion which requires good well ties and a reliable low-frequency model.
ABSTRACT In this study we show how we can combine basin modeling principles with rock physics mod... more ABSTRACT In this study we show how we can combine basin modeling principles with rock physics models. We apply the Walderhaug diagenetic modeling approach to simulate porosity and rock texture evolution during geological time, and use these as input to contact theory rock physics models which allows us to predict seismic properties as a function of the burial history. We also demonstrate how this approach can be used to estimate tectonic uplifts, and show an example from the Barents Sea.
Present day rock physics properties and associated seismic signatures are controlled by burial hi... more Present day rock physics properties and associated seismic signatures are controlled by burial history, tectonic events and temperature changes, diagenetic alterations and pressure modifications. Hence, in order to fully understand the seismic signatures of a prospect, we should not only relate rock properties to present day geologic factors (porosity, clay content, mineralogy, in situ pressure, etc.), but honour the geologic processes through time (Avseth and Drge, 2011). There are very few studies documenting the effect of complex tectonic and uplift on rock physics and seismic properties (e.g., Brevik et al., 2011). Taking into account diagenesis and tectonic events, we can predict compaction trends and associated seismic velocities in areas with more complex burial history involving both mechanical and chemical compaction, as well as uplift episodes and corresponding erosion. The resulting rock physics trends help us to better constrain AVO inversion and classification under such circumstances. We propose a methodology for statistical classification of fluids and facies in uplifted areas. The robustness of the method was validated by comparing the results to the information obtained from newly drilled well situated near the Loppa High in Barents Sea.
Summary We have developed a method for joint inversion of PP and PS data with the goal of unique ... more Summary We have developed a method for joint inversion of PP and PS data with the goal of unique and accurate determination of important layer parameters, with particular focus on Vp/Vs ratios across reflection interfaces. These can be used in quantitative seismic interpretation for improved lithology and fluid discrimination. Our results show that by exploring the full Zoeppritz equation jointly for PP and PS reflectivities, instead of approximative equations, we obtain more accurate and unique estimates of Vp/Vs ratios. Moreover, the method gives reliable estimates of density ratio, a parameter that can be directly linked to hydrocarbon saturations.
We have demonstrated how burial history and modeling of compaction and diagenesis can be combined... more We have demonstrated how burial history and modeling of compaction and diagenesis can be combined with rock physics modeling in order to predict expected AVO signatures for a given reservoir. This method can be used in a forward modeling way to predict seismic signatures given a known burial history. We show examples from the Barents Sea, offshore Norway, and demonstrate that the proposed methodology can be useful to screen for hydrocarbons from seismic AVO data if knowledge/information about net erosion (uplift) is available.
C004 Seismic Reservoir Mapping Using Rock Physics Templates: Example from a North Sea Turbidite S... more C004 Seismic Reservoir Mapping Using Rock Physics Templates: Example from a North Sea Turbidite System Abstract 1 Rock physics templates (RPTs) are locally calibrated rock physics models that can be used for interpretation of well log and seismic data. Odegaard and Avseth (2003) showed how to construct the templates and presented examples of how to apply RPTs for well log and seismic data analysis. In this case study we present an example of the full workflow of RPT analysis starting with the selection of the most appropriate RPT using well log cross-plot analysis followed by rock physics interpretation of elastic
We explore some new rock physics attributes that can be used to screen for reservoir sandstones a... more We explore some new rock physics attributes that can be used to screen for reservoir sandstones and hydrocarbon pore-fill from seismic inversion data. We honor the physical properties of the rocks by defining attributes, in general referred to as fluid- and rock-impedances, that comply with calibrated rock physics models. Two different approaches are presented, both of which attempt to separate out the effects of fluids and rock stiffness. The first set of attributes include the fluid saturation sensitive CPEI (Curved pseudo-elastic impedance for saturation), and the rock stiffness/lithology attribute PEIL (Pseudo-elastic impedance for lithology), which are near orthogonal to each other in a Vp/Vs versus acoustic impedance (AI) domain. The second set of attributes includes a porosity attribute derived from shear impedance, and the fluid deviation attribute from a background trend in acoustic impedance. The latter set of attributes shows almost perfect orthogonality for any burial depth. We demonstrate the use of these attributes on well log and seismic inversion data from the Norwegian Sea, and successfully screen out reservoir rocks filled with hydrocarbons.
In this paper the effect of several factors influencing the porosity determination from seismic i... more In this paper the effect of several factors influencing the porosity determination from seismic inversion as well as their interplay have been discussed. The main findings may be summarized in three points: 1) Porosity cannot be uniquely determined from acoustic impedance without having information about what fluid saturates the pore space. As the AI is decreasing, the range of the possible porosity values is increasing in function of effective fluid density. 2) Upscaled (Backus averaged) and the well data show the same (quadratic) trend, i.e. the scale does not affect the relationship between AI and porosity. However, high and low porosity tails are lost as a consequence of averaging a finite thickness into a single point, thus a high resolution information is lost. 3) Upscaled well log data with noise is showing a different trend in high porosity part when compared with quadratic trend from well log data and upscaled well log data.
Purely physics-driven inversion scheme free of empirical relationships has been developed with th... more Purely physics-driven inversion scheme free of empirical relationships has been developed with the goal of inverting the noise-free PP-gathers for the model parameters, namely the exact Ď - AI - - quadruplet entering Zoeppritz formula. In order to achieve this goal, three scaler-independent angles were observed, two of which for the following conditions hold: 1) RC(θ) = 0, 2) RC(θ) = min. The third angle is the critical angle. Moreover, assuming that the scaler does not depend on the incident angle of the plane wave, it has been shown that its value can be accurately reconstructed.
As a result of continuous improvement in seismic data quality the lithology and fluid prediction ... more As a result of continuous improvement in seismic data quality the lithology and fluid prediction from seismic data has become more reliable during the last decade. This is also a requirement for more complex and less robust exploration targets, as well for increased hydrocarbon recovery in production. Milestones on the road for improved LFP have been Elastic Inversion and Simultaneous Inversion.
ABSTRACTRockâphysics properties of sands and sandstones are strongly affected by geological proce... more ABSTRACTRockâphysics properties of sands and sandstones are strongly affected by geological processes of the past, including deposition, compaction and exhumation. By honouring these geological processes, the rockâphysics modelling will be more predictive in areas with limited well control. This study performs rockâphysics modelling constrained by a given geological history, starting from deposition to mechanical and chemical compaction. Different geological factors, including sorting, grain size and clay coating, will affect the quartz cementation and rock stiffening of quartzose reservoir sandstones. By combining compaction models with rockâphysics contact theory, we can model the rockâphysics properties of sands/sandstones as a function of geologic time. We have demonstrated the approach on well log data from three selected wells on the Norwegian Continental Shelf, where the burial histories of the target reservoir sandstones are significantly different. We conclude that rockâphysics modelling constrained by burial history can be used to predict elastic properties quite accurately in these wells. The integrated approach presented in this study allows for more realistic rockâphysics depth trends in areas with complex burial history that can be used in AVO studies or to estimate net erosion associated with tectonic uplift.
Turbidite reservoirs, deepwater clastic systems often characterized by complex sand distribution,... more Turbidite reservoirs, deepwater clastic systems often characterized by complex sand distribution, stretch the limits of conventional seismic and geostatistical modeling and analysis. Nevertheless, such reservoirs must produce at a high rate to cover large drilling and production costs. Hence, reservoir heterogeneity must be accurately quantified and the associated uncertainty measured to determine the investment risk.
ABSTRACTWe have demonstrated an approach for dataâdriven rock physics analysis, where we first do... more ABSTRACTWe have demonstrated an approach for dataâdriven rock physics analysis, where we first do facies classification using elastic well log data from several wells, followed by faciesâconstrained regression analysis to establish local rock physics relations for the prediction of VP and VS from geological input parameters. We have applied this approach to a multiâwell log data set (53 wells, 40 of which had reliable/useful data) from the greater Alvheim area. We show how we can derive very robust local empirical rock physics relations for the prediction of Pâwave and Sâwave velocities as well as densities, for given combinations of porosity and clay volume. These locally derived empirical relations are recommended instead of universal rock physics models, even when the latter are locally calibrated. Using elastic facies with geological characteristics (cemented versus unconsolidated; normally compacted versus injectites; homogeneous versus heterogeneous) helps to improve the predictability of the regression models. The local rock physics relations that we obtain can furthermore be used to create training data for AVO classification.
We introduce a methodology for probabilistic calculation of the Vp/Vs ratios in layers 1 and 2 so... more We introduce a methodology for probabilistic calculation of the Vp/Vs ratios in layers 1 and 2 solely from calibrated PP seismic gathers without postulating Vp/Vs1 ratio to be known. We utilize the Zoeppritz equation for calculation of PP-reflection coefficients at an interface of two homogeneous, isotropic, elastic layers and invert it to estimate Vp/Vs1 and Vp/Vs2. This way we obtain all Vp/Vs1-Vp/Vs2 pairs that satisfy intercept-RC(angle) at each point of interest. Moreover, we calculate statistical parameters such as standard deviation at each point. By doing so, we create uncertainty maps which can yield useful information about quality of data and depict zones of unreliable data. We test the methodology in the Barents Sea gas reservoir. It serves as an alternative to reflectivity-based AVO screening during early exploration, opening up for more quantitative interpretations without going to more cumbersome simultaneous AVO inversion which requires good well ties and a reliable low-frequency model.
ABSTRACT In this study we show how we can combine basin modeling principles with rock physics mod... more ABSTRACT In this study we show how we can combine basin modeling principles with rock physics models. We apply the Walderhaug diagenetic modeling approach to simulate porosity and rock texture evolution during geological time, and use these as input to contact theory rock physics models which allows us to predict seismic properties as a function of the burial history. We also demonstrate how this approach can be used to estimate tectonic uplifts, and show an example from the Barents Sea.
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Papers by Per Avseth