An ‘Elastic Impedance’ Approach

Contributors: Ritesh Sharma

A detailed investigation of seismic amplitudes can yield information pertaining to lithological variation in subsurface sedimentary rock formations and the existence and extent of some hydrocarbon zones.

This objective can be facilitated in a process called seismic inversion, which transforms seismic amplitudes into acoustic impedance values.

In doing so, the seismic reflection response gets transformed into layered impedance response, which makes the interpretation of the lithological and fluid information more convenient – each transformed impedance trace can now be considered as an impedance log curve and the seismic volume as logs recorded in wells drilled at every seismic trace location.

Just as the changes in the character of impedance log curves are indicative of changes in lithology, porosity and fluid content, similar changes seen on inverted impedance traces are interpretable of these properties in a lateral sense over an area and so over a volume.

Acoustic impedance inversion has now become an integral part of most interpretation projects today.

While this is a beneficial tool for the seismic interpreter, acoustic impedance inversion is usually run on stacked seismic traces – that is, the individual prestack time migrated offset gathers are stacked and then transformed into impedance. To better exploit the fluid effects that manifest on prestack gathers as variation of amplitudes with offset or angle, prestack impedance inversion also can be carried out.

Of course, it would take longer – and so the trade-off is usually between the cost, time and the method to be used.


A simple way to examine the variations of amplitude as a function of offset is to generate the offset-limited seismic volumes, such as the near-, mid- and far-offset (or angle) volumes. Variations seen on these volumes in desired zones could then be indicative of the fluid information.

For example, a low-impedance gas-sand sandwiched between shale would yield an increase of amplitude with offset. Such a variation can be detected on comparing the near-offset seismic volume with the equivalent far-offset volume, and noticing high amplitude anomalies on the latter corresponding to the gas samples.

As amplitudes of the near-offset traces are related to the changes in acoustic impedance they can be calibrated with well log curves or synthetic seismograms. However, if a far-offset or a far-angle stack has to be calibrated with the log data or synthetic seismograms, there is no analogous set of log curves that could be used for the purpose.

Back in 1999, Patrick Connolly from BP pointed this out and suggested the generalization of acoustic impedance for variable incidence angle using a linearized version of Zoeppritz equations. He called this “elastic impedance,” and it provides the framework to calibrate and invert non-zero offset data.

The elastic impedance approach is strongly dependent on the medium parameters (VP, VS, and density and angle of incidence), and so is often regarded as the rock attribute analog of acoustic impedance for varying angles of incidence.

In actual practice, the CMP gather at the position of the well is picked up, different angle ranges are selected and angle stacks generated. Given the VP, VS and density log curves, the elastic impedance is calculated for different angles of incidence. The angle stack traces from the gather and those derived from the log curves (elastic impedance, or “EI”) are compared for a visual assessment and interpretation.


Another useful and meaningful display is the comparison of the acoustic impedance log curve with the elastic impedance curve at the far-angle that is admissible for the given data.

In figure 1, the acoustic impedance log is compared with the EI (30 degree) log curve for a discovery well from Colombia. The target is related to Eocene fluvial deposits, mainly composed of interbedded medium to fine grained quartz sandstones and clay stones.

The gas was detected during mud logging and on the electric log curve; however, the density and neutron curve crossover is not as high as expected, probably due to low saturation as well as its position. The saturation is expected to increase in the up-dip direction.

Notice that there is a decrease of impedance at the gas-sand interface, and so it will show up as higher amplitudes on the seismic data.

It may be mentioned that the elastic impedance values vary significantly with the incidence angle – and because of this, when elastic impedance logs have to be displayed alongside with or overlaid on acoustic impedance, they have to be scaled in such a way that the EI values for all angles fall in the range of the normal acoustic impedance values.

In figure 1, such a scaling has been applied to EI (30 degrees).

As stated, elastic impedance also provides a convenient way of producing synthetic seismograms for variable angles of incidence. The computed EI (30 degree) log curve can be used for producing synthetic seismograms, which could now be used for correlation with far-offset/angle stack.

In figure 2 we show the correlation of a synthetic seismogram (generated with the acoustic impedance log curve) with the near-stack from the Magdelena Valley, Colombia. The tie seems to be reasonably good. Figure 3 shows a similar synthetic seismogram (generated by using the EI log curve) tie with the far-offset stack. This tie again seems to be good – but, as expected, the amplitudes at the indicated locations on the far-stack are weaker and seem to tie accordingly with the synthetic.

In figure 4 we show a comparison of a segment of an acoustic impedance section derived from post-stack AI inversion and the equivalent EI (30 degree) section. Notice the differences in the yellow highlighted zones that enclose the gas-producing reservoir. While the hydrocarbon-bearing zone is indicated on the AI section, it appears more pronounced and convincing – and its correlation with the overlaid impedance curve also is much better than the correlation with the AI section.

Thus elastic impedance attribute serves to combine the benefits of working with inverted data with far-angle data where the fluid information resides.


We thank PetroNorte, Colombia, for giving us permission for presentation of the results shown in this study. We also thank Arcis Seismic Solutions for permission to present this work.

Comments (0)

 

Geophysical Corner

Geophysical Corner - Satinder Chopra
Satinder Chopra, chief geophysicist (reservoir), at Arcis Seismic Solutions, Calgary, Canada, began serving as the editor of the Geophysical Corner column in 2012.

Geophysical Corner - Ritesh Kumar Sharma

Ritesh Kumar Sharma is with Arcis Seismic Solutions, Calgary, Canada.

Geophysical Corner

The Geophysical Corner is a regular column in the EXPLORER that features geophysical case studies, techniques and application to the petroleum industry.

VIEW COLUMN ARCHIVES

Image Gallery

See Also: Bulletin Article

Data derived from core and well-logs are essentially one-dimensional and determining eolian system type and likely dimensions and orientation of architectural elements present in subsurface eolian reservoir successions is typically not possible from direct observation alone. This is problematic because accurate predictions of the three-dimensional distribution of interdune and dune-plinth elements that commonly form relatively low-permeability baffles to flow, of net:gross, and of the likely distribution of elements with common porosity-permeability properties at a variety of scales in eolian reservoirs is crucial for effective reservoir characterization.

Direct measurement of a variety of parameters relating to aspects of the architecture of eolian elements preserved as ancient outcropping successions has enabled the establishment of a series of empirical relationships with which to make first-order predictions of a range of architectural parameters from subsurface successions that are not observable directly in core. In many preserved eolian dune successions, the distribution of primary lithofacies types tends to occur in a predictable manner for different types of dune sets, whereby the pattern of distribution of grain-flow, wind-ripple, and grain-fall strata can be related to set architecture, which itself can be related back to original bedform type.

Detailed characterization of individual eolian dune sets and relationships between neighboring dune and interdune elements has been undertaken through outcrop studies of the Permian Cedar Mesa Sandstone and the Jurassic Navajo Sandstone in southern Utah. The style of transition between lithofacies types seen vertically in preserved sets, and therefore measurable in analogous core intervals, enables predictions to be made regarding the relationship between preserved set thickness, individual grain-flow thickness, original bedform dimensional properties (e.g., wavelength and height), the likely proportion of the original bedform that is preserved to form a set, the angle of climb of the system, and the likely along-crest variability of facies distributions in sets generated by the migration of sinuous-crested bedforms. A series of graphical models depict common facies arrangements in bedsets for a suite of dune types and these demonstrate inherent facies variability.

Desktop /Portals/0/PackFlashItemImages/WebReady/reconstruction-of-three-dimensional-eolian-dune-architecture.jpg?width=50&h=50&mode=crop&anchor=middlecenter&quality=90amp;encoder=freeimage&progressive=true 3249 Bulletin Article

See Also: DL Abstract

Production from the Marcellus gas shale generated international interest when methane accumulated in the surface housing of a water well pump and exploded.

Desktop /Portals/0/images/_site/AAPG-newlogo-vertical-morepadding.jpg?width=50&h=50&mode=crop&anchor=middlecenter&quality=90amp;encoder=freeimage&progressive=true 3104 DL Abstract

See Also: Education Conference

Desktop /Portals/0/PackFlashItemImages/WebReady/2015-FEC-hero-color.jpg?width=50&h=50&mode=crop&anchor=middlecenter&quality=90amp;encoder=freeimage&progressive=true 17699 Education Conference

See Also: Online e Symposium

Recent interest in unconventional gas resources has attracted several oil and gas explorers to sedimentary basins in Southern Quebec.

Desktop /Portals/0/PackFlashItemImages/WebReady/oc-es-sale-gas-in-quebecs-sedimentary-basins.jpg?width=50&h=50&mode=crop&anchor=middlecenter&quality=90amp;encoder=freeimage&progressive=true 1467 Online e-Symposium
Desktop /Portals/0/PackFlashItemImages/WebReady/oc-es-challenging-the-paradigm-missing-section-normal-fault.jpg?width=50&h=50&mode=crop&anchor=middlecenter&quality=90amp;encoder=freeimage&progressive=true 1462 Online e-Symposium