The Final Touch: Attributes Prove Their Worth

Contributors: Kurt Marfurt

Seismic stratigraphy requires interpreters to analyze the geometrical configurations and termination patterns of seismic reflection events.

Maps of distinct families of these reflection behaviors usually can be interpreted to determine where distinct depositional processes occur across the mapped area. Reflection patterns such as toplap, onlap, downlap and erosional truncation are used as architectural elements to reconstruct the depositional environments imaged by seismic data.

Using such seismic-depositional environment maps – together with well control and modern and paleo analogues – allows interpreters to produce probability maps of “most-likely” lithofacies.

Although coherence and curvature are excellent for delineating some seismic stratigraphic features, they have limited value in imaging classic seismic stratigraphy features such as onlap, progradation and erosional truncation.

Here we examine how newer volumetric attributes facilitate seismic stratigraphic analysis of large 3-D seismic volumes.

Reflection Convergence

Changes in reflector dip, reflection terminations, erosional unconformities and angular unconformities are relatively easy to recognize by visual inspection of vertical seismic sections.

To translate visual recognition of these features to a numerical-recognition process, a first step is to compute volumetric estimates of vector dip at each data sample.

Next, the mean and standard deviations of these vector dips are calculated in small windows about each data sample. Conformable reflections will have small standard deviations of their reflection dips, while non-parallel events such as angular unconformities will have high standard deviation.

In 2000, Barnes computed a vertical derivative of apparent dip along a user-defined azimuth, and used that calculation to define whether reflections diverged or converged. In this methodology, converging reflections show a decreasing change in dip while divergent reflections show increasing change in dip.

Marfurt and Rich (2010) built upon this method and generated 3-D estimates of reflector-convergence azimuths and magnitudes.

In order to represent the vector nature of reflector convergence in different azimuthal directions, they employed a 2-D color wheel to indicate reflector dip and azimuth.

Reflection Rotation

Compressive deformation and wrench faulting cause fault blocks to rotate. The extent of rotation depends on the size of the block, the lithology and the stress levels.

As individual fault blocks undergo rotation, higher stresses and fracturing may occur at block edges. Natural fractures are partially controlled by such fault-block rotation and partially depend on how individual fault segments intersect.

Fault-block rotation also can control depositional processes by providing increased accommodation space in subsiding areas and enhancing erosional processes in uplifted areas.

In view of the importance of fault block rotation, interpreters need a seismic attribute that allows the rotation of fault blocks to be better analyzed.


In figure 1, we show the behavior of reflection convergence for a channel with and without levee/overbank deposits for four scenarios:

Deposition within the channel that shows no significant convergence.

Deposition within the channel such that the west channel margin converges toward the west and the east channel margin converges toward the east.

Deposited sediments within the channel that do not converge at the margins, and levee/overbank deposits that converge toward the channel (west deposits converge toward the east and vice-versa).

Strata within the channel and levee/overbank deposits that converge to the channel margins.

We carried out the computation of both reflector convergence and reflection rotation for a suite of 3-D seismic volumes from Alberta, Canada. Figure 2 shows a 3-D chair view of a coherence time slice spanning a channel system, co-rendered with reflector-convergence attributes.

Using the scenarios presented in figure 1, our interpretation of the zone within the yellow dotted ellipse is that levee/overbank deposits converge toward the channel margin to the northeast (magenta) and southwest (green).

In figure 3 we show a time slice through a reflector-rotation volume. Notice the horst and graben features show considerable contrast and can be interpreted as distinct geologic regimes.

An equivalent display is shown in figure 4, with a time slice through a reflector-convergence attribute. In this case, the thickening and thinning of reflectors appear to be controlled by rotated fault blocks.


Application of two attributes, namely reflector convergence and reflector rotation, are shown for two different 3-D seismic volumes. These attributes provide complementary information to that provided by amplitude, coherence and curvature attributes.

Reflector-convergence measures the magnitude and direction of thickening and thinning of reflections.

Reflector rotation about faults is demonstrated to be valuable for mapping wrench faults.

We thank Arcis Corporation for permission to show the data examples, as well as for the permission to publish this work.

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Geophysical Corner

Geophysical Corner - Satinder Chopra
Satinder Chopra, award-winning chief geophysicist (reservoir), at Arcis Seismic Solutions, Calgary, Canada, and a past AAPG-SEG Joint Distinguished Lecturer began serving as the editor of the Geophysical Corner column in 2012.

Geophysical Corner - Kurt Marfurt
AAPG member Kurt J. Marfurt is with the University of Oklahoma, Norman, Okla.

Geophysical Corner

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


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Predicting the presence and connectivity of reservoir-quality facies in otherwise mud-prone fluvial overbank successions is important because such sand bodies can potentially provide connectivity between larger neighboring sand bodies. This article addresses minor channelized fluvial elements (crevasse-splay and distributary channels) and attempts to predict the connectivity between such sand bodies in two interseam packages of the Upper Permian Rangal Coal Measures of northeastern Australia. Channel-body percent as measured in well logs was 2% in the upper (Aries-Castor) interseam and 17% in the lower (Castor-Pollux) interseam. Well spacing were too great to allow accurate correlation of channel bodies. The Ob River, Siberia, was used as a modern analog to supply planform geometric measurements of splay and distributary channels so that stochastic modeling of channel bodies was possible. The resulting models demonstrated that (1) channel-body connectivity is more uniform between minor distributary channels than between crevasse-splay channels; (2) relatively good connectivity is seen in proximal positions in splays but decreases distally from the source as channel elements diverge; and (3) connectivity tends to be greater down the axis of splays, with more isolated channel bodies occurring at the margins.
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Organic-carbon–rich shales of the lower Marcellus Formation were deposited at the toe and basinward of a prograding clinothem associated with a Mahantango Formation delta complex centered near Harrisburg, Pennsylvania. Distribution of these organic-carbon–rich shales was influenced by shifts in the delta complex driven by changes in rates of accommodation creation and by a topographically high carbonate bank that formed along the Findlay-Algonquin arch during deposition of the Onondaga Formation. Specifically, we interpret the Union Springs member (Shamokin Member of the Marcellus Formation) and the Onondaga Formation as comprising a single third-order depositional sequence. The Onondaga Formation was deposited in the lowstand to transgressive systems tract, and the Union Springs member was deposited in the transgressive, highstand, and falling-stage systems tract. The regional extent of parasequences, systems tracts, and the interpreted depositional sequence suggest that base-level fluctuations were primarily caused by allogenic forcing—eustasy, climate, or regional thermal uplift or subsidence—instead of basement fault reactivation as argued by previous workers. Paleowater depths in the region of Marcellus Formation black mudrock accumulation were at least 330 ft (100 m) as estimated by differences in strata thickness between the northwestern carbonate bank and basinal facies to the southeast. Geochemical analysis indicates anoxic to euxinic bottom-water conditions. These conditions were supported by a deep, stratified basin with a lack of circulation.
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Over the last two decades, numerical and physical experiments have repeatedly generated insights that contradict the sequence stratigraphic model that is near-universally used to interpret ancient strata in terms of relative changes in sea-level. This presentation will re-examine Upper Cretaceous strata (Blackhawk Formation, Castlegate Sandstone, Mancos Shale) exposed in the Book Cliffs, east-central Utah, USA, which are widely used as an archtype for the sequence stratigraphy of marginal-marine and shallow-marine strata. Stratigraphic architectures in these strata are classically interpreted to reflect forcing by relative sea level, but key aspects can instead be attributed to autogenic behaviors and variations in sediment flux.

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