Shining Light on a Shady Situation

Contributors: Arthur Barnes

Seismic reflection data come alive when displayed with shaded relief.

With shaded relief, time slices look like illuminated topography, and vertical sections look like rugged canyon walls; faults, domes, anticlines, synclines, channels and even gas clouds stand out boldly.

Shaded relief displays are ubiquitous in geology and geophysics. Elevations, bathymetry, gravity, magnetic and other kinds of map data are routinely displayed with shaded relief to make maps that look like photographs of apparent topography.

Such maps are powerful aids to geologic intuition because apparent topography often suggests true underlying geology.

Though contour maps offer the same information, shaded relief maps present the information in a way that is more natural – and so more readily comprehensible.

Adding shaded relief to 3-D seismic data is similar to adding shaded relief to maps, with the difference that shading is applied to all reflection surfaces in the seismic volume, not to a single horizon. Thus seismic shaded relief is inherently 3-D, so that both time slices and vertical sections appear illuminated.

The process of adding shaded relief to seismic data is simple: create a shaded relief seismic attribute and blend it with the seismic data (figure 1). A shaded relief seismic attribute quantifies the amount of light that seismic surfaces reflect when illuminated by a distant light source (figure 2).

This quantity – the shading – is a function of the angle of incidence of the illumination, which depends on reflection orientation and the position of the sun. Shading can be controlled by exaggerating reflection slopes to enhance contrasts, or by adjusting surfaces to appear dull like shale, shiny-like water or moderately shiny-like quartz sand.

Because shaded relief depends on the sun position, it acts as a directional filter. Features that trend perpendicular to the illumination direction are highlighted, while features that trend parallel are hidden.

To capture all trends, it is necessary to create two shaded relief attribute volumes using orthogonal illumination directions.

Blending seismic data with shaded relief complements blending data with a discontinuity attribute because shaded relief reveals different structural features than continuity, principally anticlines, synclines and domes (figure 3).

Like discontinuity, shaded relief also reveals faults and channels (figure 4), with the advantage that it can indicate the direction of throw on a fault and show the internal geometry of the channel.

A shaded relief seismic attribute can have arbitrary resolution, but it tends to provide better results when it is fairly smooth and clean (as in the data examples presented here). Smoothed shaded relief highlights large features and trends that might otherwise be obscured by details in the data; it lets one see the forest for the trees. In this way shaded relief can serve as a useful tool for rapid reconnaissance of structure in a seismic volume.

Of course, smoothing reduces the resolution of the shaded relief so that small features, such as narrow channels and minor faults, will not be seen. These features are often best imaged by discontinuity and curvature attributes.

Almost everything we do to prepare seismic data for conventional interpretation is designed to make images that look as much like geology – and as little like seismic waves – as we can. Seismic shaded relief is another small step in this direction.

Can shaded relief aid our understanding of seismic data as much as it aids our understanding of geologic maps?

Only time will tell.

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Geophysical Corner - Rongfeng Zhang

Rongfeng Zhang is a senior geoscientist with Geomodeling Technology Corp.

Arthur Barnes, an AAPG member, is with Landmark Graphics Corp., Highlands Ranch, Colo. He can be contacted at Landmark .

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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Oil degradation in the Gullfaks field led to hydrogeochemical processes that caused high CO2 partial pressure and a massive release of sodium into the formation water. Hydrogeochemical modeling of the inorganic equilibrium reactions of water-rock-gas interactions allows us to quantitatively analyze the pathways and consequences of these complex interconnected reactions. This approach considers interactions among mineral assemblages (anorthite, albite, K-feldspar, quartz, kaolinite, goethite, calcite, dolomite, siderite, dawsonite, and nahcolite), various aqueous solutions, and a multicomponent fixed-pressure gas phase (CO2, CH4, and H2) at 4496-psi (31-mPa) reservoir pressure. The modeling concept is based on the anoxic degradation of crude oil (irreversible conversion of n-alkanes to CO2, CH4, H2, and acetic acid) at oil-water contacts. These water-soluble degradation products are the driving forces for inorganic reactions among mineral assemblages, components dissolved in the formation water, and a coexisting gas at equilibrium conditions.

The modeling results quantitatively reproduce the proven alteration of mineral assemblages in the reservoir triggered by oil degradation, showing (1) nearly complete dissolution of plagioclase; (2) stability of K-feldspar; (3) massive precipitation of kaolinite and, to a lesser degree, of Ca-Mg-Fe carbonate; and (4) observed uncommonly high CO2 partial pressure (61 psi [0.42 mPa] at maximum). The evolving composition of coexisting formation water is strongly influenced by the uptake of carbonate carbon from oil degradation and sodium released from dissolving albitic plagioclase. This causes supersaturation with regard to thermodynamically stable dawsonite. The modeling results also indicate that nahcolite may form as a CO2-sequestering sodium carbonate instead of dawsonite, likely controlling CO2 partial pressure.

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The Molasse Basin represents the northern foreland basin of the Alps. After decades of exploration, it is considered to be mature in terms of hydrocarbon exploration. However, geological evolution and hydrocarbon potential of its imbricated southernmost part (Molasse fold and thrust belt) are still poorly understood. In this study, structural and petroleum systems models are integrated to explore the hydrocarbon potential of the Perwang imbricates in the western part of the Austrian Molasse Basin.

The structural model shows that total tectonic shortening in the modeled north–south section is at least 32.3 km (20.1 mi) and provides a realistic input for the petroleum systems model. Formation temperatures show present-day heat flows decreasing toward the south from 60 to 41 mW/m2. Maturity data indicate very low paleoheat flows decreasing southward from 43 to 28 mW/m2. The higher present-day heat flow probably indicates an increase in heat flow during the Pliocene and Pleistocene.

Apart from oil generated below the imbricated zone and captured in autochthonous Molasse rocks in the foreland area, oil stains in the Perwang imbricates and oil-source rock correlations argue for a second migration system based on hydrocarbon generation inside the imbricates. This assumption is supported by the models presented in this study. However, the model-derived low transformation ratios (lt20%) indicate a charge risk. In addition, the success for future exploration strongly depends on the existence of migration conduits along the thrust planes during charge and on potential traps retaining their integrity during recent basin uplift.

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Carbonate submarine slopes have a tendency to be steeper than their siliciclastic counterparts, an observation that is generally attributed to microbial binding and early cementation in carbonates. However, careful comparison of gross development, curvature, and angle of dip in similar settings shows surprising similarities between siliciclastic and carbonate slopes. This paper presents examples of the various systems from seismic and outcrop and proposes a workflow that facilitates more systematic and improved prediction of carbonate and siliciclastic depositional systems ahead of drill.

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 SONAR, historical and aerial photographs, and vibracoring were used to assess the type and thickness distribution of sediments impounded by Gold Ray Dam on the Rogue River in southern Oregon. From these data, a volume of about 400,000 cubic yards (

Equation EG13006eq1

) of sediment was determined for the inundated area of the reservoir.

Overall, sediment volumes in the impounded part of the reservoir were less than expected. There are three possibilities that may explain the perceived absence of sediment: (1) the gradient of the Rogue River in this stretch is less, and therefore sediment yields are less; (2) the extraction of gravels and/or other impediments upstream decreased the availability of sediments delivered into the reservoir; and/or (3) sediment was deposited by a prograding delta that filled in the inundated area of the floodplain upstream from Gold Ray Dam. The amount of sediment deposited on this inundated floodplain may have been as much as 1,800,000 cubic yards (Equation EG13006eq2), bringing the total amount of sediment impounded by Gold Ray Dam to Equation EG13006eq3 yards (Equation EG13006eq4).

Applied sedimentology is not only vital to developing a depositional model for the filling of a reservoir, but also providing insights into depositional and erosional changes that will occur upon the removal of a dam. In particular, the processes of delta formation, reoccupation of abandoned channels, and avulsion are paramount in determining sediment accumulation and distribution in reservoirs.

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