Differential Compaction: 3-D Attributes Can Help

Contributors: Kurt Marfurt

Differential compaction has been used by seismic interpreters to map features of exploration interest, such as carbonate build-ups, fans and fluvial channels. However, the 3-D mapping of compaction can take considerable time – and so many times it is not pursued.

But 3-D seismic attributes such as coherence and curvature can be used to map compaction features.

This month we illustrate the more common differential compaction expressions over paleo channels and carbonate reefs using examples from the Western Canadian Sedimentary Basin.

Seismic attributes such as coherence and curvature are routinely used for mapping structural features such as faults, folds and flexures:

  • The coherence attribute, which is a measure of similarity of seismic traces, has been used for mapping discontinuities that arise at channel edges, or boundaries, of reefs.
  • Similarly, curvature is a measure of bending of seismic reflections and has been used to map faults and folds. Calibrated with borehole image log data, curvature can be used to predict fractures. As differential compaction causes the deformation of overlying, previously flat surfaces, such surfaces can be used to map underlying features of interest.

As the name implies, “differential” compaction is a topological change due to lateral changes in the loss of rock volume as it lithifies. Channels that are filled with shale and are separated by sandier interfluves will appear as structural lows. Channels that are filled with sand cut through a shale matrix may appear as structural highs.

Channels incised in older, previously compacted rock, will usually appear as lows regardless of their fill.

In figure 1 we show a chair display for a strat-cube constructed from the coherence attribute volume and a vertical slice through seismic amplitude perpendicular to the thalweg of the channel. The seismic signature of the incised channel is seen at the positions of the colored arrows, the incision being deeper at the location of the yellow arrow, less deep at the location of the orange arrow and lesser at the green arrow.

The disposition of the channel is clearly indicated on the coherence strat-slice.

In figure 2 we show a chair display with seismic amplitude as the vertical section, and for the horizontal section we have overlaid the most-positive (red) and the most-negative curvature (blue) attributes using transparency.

Notice the edges of the channel are seen in red and the axis of the channel is defined in blue. Segments of a deeper channel also are seen in the display and these have been marked with light blue arrows.

In general, it is easier to reduce pore space – and therefore compact shales – than sand. If the channel is filled with sand, it will appear to have a positive relief feature seen over the length of the channel and slight negative relief feature at the edges of the channel. The coherence attribute picks up the edges of such a channel – however, the most-positive curvature attribute shows the central mound over the channel (figure 3a) and the most-negative curvature defines the edges of the channel as shown with the help of yellow arrows in figure 3b. Such patterns serve as a lithologic indicator.

Carbonates are stronger and more difficult to compact than sands and shales. While living, the carbonate mound was structurally higher than the surrounding sediments. Once it died it was covered by flat overlying, typically softer, more easily compacted sediments.

Differential compaction between the reef carbonate facies and the off-reef facies results in an image whereby the overlying sediments appear to drape across the carbonate buildup. The extent of the drape would depend on the variation in the compaction of the reefal and the off-reef material as well as the thickness of the overlying sediments.

In figure 4a we show a chair display with the horizon slice from coherence and correlated with the vertical seismic section.

Notice the prominent reef feature on the coherence and the drape of the seismic reflections over it – the most-positive curvature (figure 4b) defines the mound and corresponds to the edges of the reef clearly.

Thus, a clear understanding of the effects of differential compaction can facilitate the rapid interpretation of otherwise subtle, and perhaps otherwise overlooked geological features of interest.

We thank Arcis Seismic Solutions for encouraging this work and for permission to present these results. 

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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 - Rob Vestrum

Rob Vestrum is chief geophysicist at Thrust Belt Imaging, Calgary, Alberta, Canada.

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

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.

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Sequence stratigraphy and coal cycles based on accommodation trends were investigated in the coal-bearing Lower Cretaceous Mannville Group in the Lloydminster heavy oil field, eastern Alberta. The study area is in a low accommodation setting on the cratonic margin of the Western Canada sedimentary basin. Geophysical log correlation of coal seams, shoreface facies, and the identification of incised valleys has produced a sequence-stratigraphic framework for petrographic data from 3 cored and 115 geophysical-logged wells. Maceral analysis, telovitrinite reflectance, and fluorescence measurements were taken from a total of 206 samples. Three terrestrial depositional environments were interpreted from the petrographic data: ombrotrophic mire coal, limnotelmatic mire coal, and carbonaceous shale horizons. Accommodation-based coal (wetting- and drying-upward) cycles represent trends in depositional environment shifts, and these cycles were used to investigate the development and preservation of the coal seams across the study area.

The low-accommodation strata are characterized by a high-frequency occurrence of significant surfaces, coal seam splitting, paleosol, and incised-valley development. Three sequence boundary unconformities are identified in only 20 m (66 ft) of strata. Coal cycle correlations illustrate that each coal seam in this study area was not produced by a single peat-accumulation episode but as an amalgamation of a series of depositional events. Complex relations between the Cummings and Lloydminster coal seams are caused by the lateral fragmentation of strata resulting from the removal of sediment by subaerial erosion or periods of nondeposition. Syndepositional faulting of the underlying basement rock changed local accommodation space and increased the complexity of the coal cycle development.

This study represents a low-accommodation example from a spectrum of stratigraphic studies that have been used to establish a terrestrial sequence-stratigraphic model. The frequency of changes in coal seam quality is an important control on methane distribution within coalbed methane reservoirs and resource calculations in coal mining. A depositional model based on the coal cycle correlations, as shown by this study, can provide coal quality prediction for coalbed methane exploration, reservoir completions, and coal mining.

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