In Overthrust Settings, Tie, Tie (2-D) Again

Contributors: Rob Vestrum

In the rough terrain of overthrust settings, 2-D seismic data continues to be a standard tool for subsurface mapping – and not only because of economic reasons. Two-D and 3-D seismic surveys are complementary in land environments, because each data type has its own strength and weakness.

Three-D seismic data gives us a three-dimensional image volume of the subsurface, with no out-of-plane energy problems or potential to miss structural details between 2-D profiles. With such limitations in 2-D seismic data, one might argue that a better exploration strategy would be to just shoot 3-D surveys and not bother with 2-D seismic data, which may be getting obsolete.

However, in land seismic acquisition with rough terrain and heavy vegetation, access restrictions make the logistics difficult and expensive to acquire 3-D seismic data with high density. Two-D surveys give us overall higher fold and much higher resolution – and the improved resolution in the shallow section helps us tie surface geology to the subsurface reflectors.

Where 2-D and 3-D data overlap, the 2-D lines can complement the 3-D interpretation with a higher-resolution perspective.

So, for scientific as well as economic reasons, 2-D seismic data will continue to be a mainstay in resource exploration in compressional and transpressional geologic settings.

One of the major pitfalls when interpreting 2-D seismic data is dealing with out-of-plane reflections, especially when trying to tie intersecting lines in structured areas.

Structural geologists and interpretation geophysicists can understand the problem of reflection event correlation across intersecting depth profiles and overcome the difficulty by considering the direction of propagation of seismic energy.

Tying 2-D Profiles in Structure

When processing seismic images in thrust-belt areas, it is rare that we are able to make a perfect tie between intersecting 2-D lines.

It is possible to manage the mistie in the time shifts and wavelet character differences between lines, but when we have dipping reflectors on our seismic data, the reflection energy will be coming from out of the 2-D plane of acquisition, resulting in a mistie in time that a simple static shift cannot repair.

\Figure 1 shows two intersecting depth-migrated lines over a thrusted structure in the foothills of the Andes. The left half of the figure shows the dip line. The dips in the overthrust range between 10 degrees and 30 degrees. The right side if the figure is the intersecting strike line.

Note that there is a reasonably good tie between the two lines below 3.5 kilometers depth, where there are relatively flat layers in the footwall. Above the fault (~3.3 kilometers depth at the intersection), the reflectors on the strike line do not line up with the reflectors on the dip line. The layers in the shallow section are dipping, so the reflectors on the strike line are imaged from out of the 2-D plane.

Since we illuminate the reflectors at angles near the bedding-plane normal, if one wanted to correlate these dipping reflectors, then one would need to align the sections along the bedding-normal direction.

Figure 2 shows the improvement in reflector alignment in the shallow section if we rotate the strike line 10 degrees counter-clockwise about the intersecting point at the surface. In this orientation, the correlation is along a direction normal to bedding on the dip line.

After the rotation (figure 2), the reflector alignment is significantly improved between dip and strike lines in the hanging wall. The footwall reflectors, which are more flat, do not tie as well in figure 2 as with the vertical tie in figure 1,because the normal-to-bedding direction of these layers is near vertical.

Even though the strike line imaged the subsurface reflector outside of its 2-D plane, we still can correlate the two lines by orienting the strike line in the direction normal to bedding.

There will still be challenges in creating a 3-D structure map, but at least one may tie the reflectors between lines to ensure consistent mapping over the entire area of 2-D coverage.

Conclusions

When tying 2-D lines in structure, one must not only consider possible differences in static shifts and the phase of the seismic wavelet between intersecting lines, but we also need to consider possible problems with out-of-plane energy.

In reasonably simple geometries with gentle dips, rotating the seismic section at the surface intersection point may simplify the problem of correlating reflectors between dip and strike lines.

Comments (0)

 

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 - Ritesh Kumar Sharma

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

Geophysical Corner - Rob Vestrum

Rob Vestrum is chief geophysicist at Thrust Belt Imaging, Calgary, Alberta, 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: Book

Desktop /Portals/0/images/_site/AAPG-newlogo-vertical-morepadding.jpg?width=50&h=50&mode=crop&anchor=middlecenter&quality=90amp;encoder=freeimage&progressive=true 4384 Book

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: CD DVD

Desktop /Portals/0/images/_site/AAPG-newlogo-vertical-morepadding.jpg?width=50&h=50&mode=crop&anchor=middlecenter&quality=90amp;encoder=freeimage&progressive=true 4034 CD-DVD

See Also: Field Seminar

Join us on a trip to the Capricorn-Bunker group located around the world-famous Southern Great Barrier Reef.
Desktop /Portals/0/PackFlashItemImages/WebReady/FT8-Geology-of-Heron-Island-hero.jpg?width=50&h=50&mode=crop&anchor=middlecenter&quality=90amp;encoder=freeimage&progressive=true 19061 Field Seminar

See Also: Online e Symposium

The surprising emergence of several new exploration plays and new ideas on the basin history demonstrates that we have much more to learn and harvest from this natural laboratory of sedimentary processes.
Desktop /Portals/0/PackFlashItemImages/WebReady/oc-es-the-gulf-of-mexico-basin-new-science-and-emerging-deep-water-plays.jpg?width=50&h=50&mode=crop&anchor=middlecenter&quality=90amp;encoder=freeimage&progressive=true 5808 Online e-Symposium