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: Bulletin Article

This article describes a 250-m (820-ft)-thick upper Eocene deep-water clastic succession. This succession is divided into two reservoir zones: the lower sandstone zone (LSZ) and the upper sandstone zone, separated by a package of pelitic rocks with variable thickness on the order of tens of meters. The application of sequence-stratigraphic methodology allowed the subdivision of this stratigraphic section into third-order systems tracts.

The LSZ is characterized by blocky and fining-upward beds on well logs, and includes interbedded shale layers of as much as 10 m (33 ft) thick. This zone reaches a maximum thickness of 150 m (492 ft) and fills a trough at least 4 km (2 mi) wide, underlain by an erosional surface. The lower part of this zone consists of coarse- to medium-grained sandstones with good vertical pressure communication. We interpret this unit as vertically and laterally amalgamated channel-fill deposits of high-density turbidity flows accumulated during late forced regression. The sandstones in the upper part of this trough are dominantly medium to fine grained and display an overall fining-upward trend. We interpret them as laterally amalgamated channel-fill deposits of lower density turbidity flows, relative to the ones in the lower part of the LSZ, accumulated during lowstand to early transgression.

The pelitic rocks that separate the two sandstone zones display variable thickness, from 35 to more than 100 m (115–>328 ft), indistinct seismic facies, and no internal markers on well logs, and consist of muddy diamictites with contorted shale rip-up clasts. This section is interpreted as cohesive debris flows and/or mass-transported slumps accumulated during late transgression.

The upper sandstone zone displays a weakly defined blocky well-log signature, where the proportion of sand is higher than 80%, and a jagged well-log signature, where the sand proportion is lower than 60%. The high proportions of sand are associated with a channelized geometry that is well delineated on seismic amplitude maps. Several depositional elements are identified within this zone, including leveed channels, crevasse channels, and splays associated with turbidity flows. This package is interpreted as the product of increased terrigenous sediment supply during highstand normal regression.

Desktop /Portals/0/PackFlashItemImages/WebReady/sequence-stratigraphy-of-the-eocene-turbidite.jpg?width=50&h=50&mode=crop&anchor=middlecenter&quality=90amp;encoder=freeimage&progressive=true 5773 Bulletin Article

Criteria for recognizing stratigraphic sequences are well established on continental margins but more challenging to apply in basinal settings. We report an investigation of the Upper Devonian Woodford Shale, Permian Basin, west Texas based on a set of four long cores, identifying sea level cycles and stratigraphic sequences in an organic-rich shale.

The Woodford Shale is dominated by organic-rich mudstone, sharply overlain by a bioturbated organic-poor mudstone that is consistent with a second-order eustatic sea level fall. Interbedded with the organic-rich mudstone are carbonate beds, chert beds, and radiolarian laminae, all interpreted as sediment gravity-flow deposits. Bundles of interbedded mudstone and carbonate beds alternate with intervals of organic-rich mudstone and thin radiolaria-rich laminae, defining a 5–10 m (16–33 ft)-thick third-order cyclicity. The former are interpreted to represent highstand systems tracts, whereas the latter are interpreted as representing falling stage, lowstand, and transgressive systems tracts. Carbonate beds predominate in the lower Woodford section, associated with highstand shedding at a second-order scale; chert beds predominate in the upper Woodford section, responding to the second-order lowstand.

Additional variability is introduced by geographic position. Wells nearest the western margin of the basin have the greatest concentration of carbonate beds caused by proximity to a carbonate platform. A well near the southern margin has the greatest concentration of chert beds, resulting from shedding of biogenic silica from a southern source. A well in the basin center has little chert and carbonate; here, third-order sea level cycles were primarily reflected in the stratigraphic distribution of radiolarian-rich laminae.

Desktop /Portals/0/PackFlashItemImages/WebReady/a-sequence-stratigraphic-framework-for-the-Upper-Devonian-Woodford-Shale.jpg?width=50&h=50&mode=crop&anchor=middlecenter&quality=90amp;encoder=freeimage&progressive=true 3250 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 4061 CD-DVD

See Also: Events Blog

The Arctic Technology Conference (ATC) awarded its 2015 Spotlight on Arctic Technology Awards at the Comwell Convention Center (Bella Center) in Copenhagen, Denmark to two companies that have made innovative contributions to Arctic exploration and production.

Desktop /Portals/0/PackFlashItemImages/WebReady/atc2015-spotlight-winners.jpg?width=50&h=50&mode=crop&anchor=middlecenter&quality=90amp;encoder=freeimage&progressive=true 17122 Events Blog

See Also: Hedberg Abstract

Desktop /Portals/0/images/_site/AAPG-newlogo-vertical-morepadding.jpg?width=50&h=50&mode=crop&anchor=middlecenter&quality=90amp;encoder=freeimage&progressive=true 17141 Hedberg Abstract