Looking High and Low for References

The fundamental criteria required of a seismic reflection event that is to be used as a reference surface for interpreting thin-bed geology are that the seismic reflection should:

  • Extend across the entire seismic image space and have a good signal-to-noise character.
  • Be reasonably close (vertically) to the geology that is to be interpreted.
  • Be conformable to the strata that need to be analyzed.

Criterion 3 is probably the most important requirement on this list.

Figure 1 shows a data window from a vertical slice through a 3-D seismic volume that is centered on a channel system that is to be interpreted.

The seismic reflection event labeled “reference surface” was selected as an appropriate conformable reflection for interpreting the thin-bed channel system identified on figure 1a. The reference surface in this case follows the peak of the seismic reflection event on which it is positioned.

Four horizon surfaces labeled A, B, C and D, each conformable to the reference surface, pass through the targeted channel system on figure 1b.

Each of these horizon surfaces can tentatively be assumed to be a reasonable approximation of a stratal surface that intersects the channel system because each horizon is conformable to the selected reference reflection event, and a fundamental thesis of seismic stratigraphy is that seismic reflection events are chronostratigraphic by definition.

Figure 2a shows reflection amplitude behavior on horizon surface B. This horizon surface does a reasonable job of defining the targeted channel system (channel 1) across the lower right quadrant of the display and also depicts a second channel system (channel 2) at the top of the image display.

The image on figure 2a is a horizon-based image, meaning that the seismic attribute that is displayed is limited to a data window that vertically spans only one data point.

In challenging interpretation problems, it is important to try to define two seismic reference surfaces that bracket the geological interval that is to be interpreted – one reference surface being below the geological target and the other being above the target. An interpreter can then extend conformable surfaces across a targeted interval from two directions (from above and from below).

Sometimes one set of conformable surfaces will be more valuable as stratal surfaces than the other at the level of a targeted thin bed.

To illustrate the advantage of this opposite-direction convergence of seismic horizon surfaces, a second reference surface was interpreted above the targeted channel system and was placed closer to the target interval. This second reference surface followed the apex of the reflection troughs immediately above the channel system. The two bracketing reference surfaces are shown on figure 3.

The reflection amplitude response on a horizon surface conformable to reference surface 2 and positioned 26 milliseconds below that reference surface is displayed on figure 2b. This image is again a one-point-thick attribute display (i.e. a horizon-based attribute).

The channel systems are a bit crisper in appearance and their geometries are more definitive on this second imaging attempt than they were on the first effort (figure 2a).

This dual-direction approach to constructing horizon surfaces that traverse thin-bed targets is a concept that often will provide valuable results. An even better approach would be to calculate stratal slices through a bracketed data window – a concept discussed and illustrated in the article published June 2006.

Unfortunately, not all interpretation software provides a stratal slicing option.

In those cases, a dual-direction-approach strategy such as described here can be valuable for constructing horizon slices that approximate stratal slices.

Comments (0)


Geophysical Corner

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


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 3971 Book

See Also: Bulletin Article

Thus far, the subject of deep-marine sands emplaced by baroclinic currents associated with internal waves and internal tides as potential reservoirs has remained an alien topic in petroleum exploration. Internal waves are gravity waves that oscillate along oceanic pycnoclines. Internal tides are internal waves with a tidal frequency. Internal solitary waves (i.e., solitons), the most common type, are commonly generated near the shelf edge (100–200 m [328–656 ft] in bathymetry) and in the deep ocean over areas of sea-floor irregularities, such as mid-ocean ridges, seamounts, and guyots. Empirical data from 51 locations in the Atlantic, Pacific, Indian, Arctic, and Antarctic oceans reveal that internal solitary waves travel in packets. Internal waves commonly exhibit (1) higher wave amplitudes (5–50 m [16–164 ft]) than surface waves (lt2 m [6.56 ft]), (2) longer wavelengths (0.5–15 km [0.31–9 mi]) than surface waves (100 m [328 ft]), (3) longer wave periods (5–50 min) than surface waves (9–10 s), and (4) higher wave speeds (0.5–2 m s–1 [1.64–6.56 ft s–1]) than surface waves (25 cm s–1 [10 in. s–1]). Maximum speeds of 48 cm s–1 (19 in. s–1) for baroclinic currents were measured on guyots. However, core-based sedimentologic studies of modern sediments emplaced by baroclinic currents on continental slopes, in submarine canyons, and on submarine guyots are lacking. No cogent sedimentologic or seismic criteria exist for distinguishing ancient counterparts. Outcrop-based facies models of these deposits are untenable. Therefore, potential exists for misinterpreting deep-marine baroclinic sands as turbidites, contourites, basin-floor fans, and others. Economic risks associated with such misinterpretations could be real.
Desktop /Portals/0/PackFlashItemImages/WebReady/Modern-internal-waves-and-internal-tides.jpg?width=50&h=50&mode=crop&anchor=middlecenter&quality=90amp;encoder=freeimage&progressive=true 3726 Bulletin Article

See Also: Field Seminar

The field trip will take you to outcrops of the Musandam Peninsula of Ras Al-Khaimah, UAE equivalent in age and architecture to the producing Shuaiba and Kharaib formations in the subsurface of the Arabian Peninsula. Outcrops are of seismic scale and thus provide field-scale cross-sections that help to refine sequence stratigraphic and facies models, and aid to the understanding of reservoir geometry distribution, and reservoir continuity in the subsurface.
Desktop /Portals/0/PackFlashItemImages/WebReady/fs-field-trip-to-the-kharaib-and-shuaiba-formations-11nov-2015-hero.jpg?width=50&h=50&mode=crop&anchor=middlecenter&quality=90amp;encoder=freeimage&progressive=true 20580 Field Seminar

See Also: Online e Symposium

Technical Writing Triage is a condensed course which identifies the key professional and technical writing in today’s workplace, discusses the most common problems/issues, and provides quick, easy-to-implement solutions for producing high-quality, effective communications.

Desktop /Portals/0/PackFlashItemImages/WebReady/oc-es-technical-writing-triage.jpg?width=50&h=50&mode=crop&anchor=middlecenter&quality=90amp;encoder=freeimage&progressive=true 3811 Online e-Symposium

This e-symposium covers how to conduct an interdisciplinary evaluation of mature fields to determine the best approach to recover remaining reserves.

Desktop /Portals/0/PackFlashItemImages/WebReady/oc-es-evaluating-mature-fields.jpg?width=50&h=50&mode=crop&anchor=middlecenter&quality=90amp;encoder=freeimage&progressive=true 1464 Online e-Symposium