EXPLORATION/EXPLOITATION

Interpretation of thin beds using stratal surfaces

When two well-defined seismic surfaces are conformal and separated by a uniform thickness, extrapolation into thin beds that lie between them can reveal subtle details

Bob A. Hardage and Randy L. Remington, University of Texas, Austin, Texas

hen interpreting thin-bed reservoirs in 3-D seismic data volumes, finding where a reliable geologic time surface occurs can be challenging. If the area of interest is bracketed by smooth, well-defined seismic reflections that follow certain criteria, then these constant-time surfaces can be extrapolated into the target area.

The purpose of this article is to illustrate that this stratal-surface seismic-interpretation concept becomes valid when applied to a complex channel system along the margin of the Gulf of Mexico.

Introduction

A stratal surface is a depositional bedding plane, that is, a depositional surface that defines a fixed geologic time. A fundamental thesis of seismic stratigraphy is that a seismic reflection event follows the impedance contrast associated with a stratal surface, that is, a surface that represents a fixed point in geologic time.^1,2

As the reflection traverses a prospect area, lithology varies across the region spanned by a large depositional surface. The implication is that a widespread seismic-reflection event does not necessarily mark an impedance contrast boundary between two fixed rock types. The application of this fundamental concept about the genetic origin of seismic reflections to seismic interpretation is referred to as stratal-surface seismic interpretation.

If two seismic reflection events, A and B, are separated by an appreciable seismic-time interval (say a few hundred milliseconds) yet are conformable to each other, then the uniform seismic-time thickness between these two seismic stratal surfaces represents a constant and fixed period of geologic time. A second implication is that any seismic surface intermediate to A and B, which is also conformable to A and B, is also a stratal surface.

Bracketing The Thin Beds

In challenging thin-bed interpretations such as the fluvial channel system considered here, it is important to define two seismic reference surfaces that bracket the thin-bed system that is to be interpreted, one reference surface being below the interpretation target, the second above the target.

By creating conformable reference stratal surfaces above and below a thin-bed system, conformable seismic stratal surfaces can be extrapolated from two directions to sweep across a thin-bed target. One set of seismic stratal surfaces is commonly a better approximation of constant-depositional-time surfaces within the targeted thin-bed sequence than the other set, and produces more accurate images of facies patterns within the thin-bed unit.

Choosing Reference Strata

To illustrate the advantage of this opposite-direction convergence of seismic stratal surfaces onto a thin-bed target, two reference surfaces were interpreted above and below a targeted fluvial system. The fluvial system that is to be imaged is about 24 to 30 ms below, and conformable to, reference surface 2 shown in Fig. 1.

A good-quality reflection peak (reference surface 2) and a good-quality reflection trough (reference surface 1) are labeled. Either the reflection peak or the trough satisfies fundamental criteria that are required of a reference stratal surface used to study thin-bed sequences such as this targeted fluvial channel system, namely:

1. Each event must extend over the total 3-D image space and have a high signal-to-noise character.
2. Each event must be reasonably close to the targeted thin-bed sequences (within 100 ms in this example).
3. Each event must be conformable to the targeted thin-bed sequence.

Criterion 3 is the most important requirement for any seismic stratal surface that is to be used as a reference surface — one from which constant-depositional-time surfaces are to be made that span targeted thin-bed sequences.

Extrapolation Into Target Area

Because each horizon labeled "reference surface" in Fig. 1 follows the apex of an areally continuous reflection peak (trough), the basic premise of seismic stratigraphy is that each reference surface follows an impedance contrast that coincides with a stratal surface.

Downward extrapolation. The reflection-amplitude response across the channel systems, observed on a stratal surface 26 ms below and conformable to the overlying reference surface (surface 2), is displayed in Fig. 2. A high-quality channel image occurs in this case because stratal surfaces that pass through the thin bed and are conformable to the overlying seismic stratal surface are good approximations of constant-depositional-time surfaces for this particular channel system.

Upward extrapolation. Crossline 200 traverses the northern, smaller channel system at a rather oblique angle and then transects at least three loops of the southern, larger channel system, Fig. 2. Data along the other two labeled crosslines (174 and 222) will not be shown. A 200-ms-wide window extending along crossline 200 is displayed in Fig. 3.

Ideally, a reference surface should be smooth. The ripples on this particular surface, such as the three peaklike distortions between inline coordinates 125 and 145, are vertical displacements of only one time sample point (2 ms). The expanded time scale used in this wiggle-trace display magnifies these small irregularities.

Four conformable surfaces, A, B, C and D, are added to this profile. These four surfaces are, respectively, 92, 90, 88 and 86 ms above — and conformable to — the reference surface. Visual inspection of the reflection events above and below surfaces A, B, C and D show that all of these reflection peaks and/or troughs are reasonably conformable to the reference-surface event. These surfaces can thus be assumed to be stratal surfaces, or constant-depositional-time surfaces. This is because they are conformable to a known stratal surface (the reference surface), and embedded in a seismic window in which all reflection events are conformable to the selected reference surface.

Waveform analysis. The circled features in Fig. 3 identify locations where stratal surfaces A, B, C and D intersect obvious variations in reflection waveform. These waveform changes are the critical seismic attributes that distinguish channel facies from nonchannel facies. This can be verified by comparing the inline coordinates spanned by the circled features with the inline coordinates where crossline 200 intersects channel features in Fig. 2.

The two channel systems (northern and southern) are not completely imaged on any of these four horizons, which implies that none of the four surfaces is a perfect stratal surface. Surface B is probably the best approximation of a stratal surface that coincides with the deposition time during which both channel systems were active, Fig. 4.

That considerable portions of the channels appear on this surface is evidence that this horizon is a reasonably good approximation of a stratal surface passing through the thin-bed target. However, B is not as good an approximation of a thin-bed stratal surface as is the surface used to generate the image in Fig. 2.

Conclusions

The interpretation of thin-bed reservoirs in 3-D seismic data volumes can be achieved by: 1) interpreting a reference surface that is conformable to the areal geometry of the thin-bed sequence, and 2) creating seismic stratal surfaces conformable to this reference surface that pass through the thin-bed target.

If the seismic stratal surfaces constructed according to this logic are satisfactory approximations of constant-depositional-time surfaces that existed during deposition of the thin-bed sequence, then seismic attributes across these stratal surfaces can be valuable indicators of facies distributions within the sequence.

A two-pronged approach to thin-bed interpretation can be done by extrapolating seismic stratal surfaces onto the thin-bed target from opposite directions, that is, from both below and above the thin-bed target. The logic in this dual-direction extrapolation is that one of the seismic reference surfaces is generally more conformable to the thin-bed sequence than is the other reference surface. This improved conformability leads to improved attribute imaging of facies distributions within the thin bed.

Literature Cited

¹ Mitchell, R. M., Jr., P. R. Vail and S. Thompson III, "Seismic stratigraphy and global changes in sea level, Part 2, the depositional sequence as a basic unit for stratigraphic analysis," in Dayton, C. E., ed., Seismic stratigraphy — applications to hydrocarbon exploration: American Association of Petroleum Geologists Memoir 26, 1977, pp. 53–62.

² Vail, P. R., and R. M. Mitchum, Jr., "Seismic stratigraphy and global changes of sea level, Part 1, overview," in Payton, C. E., ed., Seismic stratigraphy — applications to hydrocarbon exploration: American Association of Petroleum Geologists Memoir 26, 1977, pp. 51–52.

Copyright © 1999 World Oil
Copyright © 1999 Gulf Publishing Company