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The AAPG/Datapages Combined Publications Database

AAPG Special Volumes

Abstract


Pub. Id: A165 (1977)

First Page: 99

Last Page: 116

Book Title: M 26: Seismic Stratigraphy--Applications to Hydrocarbon Exploration

Article/Chapter: Seismic Stratigraphy and Global Changes of Sea Level: Part 5. Chronostratigraphic Significance of Seismic Reflections: Section 2. Application of Seismic Reflection Configuration to Stratigraphic Interpretation

Subject Group: Seismic Stratigraphy, Sequence Stratigraphy

Spec. Pub. Type: Memoir

Pub. Year: 1977

Author(s): P. R. Vail, R. G. Todd (2), J. B. Sangree (3)

Abstract:

Primary seismic reflections follow chronostratigraphic (time-stratigraphic) correlation patterns rather than time-transgressive lithostratigraphic (rock-stratigraphic) units. Physical surfaces that cause seismic reflections are primarily stratal surfaces and unconformities with velocity-density contrasts. Stratal surfaces are major bedding surfaces and thus represent ancient depositional surfaces. Unconformities are surfaces of erosion or nondeposition that represent significant chronostratigraphic gaps.

Both stratal surfaces and unconformities have time significance because of the Law of Superposition. In terms of geologic time, reflections from stratal surfaces approximate time-synchronous events, where reflections from unconformities are commonly time-variable. However, unconformity reflections are time-significant because all the strata below the unconformity are older than all the strata above the unconformity.

No physical surface that could generate a reflection parallel with the top of a time-transgressive formation (lithostratigraphic unit) exists in nature. The continuity of the seismic reflection follows the stratal surfaces across the time-transgressive formation boundaries, although reflection character (amplitude, cycle breadth, and waveform) will change as the reflection coefficients and spacing of the stratal surfaces change laterally. Commonly, a given seismic reflection character transgresses reflection continuity as a rock formation transgresses geologic time.

Geologic time correlations made from paleontologic data tie with seismic-reflection correlations even where the latter cross major facies boundaries. In addition, unconformities or their correlative conformities that bound sequences, also commonly bound paleontologic zones especially in the Paleozoic and Mesozoic.

Understanding the chronostratigraphic significance of seismic-reflection correlations and relating them to available well control is essential for stratigraphic trap exploration.

There also are other types of continuous physical surfaces that are locally present in sedimentary rocks. Most significant of these are gas-water, gas-oil, and oil-water fluid contacts and gas hydrate zones. Reflections from these types of physical surfaces cut across the reflections originating from the stratal surfaces, if they are at an angle to each other.

Chronostratigraphic correlations of seismic data with well data are accurate only to ± 1/2 cycle breadth owing to possible changes of reflection character caused by changes in bed spacing and reflection coefficients.

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