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Abstract

AAPG Bulletin, V. 83 (1999), No. 6 (June 1999), P. 952-971.

Discriminating Between Onlap and Lithologic Interfingering in Seismic Models of Outcrops1

Guido L. Bracco Gartner and Wolfgang Schlager2
 

©Copyright 1999.  The American Association of Petroleum Geologists.  All Rights Reserved

1Manuscript received February 2, 1998; revised manuscript received December 15, 1998; final acceptance January 10, 1999.
2Vrije Universiteit, Institute of Earth Sciences, De Boelelaan 1085, 1081 HV Amsterdam, Netherlands; e-mail: [email protected]

We thank Lorenz Keim for his significant contribution in the field, as well as in the laboratory. We thank Eric Verschuur of the faculty of applied physics at the Delft University of Technology for running the finite difference modeling program. Ruud de Jongh and Andy Chermak are acknowledged for reviewing earlier versions of this manuscript. AAPG reviewers R. Erlich, W. Helland-Hansen, and J. May are thanked for their constructive criticism. Support by the Industrial Associates Program in Sedimentology at the Vrije Universiteit Amsterdam was instrumental for completing the project. 
 

ABSTRACT

Seismic models of outcrops have shown that near the limits of resolution the seismic reflection tool records rapid facies changes as false lap-outs. We examined one such outcrop, the Triassic platform flank of Picco di Vallandro (Dürrenstein) in northern Italy, to develop criteria for recognizing false lap-out patterns.

Using measured sections, photo mosaics of the continuous outcrop, and petrophysical measurements on samples, two contrasting impedance models were constructed. One model, supported by field observation, shows interfingering between basin marls and slope carbonates. The second model shows onlap of basin marls against slope carbonates, but with similar overall geometries to the first model.

Vertical incidence seismic models were produced at frequencies between 50 and 150 Hz and with displays of seismic amplitude, as well as two seismic attributes-instantaneous phase and reflection strength. At 50 Hz, the two lithologic models are seismically indistinguishable, both showing onlap of basin units onto the slope. At 90 Hz, displays of instantaneous phase begin to reveal the stratigraphic interfingering, whereas seismic amplitude displays require frequencies of 150 Hz to do so. Reflection strength accurately portrays the distribution of the two major facies in the entire range of examined frequencies, but does not show sufficient detail to reveal interfingering vs. onlap.

The same trends were visible with finite difference modeling of the full waveform. Here, the differences between interfingering and onlap become visible at 150 Hz in the phase model, where seismic amplitude displays at this frequency are still ambiguous. The phase model, however, fails to differentiate between slope dip and the basin sediment wedge, whereas this wedge is clearly mapped by the reflection strength. A combination of instantaneous phase and reflection strength provides a decisive advantage over conventional amplitude displays in recognizing rapid facies changes in seismic reflection data. 

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