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Seismic Model of a Mixed Siliciclastic-Carbonate Ramp to Basin Transition: Southwest Paradox Basin, Colorado and Utah, Usa

Wunderlich, Alexander 1; Kiefer, Ernst 3; Egenhoff, Sven 2; Schneider, Jörg 1; Bohlen, Thomas 1
1 Department of Geosciences, TU Bergakademie Freiberg, Freiberg, Germany.
2 Department of Geoscience, Colorado State University, Fort Collins, CO.
3 VNG - Verbundnetz Gas AG, Leipzig, Germany.

In the southwestern part of the Paradox Basin, Colorado and Utah, USA a Carboniferous mixed siliciclastic-carbonate ramp developed in an intraforeland flexural basin. The transition of the ramp to the evaporite basin was used the produce a seismic model that represents 300 m of stratigraphic thickness over a distance of 4 km.

Detailed lithological models were constructed using well logs from boreholes that penetrate strata which were described in detail from high-resolution core description. With the help of GOCAD® a lithological model was produced which corresponds to an oblique proximal-distal transect through the basin, which was used as the input model for computing synthetic seismic sections. Petrophysical properties were derived from Neutron and Sonic

Logs and were assumed to be homogenous and invariant throughout the derived lithological units. With the help of the velocity-stress-formulation of the seismic wave equation two time section were constructed, using a 25 and 100 Hz Ricker source wavelet. This approach of seismic modeling allows the computing of real seismic data and compared to different techniques it is better suited for further processing.

In the input model five depositional sequences were mapped. Comparison with the synthetic seismic section shows that, at a frequency of 25 and 100 Hz, not even one unconformity was imaged. The synthetic seismic section also reveals that, like in carbonate platforms, pseudo-unconformities, due to rapid changes of dip were not imaged. Both, reflections and unconformities were not correctly imaged according to two main facts: a) thickness of rock units is below seismic resolution and b) destructive interference as well as out of phase shift prevents perfect imaging of given geological features.

These limitations must be taken into account when making sequence interpretation in such environments based solely on seismic data. The use of biostratigraphic data tied-in with synthetic sections is highly recommended to resolve nonangular or concordance unconformities and their correlative conformities.


AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009