Modeling Procedure and Related Statements
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normal-incidence, ray-theory program. Neidell and Poggiagliolmi (1977) summarized the general principles and use of ray-theory models. Several seismic models were also generated by a wave-equation solution, but these models did not differ significantly from the models produced by the ray-theory program.
Each seismic model begins with a restored stratiraphic section through the stratigraphic-trap field of interest. All the stratigraphic sections in the text were constructed by the authors from available borehole logs, core data, and lithology logs. Where a section is modified from a previously published section, it is referenced accordingly. A few borehole logs from each field are presented in the text to acquaint the reader with the evidence on which the geologic interpretation was based. To validate the entire stratigraphic section, the reader can assemble all of the referenced log data and reconstruct the section.
From the geologic cross sections and their associated borehole logs we extracted the following modeling parameters for each major stratigraphic interval: (1) velocity (ft/sec), (2) density (g/cu cm), (3) thickness (ft), and (4) geometry. Our models commonly consist of 5 to 10 well and dry hole locations, 10 to 20 interfaces, and over 100 velocity and density values. Most velocity and density values were obtained from hand- blocked acoustic and density logs. The remainder of the values were calculated from porosity measurements or log crossplots (Schlumberger Ltd., 1972). We assumed that the acoustic- and density-log values for the reservoir units adequately account for the hydrocarbon.
Figures & Tables
Stratigraphic-trap classifications used in this investigation, follow Rittenhouse (1972).
These subtle stratigraphic traps are representative of the type of trap that exploration geologists and geophysicists commonly pursue in the Rocky Mountain basins. If known specific stratigraphic-trap fields can be detected and identified with surface reflection seismic data, then these data may possibly serve as guides to explore for analogous but undiscovered fields.
Our approach to documenting the seismic response of the known stratigraphic traps involves three specific steps: (1) simulate the seismic response of a selected stratigraphic-trap field with digital seismic modeling, (2) run vertical seismic profile (VSP) experiments in available wells and dry holes to measure the in situ acoustic properties of both the reservoir and the trap facies, and (3) gather surface seismic data across the field, near available VSP control, to corroborate the model and VSP studies. Each successive stage of the investigation contributes more documentation of the waveform character or seismic signature accompanying the field.
The objective of this publication is to present the results of seismic model studies of 15 known stratigraphic traps that were obtained during the first part of the three-fold investigation. The modeled fields have sandstone reservoirs representative of depositional settings, diagenetic histories, and burial depths. Vertical seismic profile and surface seismic experiments will follow where the model studies indicate convincing seismic anomalies. For one of the models, the Red Bird field in the Powder River basin of Wyoming, an entire seismic-stratigraphic investigation is already (Balch et al, 1981).