The research was conducted to produce 3-D vector-wave field images of geologic systems and then to analyze these 3-D images to determine what new geologic information can be determined by combining a shear (S)-wave based seismic stratigraphy interpretation with a compressional (P)-wave-based seismic stratigraphy analysis.
This project was selected under the Historical Black Colleges and Universities program.
Prairie View A & M University
Prairie View, TX
Bureau of Economic Geology
University of Texas
The data used in the study were acquired by 9-C vertical seismic profile (VSP) using three orthogonal vector sources. The 9-C VSP is capable of generating P-wave mode and the fundamental S-wave mode (SH-SH and SV-SV) directly at the source station and permits the basic components of elastic wavefield (P, SH-SH, and SV-SV) to be separated from one another for the purposes of imaging. Analysis and interpretation of data from the study area show that incident full-elastic seismic wavefield is capable of reflecting four different wave modes-P, SH, SV and C-that can be utilized to fully understand the architecture and heterogeneities of geological sequences. The conventional seismic stratigraphy utilizes only reflected P-wave modes. The notation SH mode is the same as SH-SH; SV mode means SV-SV and C mode, which is converted sheer wave is a special SV mode and is the same as P-SV.
The internal complexities and heterogeneities within an oil reservoir can be characterized with seismic stratigraphy. Traditionally, most oil reservoir characterization studies are done with only compressional P-wave seismic data. This project demonstrated that the full science of reservoir characterization can be achieved by incorporating the principles and application of vector-wave field seismic data in which geologic systems are interpreted using both P-wave and S-wave images of subsurface stratigraphy. This is because sometimes spatially coincident P and S seismic profiles do not show the same reflection sequences or the same lateral variations in seismic facies character.
The utilization of full-elastic seismic wavefield can be maximized in oil and gas exploration using knowledge of 9-component (9-C) seismic profiling in order to optimize the search for hydrocarbons.
The research program was designed to attempt to answer the following questions:
The 3-D, 9-C data were recorded using midpoint imaging concepts that are standard practice in the oil and gas industry. Three orthogonal vibrators were used to generate a 9-C VSP-vertical, inline horizontal, and crossline horizontal. The geometry of the three orthogonal vibrators created stacking bins measuring 110 feet x 82.5 feet across the image space, with a stacking fold of 20-24 in the full-fold area of each data acquisition grid. The recording template that moved across the image space consisted of six parallel receiver lines, each spanning 96 receiver stations. Three-component geophones were deployed at each receiver station of this 3-D grid. Each receiver string deployed at a receiver station contained three 3-C geophones, and all three geophones were positioned in an area spanning 3-5 feet to form a point array. The geophones were planted carefully to position one horizontal element in the inline direction (the direction in which the receiver line was oriented) and the second horizontal element in the crossline direction. Large (52,000 lb) vibrators were used to generate the 9-C data.
These four wave modes image unique geologic stratigraphy and facies and at the same time reflect independent stratal surfaces because of the unique orientation of their particle-displacement vectors. As a result of the distinct orientation of an individual mode's particle-displacement vector, one mode may react to a critical subsurface sequence more than the other. It was also observed that P-waves and S-waves did not always reflect from the same stratal boundaries.
The P- and C-waves are capable of imaging different stratal surfaces because P and C modes have different reflectivities at impedance boundaries. It was observed that it is possible for either P or C mode to have a zero or near-zero reflectivity at a given stratal geologic surface, while the other mode has a large reflectivity.
It was concluded that, in a complex geologic environment, it is necessary that the sedimentary record be described by one set of P-wave seismic sequences (and facies), and by a second, distinct set of S-wave sequences (and facies). The application of both P-and S-wave images to oil reservoir characterization is the current trend in most oil and gas companies and will sooner or later overtake the conventional seismic stratigraphy of only P-wave imaging.
The project is complete.
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