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Abstract: Litho-Stratigraphy of the Miocene in the Texas and Louisiana
Transitional Zone: How to
Recognize Sequence Boundaries and Hydrocarbons from Seismic Data
1Geophysical Development Corporation
3Colorado School of Mines
Throughout the Tertiary basins in the Gulf Coast, there are areas where the acoustic-impedance values of shales, wet sands and gas sands are approximately equal. Hydrocarbon zones do not appear as bright spots and are difficult to detect with conventional 3-D seismic data. In some areas, even AVO has not been successful. This difficulty normally occurs when the rock properties are not calibrated to the various local AVO attributes.
A 3-D AVO study in the Texas and Louisiana transitional zone has been completed using well log suites, core analyses, and field production histories. Results from this study are illustrated in Figure 1, where a conventional 3-D section is shown in the upper portion. This section is a mixture of both chrono- and litho-stratigraphic reflections. In the bottom portion of the figure only the litho-stratigraphic reflections are retained.
Correlation of the well log curves and the field production histories to the lower section indicate that all the high-amplitude events are associated with proven hydrocarbon zones. It is obvious that the reflection amplitudes in the upper conventional 3-D seismic section do not identify lithology if the high-amplitude events in the lower litho-stratigraphic section are truly hydrocarbon events.
The basis for unraveling complex AVO responses to various rock types and fluid content is a two-term seismic reflection model. The first term, the Normal Incidence reflectivity (NI), which has historically been related to chrono-stratigraphic reflections, responds to changes in acoustic impedance. The second term, defined as the Poisson Reflectivity (PR), relates to changes in Poisson's Ratio. Unlike NI, PR remains sensitive to lithologic variations within the geologic environment and is thus associated with lithostratigraphic reflections. The rock property contrasts, which generate the NI and PR response, become evident by crossplotting well log values of the natural log of acoustic impedance versus Poisson's Ratio. The crossplots show that even when the sands have the same acoustic impedance as the encasing shales, Poisson's ratio discriminates between them. To obtain a robust estimate of PR from seismic data, the AVO processing incorporates corrections for anisotropy, which extends the AVO analysis out to very-far offset traces. After calibration to log data, the resulting PR sections depict reservoir quality sands and potential pay intervals as litho-stratigraphic sections.
Figure 1. Conventional 3-D section (upper) contains both lithostratigraphic and chronostratigraphic reflections and shows little indication of hydrocarbons while the lithostratigraphic section (lower) depicts eight known gas reservoirs.
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