Characterization of Depositional Layering Based on Spectral Seismic Interpretation

Date

2017-08

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Abstract

Depositional sequences layering parameters such as net-to-gross ratio can affect seismic signatures and their frequency spectra. Various means of estimating net-sand thickness were tested in an oil field with sufficient well penetrations providing known thicknesses to evaluate the seismic predictions. Map-based detuning of the signal strength in proportion to the net-to-gross ratio allowed statistically significant estimates of net-sand thickness. The impedance-based Seismic Ney-Pay method resulted in more accurate estimates (mean absolute prediction error at well validation locations under 3.0 m ± 1.5 m) than those of a reflectivity-based detuning method (4.0 m ± 2.7 m) or multiple linear regression (5.9 m ± 5.8 m); the latter with weak statistical significance. Time-frequency and time-phase analysis showed that for a thin-bed in a binary-impedance setting, there is no observable sensitivity in preferential illumination as NTG changes, while mutual thin-bed interference can significantly change the preferred phase component for each participating target. This phenomenon is largely driven by the embedded seismic wavelet, which determines the nominal seismic response of an isolated thin layer and what phase component would preferentially illuminate it. The joint time-frequency and time-phase analysis applied to field seismic data resulted in a new interpretation of the syn-depositional character of secondary faults cutting the reservoir; raising the possibility of lateral changes in reservoir quality that had not been previously recognized. Forward modelling of depositional sequences yielded P- and S-wave impedances and broad-band reflectivities whose time-invariant spectral magnitude responses follow a power-law of frequency. This allowed using some fractal statistics to relate the spectral magnitude attributes with changes in the underlying layering characteristics, such as the empirical parametrization of the Cumulative Distribution Function. Weak global empirical correlations were found between spectral magnitude attributes and layering parameters when using a limited well-log database. Spectral-magnitude attributes were calculated for a field 3D seismic survey at each trace location for each calculated reflectivity wave mode, with poor statistical significance for correlations between these spectral-magnitude attributes and the layering parameters estimated at the collocated wellbores. These results are inconsistent with the broad-band forward modelling results.

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Keywords

Spectral analysis, Layering, Binary impedance, Time-frequency analysis, Time-phase

Citation

Portions of this document appear in: Meza, Ramses G., Juan M. Florez, Stanislav Kuzmin, and John P. Castagna. "Quantitative assessment of the seismic net-pay method: A case study." Interpretation 3, no. 2 (2015): B25-B36. And in: Meza, Ramses, Gerard Haughey, John Castagna, Umberto Barbato, and Oleg Portniaguine. "Phase decomposition as a hydrocarbon indicator: A case study." In SEG Technical Program Expanded Abstracts 2016, pp. 1839-1843. Society of Exploration Geophysicists, 2016.