An integrated study for lithology and fluid prediction in the Cretaceous sediments from Halten Terrace. - Rock Physics Templates, AVO attributes and Inversion analysis.

Abstract

AVO attributes analysis and Seismic inversion are useful tools for quantitative reservoir characterization. They are used to validate hydrocarbon presence and mitigate the exploration risk. This study has as objective the fluid and lithology discrimination in the Lysing and Lange sandstones in the Halten Terrace, Norwegian Sea, by connecting the well-logging observations to seismic scale. Post-stack 3D seismic data has been used for computing the AVO attributes and run the inversion algorithms, while several wells have been accounted for well-control and synthetic modeling.

The fluid factor and relative contrast in S-wave velocity ($V_s$) and P-wave velocity ($V_p$) have been derived from the intercept and gradient using assumptions which honor the local geology. The seismic inversion has been performed for the absolute $V_p$, $V_s$ and density, with the purpose to analyze the results using the Rock Physics Templates (RPT).

The Lysing and Lange sandstones are represented by intercalations of shale and sandstone strin-gers, with thickness below seismic resolution and clay content which varies across the field. Due to that, the sandstones have to be treated as effective medium. The AVO attributes results show a variation of fluid and Net-to-Gross across the fields. The fluid factor and the relative ($V_s$) contrast drop towards zero, when the Net-to-Gross decreases. The inversion results have been plotted in the RPT domain and the data is laying between the shale and water trends.

Hydrocarbon-bearing sandstones have been interpreted on the RPT. The obvious anomalies are correlated with clean sandstones given by high ($V_s$) contrast and high negative fluid factor. The anomalies are in Smørbukk field. The Lysing and Lange sandstones in Smørbukk Sør are shalier then the ones in Smørbukk. However, the hydrocarbon bearing sandstones can be delineated and the results are similar in shape with the anomalies given by the fluid factor. Posterior probabilities for the hydrocarbon-bearing sandstone, water-bearing sandstone and shale have been computed from the seismic inversion. The results are correlated with the with the RPT and AVO attributes.

The Lysing and Lange sandstones are challenging from reservoir characterization point of view, due to the variability of distribution of the shale within the formations and due to thickness below seismic resolution. However, the integration of AVO attributes and Seismic inversion parameters with the Rock physics template has been proved to solve the Lysing and Lange sandstones in the Halten Terrace.