Dealing With Reservoir Uncertainties in Alentejo Frontier Basin: an Integrated Geological and Geophysical Approach

Abstract

The high quality of present day seismic data integrated with rock physics information from limited well data found in frontier basins can be used to constrain the subjectivity of the interpreters volumetric estimation of the possible available resources. To better constrain the reservoir properties assumed by the geological models, geophysical methods can be used to estimate the reservoir properties necessary for input for the risk and volumetric assessment.

Constrained sparse spike acoustic inversions without wells were performed on available PSDM 3D seismic surveys with the objective to provide a bandlimited impedance volumes, in depth, to aid interpretation and help define interpreted sand bodies.

Seismic and horizons were converted to TWT using the interval velocity volume from processing and a wavelet was estimated from seismic PSDM converted to TWT. Using an old well drilled in the area nearby and the interpreted horizons, P-impedance trends were defined for input to the inversion process. The inversion results were converted back to depth.

To better define the sand bodies already identified with high P-impedance values bodychecking was performed to extract connected geobodies. Thickness maps were produced using the geobodies to calculate net and gross rock volumes for the intervals of interest as input for volumetric and risking assessment.

Additional work focused on analyzing the pre-stack seismic data, without incorporating well data. A Pseudo Gradient process was performed on Santola seismic data. This approximation of the AVO analysis could indicate changes in the lithologies. The purpose is to emphasize events where the differences between Rp and Rs could be associated with lithology anomaly effect. This display will attenuate both weak events (small Rp) and normal lithological event that have similar Rp and Rs. The result shows that the lithology or the fluid content is a dominant event while the bright event from the shale is dimmed to a background. This additional work was used to enhance the understanding of the lithology of the targets.

With these methods, the prediction of lithofacies and the definition of the stratigraphy was facilitated for the whole basin. Using the impedance volumes, a siliciclastic Upper Jurassic and Lower Cretaceous sections were recognized, improving the knowledge of the reservoirs in the area.

AAPG Datapages/Search and Discovery Article #90226 © 2015 European Regional Conference and Exhibition, Lisbon, Portugal, May 18-19, 2015