--> Subsalt Seismic Imaging Advances in the Mexican Petroleum Provinces
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Subsalt Seismic Imaging Advances in the Mexican Petroleum Provinces

Abstract

Abstract

Pemex has made a significant change on its depth imaging approach. It has fully embraced this technology for solving complex imaging problems.

On the acquisition side, data has been acquired with Wide Azimuth technology, which has resulted in substantially improved sub-surfaces images.

On the processing side, data is now re-processed from scratch specifically for depth imaging. No Radon is applied for de-multiple. Noise attenuation is applied with care in order to clean data from unwanted energy while preserving weak primary energy with complex travel paths. De-ghosting is a fundamental step, as it results in a significant improvement of the seismic character and resolution of the base of salt reflection and sub-salt and deep events.

RTM is used extensively during the model building phase and for final migrations. Previous HitKirchhoffNext Hit is used for the non-complex zones and for interpretation of the top of salt. It is essential to capture its high rugosity in the PSDM model. We are currently running RTM migrations up to 35 Hz. We expect this limit to increase to 50+ Hz in the not distant future.

We still use Beam migrations for the construction of velocity models and to generate gathers for sub-salt tomography. Unfortunately Beam tends to lose resolution and sometimes make coherent noise appear as real events. On the other hand, with the advent of cost-effective generation of RTM angle gathers, we expect to update sub-salt velocities using entirely RTM.

We have put lots of effort on helping tomography. During velocity model building, we carefully analyze the velocity model for consistency with the geological model of the area and the available well information; we identify and interpret key horizons or surfaces in order to re-trend velocities and guide the tomography inversion; we run Previous HitmigrationNext Hit velocity scans and interpretation scenarios (normally related to salt geometries). We have assembled dedicated groups of Pemex interpreters, geologists and geophysicists, who work closely with the specialists of our service companies and assure the quality of our depth migrated volumes.

After Previous HitmigrationTop, the final image is processed with algorithms based on RTM sectors for attenuating coherent noise and improving event continuity.

All of the above has helped us to produce substantially better seismic images under salt in the deep waters of the North Gulf of Mexico and in the Mesozoic Carbonates of the South Gulf of Mexico, and velocity models that are consistent with geology.