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A Case Study of Structural Prospect Validation Using Turning Ray Tomography

Ley, Robert E.*1; McNeely, Joseph 1; Bagshi, Adnan 2; Buick, Darren 2
(1) Exploration Operation, Saudi Aramco, Dhahran, Saudi Arabia. (2) Area Exploration, Saudi Aramco, Dhahran, Saudi Arabia.

The area presented in this case study is located in the Eastern Province of the Kingdom of Saudi Arabia. Karst features within the area contribute to a complex near surface with sinkholes and partially dissolved layers.

The seismic data were originally processed using static corrections computed from a near surface model based on the intercept/time method. This model requires estimation of the weathering and refractor velocities at each shot location. Static corrections are computed by traveling downward through the local weathering velocity to the base of model, and then traveling to the reference datum at the local refractor velocity. This type of near surface model assumes that any lateral velocity variations are small.

The time interpretation of the seismic data, derived using the intercept/time method for the static corrections, revealed several moderate sized anticlinal closures. Isochron mapping between interpreted horizons did not reveal significant thinning over a number of these time closures, raising doubt as to the existence of the anticlines in the depth domain. Further investigation was undertaken to assess the risk that these time closures could be caused by unresolved near surface problems.

The seismic data were reprocessed using static corrections computed from a near surface model derived from turning ray tomography. An initial model was built using velocities defined by the first arrivals. Ray tracing and inversion were run until the modeled and observed picks converged.

The horizon interpretation was then updated on the reprocessed seismic data that used the new near surface model. The anticlinal structures previously mapped were either reduced in size or completely removed by the updated statics model. This agreed with the isochron mapping and well based isopachs, which did not reveal any significant thinning over the mapped highs. Several structures previously designated as leads were shown to be false structures caused by near surface effects.

Using a more complex near surface model derived from tomography enabled the interpreters to make a more realistic assessment of the structural leads found in the study area. The advantages of a tomographic solution are that it uses a nonlinear ray tracing method, it can accommodate rapid velocity changes, and the velocity model is gradational.

 

AAPG Search and Discovery Article #90141©2012, GEO-2012, 10th Middle East Geosciences Conference and Exhibition, 4-7 March 2012, Manama, Bahrain