Application of the Rock Physics Model in Anisotropic Seismic Velocity Model Building and Quantitative Reservoir Structural Uncertainty Analysis: Gulf of Mexico Case Study
Authors: Yang Yi , Osypov Konstantin , Bachrach Ran , Nichols Dave , Woodward Marta , Zdraveva Olga , Liu Yangjun , Fournier Aime , You Yu 
Published: March 2015

Abstract
Accurate anisotropic seismic velocity model building is the key to the success of seismic depth imaging projects in complex geological settings. Tomography has been an industry standard velocity model building tool for decades, but simultaneously solving for P-wave velocity, epsilon, and delta with surface seismic data only is an underdetermined inverse problem and unstable. The ambiguity in seismic migration velocity model leads to structural uncertainty in seismic image and is carried over to uncertainty in reservoir modeling. In this paper, we introduce a new method using rock physics compaction modeling of sandy shales to constrain the anisotropic tomography. An effective-media rock model was calibrated with well data for sedimentary basin and was used to build initial vertical transverse isotropy (VTI) velocity models. By running a stochastic simulation of the rock physics model, covariance functions were extracted from possible combination of to P-wave velocity, epsilon, and delta as a priori information to constrain the following anisotropic tomography updates and uncertainty analysis. The case study area is in the Green Canyon in the Gulf of Mexico. The results show that we can successfully constrain three parameters of tomography with the prior information from rock physics. We also performed seismic uncertainty analysis to assess the non-uniqueness of the tomography solutions. 500 velocity models with equivalent residual move out were generated and used to map migrate the reservoir structures. The gross rock volume P10, P50 and P90 were calculated from these 500 realizations to demonstrate the reduction of uncertainty from the rock physics constraints.

Introduction
The correctness of seismic depth imaging plays important role in E&P for Oil and Gas industry, since we are exploring in very complex areas with more imaging challenges. For instance, in the Gulf of Mexico, the presence of salt and anisotropy requires careful earth model building and advanced imaging tool such as reverse time migration. The key for success relies on building an accurate and geological plausible migration velocity model. Reflection tomography in post-migrated depth domain has been an industry standard velocity model building tool for a decade (Stork, 1992, Woodward et al., 2008). However, tomography with surface seismic data only is an ill-posed and underdetermined problem. The surface seismic acquisition geometry decides that we can solve earth model parameterizes better in one direction than the other, because of the lack of horizontal rays. In area with complex geology like salt and carbonates, the subsurface illumination can be very poor and make the problem more underdetermined. Better seismic survey designs (Moldoveanu et al., 2009) with more azimuthal coverage and longer offset can significantly improve subsurface illumination, and thus, reduce model building uncertainty. Borehole measurements, such as check shot transit times, are widely used to provide vertical velocity information, and combined with surface seismic data at well location to derive anisotropic parameters. Traditionally the process is manually done by fixing vertical velocity and picking anisotropic parameters to flat common imaging gathers. Recent examples of localized tomography (Bakulin et al., 2009) demonstrated that anisotropic parameters can be solved simultaneously at well locations with check shot transit times.