3-D Physical Modeling of a Complex Salt Canopy

Abstract

Abstract Recent drilling has confirmed both significant reservoir potential and thepresence of commercial hydrocarbons below salt structures in the Gulf ofMexico, Obtaining definitive seismic images with standard processing schemesbeneath these salt structures is very difficult if not impossible. Because ofthe complicated seismic behavior of these structures, full volume 3-D prestackdepth migration is required l. Unfortunately, carrying out the multitude ofcalculations needed to create a proper image requires the largest and fastestsupercomputers and rather complex numerical algorithms. Furthermore, developingand testing the imaging algorithms is quite involved and requires appropriatetest data sets, To better understand the problems and issues of sub salt imaging, Marathon OilCompany and Louisiana Land and Exploration Company contracted with theUniversity of Houston's Allied Geophysical Laboratories (AGL) (o construct a"salt canopy" physical model. The model is patterned after the SEG/EAEGSalt Model and is made from synthetic materials. his a full three-dimensionalmodel with an irregularly shaped, lateral salt structure embedded in fivedistinct ‘sedimentary’ layers. The model was used to acquire a multi-offset 3-D marine style survey. Thesedata are being used to address problems of sub salt imaging. In addition tostandard processing techniques, we will be investigating algorithms formultiple removal and prestack depth migration. Introduction Recent exploration and subsequent drilling in the Gulf of Mexico hasdemonstrated significant reservoir potential below salt structures. Conventional seismic methods fail to image beneath salt structures because ofthe complex nature of the salt body and the high velocity contrast at thesalt-sediment interface. Imaging beneath salt requires full volume 3-D prestackdepth migration (Ratcliff et al 1992). The development of these algorithms isinvolved and requires appropriate test data. Numerical modeling has frequently been used to provide data for test algorithmsand to provide insights into seismic wave propagation. The SEC researchcommittee developed a salt model which used to obtain synthetic seismic data, Finite difference modeling was used to compute the seismic response of thismodel. These data can be used to provide insight into sub At the same time, Marathon Oil Company and Louisiana Land and Exploration proposed a physicalmodel for the acquisition of a conventional marine seismic data set. These datawould provide a test data set for standard processing algorithms and for mosttechniques being developed. It was felt that the physical model could beproduced and the data acquired at a fraction of the cost of the numerical modeldata set. The model would be fully 3-D, support shear waves, and dataacquisition would closely correspond to current techniques. Model Design The basic design of the physical model came from the SEG /EAEG numerical model.A cross-section of the physical model is shown in Figure 1. The sediments inthe SEG /EAEG model contain curved interfaces and a vertical velocity gradient. The physical model has flat horizons and the constant velocity layers. Also thephysical model does not have the shale sheath, the sub salt lenses or theover-pressure region.