AAPG Geoscience Technology Workshop

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Estimating and Mapping Anisotropy in the East Mediterranean Subsurface

Abstract

Recent advances in drilling technology, together with offshore gas discoveries, have led to an extensive exploration efforts below the deep water of the Levant Basin. As it was the first drilling phase in this area, there was no available information on the physical properties of the subsurface. Analysis of several of the first wells drilled in the basin indicated significant mis-ties between seismic reflections and well tops. Meeasured mis-ties ranged from tens to hundreds of meters. In order to understand this phenomenon, a regional analysis of seismic anisotropy was performed in the area. Seismic anisotropy is the dependence of velocities on the direction of wave propagation. Since velocity plays such a key factor in seismic data processing and interpretation, ignoring it can adversely affect the results of most of the seismic data techniques such as velocity analysis, migration, time-to-depth conversion and amplitude versus angle (AVA) analysis. From the wells drilled in the offshore area of the Levant coast, it has been shown that the litho-stratigraphic column of the Levant Basin contains rich deposits of shales, while studies performed in the last 20 years around the world have shown a clear connection between the existence of seismic anisotropy and the presence of massive shale layers at the subsurface. The main goals of this study was to perform a regional characterization of the anisotropy in the eastern part of the Mediterranean Sea and for the first time, provide general maps of the anisotropic parameters attributed to geological layers that exhibit the anisotropic effect. As a result eight 2D lines together with well data were processed using conventional methods in order to estimate the anisotropic parametrs. The main challenge of this investigation was the insufficient number of wells available for academic research below the deep water of the East Mediterranean Sea (EMED). The borehole data used in this work comprised three wells: Delta-1, Myra-1 and Aphrodite. The seismic processing workflow resulted in the detection of three regions which exhibited clear anisotropic behavior. The first anisotropic layer was detected in the shallow part of the basin, in the young sediments from the Plio-Pleistocene age. An additional layer was observed in a deeper part, below the Messinian salt layer. This layer is the mass transport complex, in the Middle Miocene section, the same layer that overlays the hydrocarbon bearing reservoirs in the EMED. The third layer was found in the shallow water environment where anisotropy is observed due to carbonates buildups. This study, for the first time, demonstrates evidence for the presence of seismic anisotropy in the Middle Miocene section and provides regional maps of anisotropic distribution in the Levant Basin. The contribution of these anisotropic maps is significant for the future investigation of the area. It provides information which was not available before and is needed for more precise depth imaging. The initial maps, provided by this study, should be updated as new data becomes available, producing a more detailed definition of the anisotropic distribution in the region. The main conclusion of this study is that any future analysis of seismic data in the EMED deep-water environment, must take into accpunt anisotropy.