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Abstract Offset VSP data in the Deir Ez Zor area contains significant mode-converted shear wave energy. This allows us to obtain two results: a P-wave and an S-wave OVSP image. The additional information contained in the shear wave image can significantly aid interpretation. There are however differences in character, amplitude and polarity. Interpreted in conjunction with Zoeppritz equation modeling, these can provide information about lithology and fluid content that cannot be extracted from P waves alone.
We now believe, based mainly on evidence from offset VSP data, that part of the noise in our surface seismic consists of mode converted shear waves, rather than P-wave multiples. NMO velocities and AVO anomalies in prestack surface seismic data corresponded with modeled PS responses. Such modeling thus provides a method of identifying suspected PS reflections in the vicinity of wells with OVSP data or full waveform sonic and density logs. Depending on the importance of a particular event, if there is a suspicion that it may be a PS reflection, it will often be justified to obtain OVSP data and to re-evaluate pre-stack surface seismic data.
Introduction The seismic data in Deir Ez Zor area in Eastern Syria typically are of reasonable quality for the part down to about 1500 ms in seismic two-way travel time. For this part, layering of geological formations is predominantly horizontal. The main reservoirs are located in the faulted strata below, typically from 1500 to 2000 ms. There are also deeper areas of interest but it becomes difficult to map reflections there (Fig. 1).
The faulted formations below 1500 ms are the main targets for seismic imaging. The image here is heavily contaminated by noise, hitherto assumed to be mostly multiples. Various attempts to suppress this noise by using multiple elimination tools have not been successful.
New wells that are drilled in these fields are aimed at attic oil, shadow oil not drained in current completions, and field extensions. This often means that wells are drilled as close as possible to faults, in order to reach bypassed oil. The 3D seismic leaves considerable uncertainty for wells drilled close to faults. Often, AFPC's strategy is to include a side-track option for these wells, which is drilled if they miss the target. We then have an opportunity to improve our knowledge of the subsurface, and select a better location for the side-track, by means of imaging VSP. Such Offset VSPs (OVSP) have receivers closer to the target than surface seismic (the geophones are inside the well) and can therefore provide data that is more accurate in terms of positioning of events and higher in resolution.
We have found that our OVSP data contains significant amounts of mode-converted Shear waves. 50 % or more of reflected energy may be shear. We have investigated ways of making use of this shear wave energy, rather than suppressing it in processing. Shear wave reflections can be processed to image the subsurface, similar to P-wave VSP imaging. A P-wave image and an S-wave image can thus be obtained from the same raw VSP data. This provides additional information, which can be very important and either increase confidence in the interpretation or significantly change the interpretation from what it would have been, based on the P-wave image alone. In certain cases, the Shear wave image can be of rather better quality than the P-wave image, and it will be used instead of the P-wave image.
Mode converted shear waves in OVSP data also show where P-to-S conversion takes place, and with which amplitudes. We now believe, based on evidence from offset VSP data, that much of the noise in our surface seismic consists of mode converted shear waves, rather than P-wave multiples. It is even possible that some events, thus far interpreted as regular PP reflections, in fact represent PS reflections.
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