Abstract
In seismic terms, three distinct zones of sandstone reservoirs can be defined (Neidell, SEG Distinguished Lecture, 1986): Zone I sands with low acoustic impedance when wet and, in contrast with contemporary aged shales, gas presence is signaled with Bright Spots; Zone III sands have impedance values higher than associated shales (The Compacted Zone) and indicate gas presence with subtle amplitude dimming. Zones I and III are consistent in that their greatly reduced seismic velocities indicate the presence of hydrocarbons and the porosity of the structure. It is the sand reservoirs of the Crossover Zone II which are quite enigmatic yet offer excellent opportunities for the discovery of new reserves. Experience with Zone II type sands, via enhanced seismic inversion imaging, shows that anomalous sands (unpredictable in seismic terms) appear to constitute from one-third to one-half of reservoir sands. The Zone I and III type components may be mixed with these anomalous units in any manner. Such inconsistency leads to poor correlation with simulated seismograms. It is particularly noteworthy that gas filled sands of all three types show velocity drops of over 1000 ft/sec which are unrelated to porosity. Such velocity drops increase only with reduced shale content. Techniques are shownmore » for working effectively in all three sand reflectivity regimes. These include optimal seismic imaging and recognition of the three zones from well log data and seismic data, both independently and in combination. The economic impact of Zone II will be equal to Zones I or III. Zone II offers remarkable resource potential, particularly at intermediate depths. Detecting and defining hydrocarbon reservoirs using this technique should be carefully considered.« less
Published Version
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