NEIDELL, N. S., W. C. LEFLER, W. R. LANDWER, and M. SMITH, N.S. Neidell & Associates, Houston, TX

ABSTRACT: Seismic Characteristics of Sandstone Hydrocarbon Reservoirs

In seismic terms, three distinct zones of sandstone reservoirs can be defined (Neidell, SEG Distinguished Lecture, 1986): Zone I sands having low acoustic impedance when water wet in contrast with contemporary aged shales, signal gas presence with Bright Spots; Zone III sands with impedance values higher than associated shales (The Compacted Zone) indicate gas presence with subtle amplitude dimming. Zones I and III are quite consistent in their seismic responses for hydrocarbon presence and porosity showing greatly reduced velocities. It is the sand reservoirs of the Crossover Zone II which are quite enigmatic yet offer excellent opportunities for finding new reserves.

Experience with Zone II via enhanced seismic inversion imaging shows that anomalous sands (unpredictable in seismic terms) appear to constitute from one-third to one-half of this sand population. The Zone I and III-type components may be mixed with these anomalous units in any manner. Such inconsistency leads to poor correlation with usual synthetic seismograms. It is particularly noteworthy that gas filled sands here as well as in the other zones show velocity drops of over 1000 ft/sec which are unrelated to porosity. Such velocity drops increase only with reduced shale content.

Techniques are shown for working effectively in all three sand reflectivity regimes. These include optimal seismic imaging as well as recognizing all three zones from both well log data and seismic data, these being used independently and in concert.

The economic impact of zone II will be at least as great as in Zones I or III. Where Zone II will be at least as great as an average low level of reflectivity and prior misunderstanding of its properties, it offers remarkable potential particularly in view of its intermediate depths. Detecting and defining hydrocarbon reservoirs in this environment is therefore worthy of detailed consideration.

AAPG Search and Discovery Article #90988©1993 AAPG/SVG International Congress and Exhibition, Caracas, Venezuela, March 14-17, 1993.