Qualitative Stratigraphic Correlations
With the tools available, we may profitably consider case studies both simulated and real to develop guidelines and procedures for stratigraphic studies. As a first example, a complex stratigraphic situation described by four lithology logs will be considered. The particular case is termed the sand-shale interfingering model.
As all geologists know lithologlc boundaries are not always mirror smooth in the subsurface. Depositional characteristics of interbedding gradlatlon can cause a diffuse boundary in depth. This lack of a well-defined boundary will understandably influence the seismic reflections as was already noted. Additionally, there will be a lateral averaging over the Fresnel zone which usually encompasses a region of between 300–1500 feet in diameter.
When a diffuse boundary is involved we have noted that to first order there is some weakening of the reflection response. Also, there will be some measure of weakening in lateral continuity. Further, we may anticipate that the seismic response may differ from a response predicted from well log measurements via simple seismogram synthesis even if a common seismic waveform is used. The log measures the vertical lithologic sequence using a few inches of lateral penetration at the most. Seismic waves average several hundred feet laterally and thus reveal different properties from the log about the boundary.
This model attempts to portray some of the effects of a diffuse boundary. A sand of approximately 100 feet thickness is embedded in a shale. The sand base is very uniform over a large area, but the sand top is locally interfingered