Page 111 - Petroleum Geology

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Fig. 4-10, Seismic record section showing stratigraphic detail. (Courtesy of Esso Australia
Ltd.)

will not necessarily extend over the whole area of a sedimentary basin be-
cause the sediment that cannot accumulate in the area of a diastem will
move on until it reaches a position in which it can accumulate, and that posi-
tion will normally lie within the sedimentary basin.
From a study of many seismic record sections in many areas of the world,
drawing on well control, Vail and his colleagues concluded that most reflec-
tors are isochronous surfaces (and so, in effect, diastems and disconformi-
ties)*; and that many of the mappable hiatuses were indeed correlatable
around the world, and were! t erefore due to eustatic sealevel falls. It can
perhaps be argued that the f P equency of diastems and disconformities in
Barrell’s concept is such that a degree of coincidence is inevitable, and
precise correlation difficult to prove. Nevertheless, the value of seismic strati-
graphy in elucidating the stratigraphy and structure of an area is well demon-
strated.

* Some years ago I saw a record section from the US. Gulf Coast that had many good
reflectors, suggesting a sequence of sandstones alternating with mudstones. The interval
velocities were rather low, suggesting more mudstone than sandstone. The well drilled an
almost continuous sequence of mudstones more than 3 km thick. This illustrates the
point that not all reflectors are lithological boundaries in the accepted sense, despite the
fact that reflection is a function of density contrasts.