Exploration Insights Great Geos ebook | Page 132

132 | Great Geologists
A version of the classic “slug diagram” (from AAPG Studies in Geology 27, 1987) illustrating Vail’s concepts of stratal geometries as seen
in seismic data and how they can be interpreted in terms of chronostratigraphy. AAPG ©1987, reprinted by permission of the AAPG whose
permission is required for further use.
By the early 1960s, seismic interpretation had become
an important component of Vail’s pursuit of time-based
correlation and mapping. Against the advice of colleagues
he joined a geophysics team. A project integrating wells,
seismic and biostratigraphy from Guinea Bissau led to an
important revolutionary revelation – many seismic reflections
did not follow time transgressive formational boundaries
but instead followed bedding patterns or the real physical
surfaces in the rock. In other words they are time lines. Prior
to this seismic reflectors had been thought to solely represent
impedance contrasts at lithological boundaries. Vail would
subsequently write “primary seismic reflectors are generated
by stratal surfaces which are chronostratigraphic, rather than
by boundaries of arbitrary defined lithostratigraphic units”. This
new understanding (although still debated today) elevated
seismic data to being a new tool in the methodology to make
regional chronostratigraphic frameworks for mapping and
understanding palaeogeography, especially when coupled
with well logs and biostratigraphy. Moreover, the depositional
geometries displayed within seismic data gave insight into
the pattern of relative sea-level change through recognition of
onlap, downlap, etc.
Vail examined seismic data from various parts of the world
and was struck by the apparently synchronous nature of
many of the relative sea-level changes seen in the sections
he studied, calibrated by biostratigraphic data from wells. In
1959 he had drawn his first Phanerozoic sea-level cycle chart
based solely on well log correlations. In 1963 he presented this
at a company forum augmented by seismic data and in 1966
publically presented his ideas on eustasy at the AAPG annual
convention in St. Louis. At this time he was actively using
seismic/sequence stratigraphy to guide exploration for Exxon in
the newly licensed North Sea.
By his own account, not all his peers in Exxon were enamored
with these geoscience breakthroughs. He recounts that at
one internal research conference a senior member of staff
goaded the audience into laughter after a presentation by Vail
with suggestions that he was implying that seismic reflectors
must bounce back off fossils. But Vail was not disheartened
and persisted with his research eventually being given the
opportunity to lead a small research team of geologists,
geophysicists and computer scientists working on seismic
stratigraphy. As much effort was put into processing seismic