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The AAPG/Datapages Combined Publications Database
Houston Geological Society Bulletin
Abstract
Abstract: Compaction and Overpressure in Shales:
Practice and Theory
Pressures in the subsurface control the migration of fluids,
including hydrocarbons, and hence are of interest not only to
drillers (whose wells must deal with these pressures) but also
to explorationists generally. Away from well control, the most
common source of pressure information is P-wave seismic
velocities. Converting shale velocities to pressures requires an
understanding of the normal (hydrostatic) compaction curve for
shales in a given region. Absent a normal compaction curve, it is
impossible to state whether a given shale velocity represents
normal pressure, overpressure, or 
underpressure
. We will show
the expected range of normal compaction curves and discuss the
driving factors that influence compaction. A quantitative model
of shale compaction has been developed that accounts for many
of the features of shale porosity evolution with depth, including
predictions of P-wave and S-wave velocities. We conclude with
a big picture review of the place of pressure analysis in
hydrocarbon exploration.
Smectite Dehydration and Mudrock Modeling
A depth versus velocity plot
of normally compacting clay
rocks (shale) normalized to the
sea floor from seven basins
including Beaufort-McKenzie,
eastern offshore Canada, USA
Gulf of Mexico, offshore
Trinidad, offshore Nigeria,
offshore Indonesia, and NW
Australia from Recent to
Jurassic age rocks. The normally
pressured clay rock from Gulf
of Mexico shown in green are
smectite-rich and noticeably
slower than the other, mixedclay
mineral shales. Previous
authors who have discussed clay
diagenesis, log response and
compaction are Lahann (2002,
2004), Alberty and McLean
(2003), and Katahara (2003,
2006).