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).