Previous Talks

Wednesday February 10th, 2010

The normal compaction of clay rocks is fundamental for understanding overpressure

Philip D. Heppard, ConocoPhillips Company, Houston


Porosity reduction during the burial of clay rocks, compaction, has been a subject of interest for many years (Athy, 1930, Mondol et. al., 2007).  Besides just the normal curiosity of geologists and geophysicists about how rocks get to their present state, being able to predict their properties has practical applications for log analysis, interpreting seismic data and seismic rock properties, and more recently shale as a hydrocarbon reservoir, but also is fundamental for interpreting the presence and magnitude of overpressure.  In overpressured shale we expect porosity to be higher than normal which should be reflected in the well log response or seismically detected velocity.  Our processes for estimating overpressure in the industry depend one way or the other on predicting how clay rocks or shale should behave as a result of varying confining and pore pressure environments usually summarized by discussion about a normal compaction trend (compaction prediction).  In the published literature interest in this subject commonly includes the sensible comparison of a number of normal compaction trends derived by other workers from many basins as well as the results from compaction studies conducted in a laboratory.  These comparisons show a wide range of compaction trends with the only commonality being increasing compaction with depth but at any depth a very wide scatter daunting to anyone who needs to predict this behavior.  An unfortunate consequence can be a general notion that it is so variable due to a dependence on a large number of geologic factors that any interpretation of overpressure can be supportable with plausible explanations.   This presentation shows that in most clastic dominated basins that clay rock compaction is actually fairly well constrained and predictable.  A brief review of the published compaction trends suggests that outliers are from basins that have undergone a complicated geologic history and are now out of place and not at maximum burial depth. Our work supports recent evaluations of clay rock compaction which indicates, however, that the composition of the clay rock does significantly affect the rate of compaction.  Lahann, (2002, 2004), Katahara (2003), and Alberty and McLean (2003) support a similar conclusion that smectite-rich clay rocks will compact at a much slower rate then illite- or mixed-clay rich clay rocks or shale.

Speaker Biography

Philip D. Heppard is a principal geologist with ConocoPhillips in Houston, Texas.  Since 1988 Philip has been a pore pressure expert supporting worldwide exploration and development efforts encompassing most known petroleum basins and has been a lecturer on pore pressure for AAPG and related professional organizations.  In 2003 he won the AAPG best international poster award for “Using shear and Vp/Vs to predict overpressure in petroleum basins” with his four co-authors.  His interest has been the integration of well and seismic data to predict overpressure in the subsurface for well planning and the evaluation of seal quality, as well operational support for drilling wells.  He has worked as a development geologist for the Permian Basin, Texas, and Trinidad, West Indies.  Philip received his B.S. in geology from Juniata College, Pennsylvania, and his M.S. in geology from the University of Akron, Ohio, some time ago.  He joined Amoco Production Co. in 1979 and then BP after the merger of the two in 1999.  He joined ConocoPhillips Company in February 2006 to become a leading member of their GeoPressure team within the Subsurface Technology group in Houston.