Evaluation of Compaction Rates for Decompaction Processes

Bernard Beaudoin, Isabelle Cojan, Gerard Fries, Bernard Pinoteau

Burial of sediment leads to mechanical compaction with diminution of both porosity and initial thickness. The compaction rate, ^tgr = h₀/h, may be either directly measured in the sediment or estimated from variation of porosity.

Examples of direct measures are given for different facies on the basis of: textural variations (density of grains, fossil debris, burrows, etc); deformation of fossils (shells, trunks, etc); or of tracks and burrows; differential compaction around nodules, fossils, etc; folding of sedimentary dikes which show their original length.

In some cases the hypothesis of initial isopacity in between two surfaces can be tested using numerous profiles giving, by a system of linear equations, relative compaction rates.

The second way of estimation is based on porosity variations ^tgr = (1 - ^phgr)/(1 - ^phgr₀). Compilation of log data allows construction for different lithologies of mean curves and then of diagrams of variation of ^tgr with depth, giving in fact families of charts according to different lithologies and thicknesses.

The obtained values (up to 2-3 for shales, 5-10 for coal) suggest that decompaction processes are essential not only for geohistory and oil migration, but also for rather classical geological approaches, such as: correlation and definition of sedimentary bodies and their continuity; interpretation of thickness variations (what is the meaning of isopach maps?); interpretation of facies evolutions (what is the meaning of sand/shale ratio maps?); restitution of paleomorphologies and paleoslopes; estimation and comparison of sedimentation rates; and subsidence reconstitutions and synsedimentary deformation analysis.

AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.