Two methods for computing compaction effects in sedimentary basins not affected by orogenic stresses are proposed.

The first is related to the case where porosity logs can be obtained from wells. The series is divided into "stages," which are bounded by dated levels. Each stage is subdivided into n "slices," every slice containing one nth of the total quantity of solid matter in the stage. The initial evolution of change in thickness for each slice is deduced from the porosity-depth curve of recent sediments similar in composition to those in the basin studied. An interpolation is made between this initial evolution and the present thickness of the slices. By summing the slice thickness, the evolution of the total thickness of the stages is obtained. An example is given for the North Sea.

The second method applies to areas where complete porosity logs are missing, but where an average porosity-depth law can be inferred. A decompaction number D is calculated for layers of variable present thickness and depth of burial, which enables the initial thickness of a layer to be evaluated. Solutions to problems concerning the correction of isopach maps, the evolution of sandstone-shale ratios, and the structural closure above buried hills are proposed.

These elementary methods are intended to help in understanding subsidence history and differential compaction structures.