As a result of differential compaction, the geometry of strata in a sedimentary basin changes continually during progressive burial. Shales and coals, in particular, undergo strong compaction relative to other lithologic units. A stratigraphic cross section constructed from present bed thicknesses may differ substantially from the cross section at the time of deposition. Therefore, decompaction study of cross sections helps stratigraphic correlation, especially in laterally intertonguing sequences of compressible and incompressible facies, and also helps define compaction-related structures.
The decompaction technique uses the porosity-depth curve of each lithology to calculate the thickness of each bed at a given burial depth; calculations and plotting are done by computer. The simplest way to visualize the technique is to consider that the stratigraphic section is moved up along the porosity-depth curve to any previous burial depth. When a bed reaches the surface, it is completely decompacted and recovers its maximum (initial) thickness. The decompaction technique may be difficult to apply where beds have undergone geopressuring or complex diagenesis.
Use of decompacted cross sections in Gulf Coast clastic sequences and in coal-bearing strata in Colorado results in more accurate and refined stratigraphic correlations. Precise correlation of sand, shale, and coal beds is economically significant, because the physical continuity of beds from one exploratory well to another is of the utmost importance in mining and petroleum geology.