* Shale: Shale is a fine-grained sedimentary rock composed of clay minerals and other very small particles. These small particles leave a lot of empty space (porosity) between them. Compaction, the process of reducing that space due to the weight of overlying sediments, is extremely effective in shale. The small particles are easily squeezed together, reducing porosity and increasing density.

* Sandstone: Sandstone, being composed of larger sand grains, has inherently less porosity than shale. While compaction still occurs in sandstone, it plays a less dominant role than in shale. The larger grains are less easily compressed.

* Conglomerate and Breccia: These rocks are composed of large, rounded (conglomerate) or angular (breccia) fragments. They generally have even less porosity than sandstone due to the large spaces between the fragments. Compaction has a minimal impact on these rocks as the fragments are too large to be significantly compressed.

In summary:

* Compaction is the most important lithification process for shale due to its high initial porosity.

* Compaction plays a lesser role in sandstone, and is even less significant in conglomerate and breccia.