The term “diagenesis” refers to essentially everything that happens to sediments and rocks after their deposition but prior to metamorphism. There are a variety of diagenetic processes, biological, chemical and physical, that ultimately convert sediments into sedimentary rocks. The earliest of those events are covered in this chapter on near-surface diagenesis; subsequent chapters cover processes and products that occur primarily during later stages of diagenesis (mainly mesogenesis). Those include mechanical and chemical compaction, cementation, dissolution, replacement and structural deformation. All these processes can profoundly affect the porosity, permeability and hydrocarbon reservoir potential of clastic terrigenous deposits, and most of them are a function of initial sediment composition and the changes in pressure, temperature and water chemistry that accompany progressive burial. Less explicitly covered, but potentially no less important, is diagenesis that can occur during one or more episodes of local or regional uplift and consequent exposure (telogenesis). These events also introduce changes in the pressure/ temperature/water chemistry regime of rocks, and thus can cause major diagenetic changes, especially grain dissolution and cementation. In an attempt to address the impacts of the various diagenetic events that rocks may experience, this book includes both a section on the recognition of porosity types as well as one on paragenesis (i.e., the placement of diagenetic events into a temporal sequence related to the burial/uplift history of rocks). Burial diagenesis is critically important in controlling the porosity of clastic terrigenous rocks and is, in the main, porosity destructive—that is, almost all rocks lose porosity with increased burial depth. Nonetheless, several factors can retard or inhibit porosity loss, including early grain-coating cements, that block later overgrowth cementation, regional overpressuring of basins that reduce effective overburden stresses and, under some circumstances, hydrocarbon entry that can reduce rock-water interactions. In addition, the processes of dissolution and fracturing may, under the right circumstance, lead to actual increases in subsurface porosity. So the discussion of porosity destruction, preservation and creation pervades all chapters in the diagenesis section, and emphasis is placed on recognition of key features associated with anomalous porosity retention or creation.
Figures & Tables
AAPG Memoir 109 is designed as a practical guide for students and professionals to learn the fundamentals of microscopic examination of sandstones, mudrocks, and associated rocks. With more than 1100 color illustrations, it covers the identification of grains, textures, and structures of clastic terrigenous rocks as well as their diagenetic alteration (compaction, cementation, dissolution, and replacement) and porosity reduction or enhancement. It also provides classification diagrams for formal description of those rocks and their porosity. Although the majority of the outcrop and subsurface examples come from the United States (35 states and Puerto Rico), there are representative photographs from 32 other countries, including many from the offshore areas. The foldout birefringence chart and an included DVD with Powerpoint files of all of the petrographic images provide additional aids for instructors and students.