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Abstract

Diagenesis encompasses any physical or chemical changes in sediments or sedimentary rocks that occur after deposition (excluding processes involving high enough temperatures and pressures to be called metamorphism). Diagenesis, thus, can begin at the sea floor (syngenetic or eogenetic alteration), continue through deep burial (mesogenetic alteration), and extend to subsequent uplift (telogenetic alteration). Diagenesis can obscure information about primary features, but diagenesis also can leave behind substantial information about the history of post-depositional settings, pore water compositions, and temperatures.

Diagenesis can reduce porosity and permeability, or it can increase them. In general, though, the trend is toward progressive loss of porosity and permeability with increased time and depth of burial, and that shift is commonly quite substantial. The top diagram (opposite page) shows the highly generalized average porosities of modern carbonate sediments, typical ancient carbonates, and the “exceptionally porous” rocks that constitute hydrocarbon reservoirs. Modern sediment porosities range from 35-45% for grainstones to 70% or more for mudstones or chalks. Typical ancient carbonates have less than 5% porosity, and even reservoir rocks average far less than half the porosity of their modern carbonate equivalents. Thus, understanding diagenetic processes, the factors that inhibit porosity loss, and the relative timing of oil migration versus porosity evolution are critical to exploration for hydrocarbons and carbonate-hosted mineral deposits.

Diagenesis typically involves a variety of physical and chemical processes — the most common of these are:

  1. Cementation (the filling of open pore space, of primary or secondary origin, with newly precipitated materials)

  2. Dissolution (the leaching of unstable minerals forming secondary pores, vugs, or caverns)

  3. Replacement of one mineral by another (or “inversion”, the replacement of one polymorph of a mineral by another)

  4. Recrystallization or strain recrystallization (changes in crystal size, strain state, or geometry without change in mineralogy)

  5. Physical or mechanical compaction (including dewatering and deformation or reorientation of grains)

  6. Chemical compaction (dissolution mainly along surfaces such as stylolites or solution seams)

  7. Fracturing

The terminology applied to such a complex range of carbonate diagenetic processes and products is understandably also complex and is generally applied with disconcerting inconsistency. Folk (1965) provided what is still the most concise, yet inclusive, terminology for diagenetic fabrics. Pore-filling cements are described based on their mode of formation (passive or displacive precipitation), crystal morphology (based on length-width ratios as shown in the middle diagram, facing page), crystal size (see table in limestone classification chapter), and relationship to foundation (overgrowth, crust, or spherulitic growth without obvious nucleus).

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