Diagenetic processes

The general diagenetic equation from which many important models have been derived (as given by Berner, 1980) is  

For steady-state conditions this equation is set equal to 0, because at a given depth x, concentration does not change with time. Steady-state models are generally more amenable to mathematical solution than are non-steady-state models. Unfortunately, diagenesis in many shoal-water carbonate sediments is significantly influenced or even dominated by non-steady-state processes. 

---

Carbonate rocks are not normally transported over long distances, and we find carbonate reservoir rocks mostly at the location of origin, in situ . They are usually the product of marine organisms. However, carbonates are often severely affected by diagenetic processes. A more detailed description of altered carbonates and their reservoir properties is given below in the description of diagenesis . 

Shallow water carbonate (reefs carbonate muds) Reservoir quality governed by diagenetic processes and structural history (fracturing). Prolific production from karstified carbonates. High and early water production possible. Dual porosity systems in fractured carbonates. Dolomites may produce H S. 

The diagenetic processes relevant to field development are compaction, cementation, dissolution and replacement. 

We have previously assumed that the principle diagenetic process is attributable to exchange or interaction of (depleted) browser apatite values with (enriched) sedimentary matrix values (Lee-Thorp and van der Merwe 1987). Another possibility is ionic or isotopic exchange with soil CO2 however in most cases soil CO2 values will be closely related to matrix carbonate values. If the former is the case, one would expect enriched grazer values (near 0%o)... 

A unique strength of concordia diagrams is their ability to reveal modes of diagenetic disturbance. If U-Th-Pa isotopic data for numerous co-eval sub-samples that have experienced different degrees of the same diagenetic process are plotted in or vs. space, they should fall on a curve or straight line... 

Hedges, R. E. M. and Millard, A. R. (1995). Bones and groundwater towards the modelling of diagenetic processes. Journal of Archaeological Science 22 155-164. 

Iron sulphides are ubiquitous in marine and freshwater sediments. They are usually present either as pyrite or as monosulphides, which can be liberated by hydrochloric acid. These acid volatile sulphides give rise to an intense black colour that is characteristic of anoxic sediments. They play an important role in recent diagenetic processes in sediments and the ratio of pyrite to acid volatile sulphides has been used as an historical indicator to determine whether sediments were formed in marine or freshwater conditions. They can be present over a wide range of... 

---