Diagenetic

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. 

Diagenetic Healing late precipitation of minerals on or near the fault plane has created a sealing surface (see diagenesis for more detail). 

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

This kind of pressure solution / precipitation is active over prolonged periods of time and may almost totally destroy the original porosity. Precipitation of material may also occur in a similar way on the surface of fault planes thus creating an effective seal via a process introduced earlier as diagenetic healing. 

To gain an understanding of the composition of the reservoir rock, inter-reservoir seals and the reservoir pore system it is desirable to obtain an undisturbed and continuous reservoir core sample. Cores are also used to establish physical rock properties by direct measurements in a laboratory. They allow description of the depositional environment, sedimentary features and the diagenetic history of the sequence. 

Abnormal Formation Pressures Origin. Four main causes are attributed to abnormal formation pressures compaction effects, diagenetic effects, differential density effects and fluid migration. 

The diagenetic effects are related to the alteration of rock mineral, shales in particular. Under certain conditions, montmorillonite clays change to illites, chlorites and kaolinites. The water of hydration that desorbs in the form of free water occupies a larger volume. This volume increase will cause abnormal pressures if the water cannot escape. 

Coleman, A. S. and Holland, H. D. (in press, January 2000). The global diagenetic flux of phosphorus from marine sediments to the oceans redox sensitivity and the control of atmospheric oxygen levels. In "Marine Authigenesis from Microbial to Global" (C. R. Glenn, L. Prevot-Lucas and J. Lucas, eds), SEPM Publication No. 66. 

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)... 

Person, A., Bocherens, H., Saliege, J.-F, Paris, F., Zeitoun, V. and Gerard, M. 1995 Early diagenetic evolution of bone phosphate an x-ray diffractometry analysis. Journal of Archaeological Science 22. 211-221. 

Sillen, A. 1986 Biogenic and diagenetic Sr/Ca in Plio-Pleistocene fossils of the Omo Shungura formation. PaZeohioZogv 12 311-323. 

Krajewski, K. P. (1984). Early diagenetic phosphate cements in the albian condensed glauconitic limestone of the Tatra mountains. Western Carpathians. Chemical Abstracts, 10, 114382. 

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