Diagenetic sequence

The whole diagenetic sequence of the rocks drilled in the boreholes in LCB may be shown diagrammatically as in Fig. 4. 

Figure 7.36. Diagenetic sequence of textural, mineralogical and geochemical changes for the carbonate sediments of the Mediterranean coast of Israel. In this case the diagenetic sequence is interpreted to be related to the progressive aging of the carbonate sediments. (After Gavish and Friedman, 1969.)...

In Table 6.1 a series of idealised decomposition reactions are listed in order of their energy yields per mole of carbon, and therefore, broadly speaking, their diagenetic sequence. The relative importance of each reaction to the overall carbon budget depends on the relative supply of each reaction s electron acceptor and the relative supply of each reaction s metabolisable carbon. For carbon supply, the rate of sedimentation is... 

Evidence for ankerite cement is that homogenization temperatures are consistent and 20-40 C lower than present reservoir temperature (Fig. 7, Table 2). As ankerite formed in the middle part of the diagenetic sequence, it has prpbably been sub-... 

If it is assumed that fluid inclusions in ankerite are primary, measured Th values would represent minimum trapping temperatures (the presence of dissolved CH4 or hydrocarbons in these inclusions could not be ascertained). However, as ankerite pre-dates calcite and quartz in the diagenetic sequence, the lowest Th yielded by fluid inclusions in calcite and quartz, i.e. 85-90°C, may represent a maximum formation temperature for ankerite. This implies that diagenetic ankerite in the Oseberg must have formed at temperatures between 65 and 85° C, i.e. essentially identical to measured Th values (Table 2). 

Compared to the anoxic sediment, the oxic surface sediment is enriched in As (up to 345 nmol g-1). The enrichment is weak where the sedimentation rate is high, but enrichment becomes stronger with decreasing sedimentation rate. Phosphorus enrichment in the surface layer is evident only where the sedimentation rate is low. However, upon burial, both P and As are released from the solid phase to the pore water, irrespective of the sedimentation rate, and the distributions of both elements show subsurface maxima that correspond to the depth where the dissolution rates of solid phases of P and As are maximum. From the relative location of these maxima, it can be concluded that As is released to the pore water at shallower depths than at which phosphate is released. Mucci etal. (2000a) proposed that As is released upon the reduction of As(V) to soluble As(III), and that this takes place earlier than the reduction of Fe(III) to Fe(II) in the diagenetic sequence. In contrast, the... 

In this sequence the authors ignored halloysite/endellite, important to dominant kaolinite minerals resulting from the alteration of feldspars in recent sediments. We propose the following diagenetic sequence ... 

Bruton CJ (1986) Predicting mineral dissolution and precipitation during burial synthetic diagenetic sequences. In Proc Worksh Geochemical modeling. Lawrence Livermore National Laboratory Publ CONF-8609134, Livermore, CA, pp 111-120 Caminiti P, Cucca P, Monduzzi M, Suba G (1984) Divalent metal acetate complexes in concentrated aqueous solutions an X-ray diffraction and NMR spectroscopy study. J Chem Phys 81 543-551... 

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. 

Although the majority of attention in discussions on the origins of BIFs has been on the oxide facies, siderite facies rocks are equally important in many BIF sequences. Reaction of Fe(II)aq and dissolved carbonate with hematite to form siderite and magnetite has been hypothesized to be an important diagenetic process in marine basins during formation of some BIFs if sulfate contents were low (e.g., Klein and Beukes 1989 Beukes et al. 1990 Kaufman 1996 Sumner 1997). In Figure 18 we assume that Fe(II)aq was derived either from MOR sources or DIR, or a combination of the two, which reacted with ferric oxide precipitates to form magnetite or dissolved carbonate to produce siderite. 

Figure 8.34. Hypothetical porosity-depth curves for various diagenetic situations. A. "Normal" burial curve of chalk sequences. B. A typical curve stemming from porosity changes brought about by dissolution and cementation processes during meteoric zone diagenesis. C. A scenario for late stage porosity development brought about by a dissolution event related to hydrocarbon maturation and destruction. D. Porosity preservation resulting from conditions of overpressuring. (After Choquette and James, 1987.)...

More deep bore holes should be drilled in carbonate sequences not directly associated with hydrocarbon accumulations to obtain diagenetic data and information on carbonate composition as a function of geologic age. 

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