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AUTHORS: R.Di Cuia, A.Riva, N.Bang, A.Harstad

YEAR: 2013


Diagenesis plays an important role in carbonate sequences because the circulation of fluids within these sequence can dramatically modify the characteristics and properties of these rocks.
The characterisation of a complete paragenetic evolution in carbonate reservoirs is fundamental to understand the evolution with time of the pore network and the distribution within the reservoir of different petrophysical properties related to the different “reservoir facies”.
To better distribute properties within the reservoir model a diagenetic study that included also geochemical and fluid inclusion analysis was carried out in a Cretaceous reservoir in Northern Iraq.
The study was carried out in different steps:
1) Petrographic analyses in transmitted light and cathodoluminescence in order to determine the mineralogical composition of the samples and to define the relative order of precipitation of the authigenic minerals (paragenesis).
2) Fluid inclusions (fis) petrography to determine fis origin (primary, secondary, pseudosecondary, uncertain), composition (“aqueous” or “hydrocarbon”), and liquid to vapor ratios, with the aim of selecting appropriate samples for microthermometry;
3) Microthermometry to achieve homogenization (th) and final ice melting temperatures (tmice)
4) Integration of the results with the other reservoir data
The study highlighted the presence of 4 main cement types (pervasive dolomite, intergranular calcite, second phase of dolomites and late calcite)
The main outcomes of the analysis can be summarised as follow:
+ the dolomites and the dolomitic cement formed at a temperature higher than that calculated with basin modelling related to a hydrothermal dolomitisation event(s) associated to the fault network present at that time
+ the late calcite precipitation occurred during cooling of the system with in some samples evidences of calcite precipitated from meteoric waters
+ oil charging occurred at the same time of the dolomitisation event probably this happened at the same time of the maximum burial (Miocene Pliocene)
The integration of these results with well log, seismic and core information allowed to better define the geological and reservoir models in particular introducing hydrothermal dolomites related to hot fluid circulating along the fault network.