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AUTHORS: Di Cuia R., Riva A., Norgard J., Haerstad A., Marre J.

YEAR: 2012

GEO2012, March 2012 Manama (Bahrain)

In order to better characterise the diagenetic evolution and the evolution and distribution of petrophysical properties with time, fluid inclusions analyses were performed on samples coming from a Cretaceous reservoir in Northern Iraq.
The results of this study were subsequently integrated with all the other reservoir data to better define and constrain the geometries and properties of the reservoir model
The study was carried out in different steps:
1) Petrographic analyses in transmitted light and cathodoluminescence (if necessary) 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 analysis 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. The impact of the data integration determined a revised model that is characterised by:
+ increase in dolomitisation with depth
+ around faults the dolomitisation intensity increases
+ Christmas-tree-like geometry of dolomite bodies
+ variable quality of the dolomites depending on their position in respect to the faults