Print this pageOpen Versus Restricted System Diagenesis Controlled By Sedimentary Architecture
In the Aguada Pichana Field, two diagenetic paths have been identified in the Mulichinco Fm. (Lower Cretaceous; Neuquen Basin, Argentina), and are directly responsible for the differences in reservoir quality. The “Kaolinite” path is restricted to the tidal channels and bars. Diagenesis is limited to little authigenic kaolinite and insignificant quartz cement. Porosity is good and permeability high due to clean pore throats. The “Chlorite+Quartz” path is characteristic of the aeolian to fluvio-tidal environments. Sand composition is similar to the other facies, indicating that sand composition is not the driving parameter of diagenesis. Porosity is variable and depends on quartz cementation, with poor to moderate permeability (pore throat reduction by chlorite).
Quartz cementation took place between 70 and 90°C, before maximum burial temperature (140°C) during the Tertiary. Quartz cementation coincides with a large increase of pore water salinity. The low temperature window and the large salinity increase suggest that chloride concentration in pore water could be the driving force in diagenesis as it has been shown by Hanor (2001). The proposed diagenetic model is therefore the following:
In the well connected tidal bars and channels, ions released by grain alteration at moderate burial depth are removed from the system faster than it is necessary to grow chlorite coats and quartz cements. Fluid expulsion due to pore volume reduction in not fully compacted sands can be the engine of ion removal (open system). In the poorly connected aeolian to fluvio-tidal sands, the time required to precipitate cements is shorter than fluid residence time: chlorite can precipitate as coatings around grains (restricted system). The reduction of permeability which resulted from chlorite precipitation in pore throats is responsible for even slower advection mass transfer, and allowed massive quartz cementation in facies where chlorite coats are poorly developed.
Exceptions to this binary behavior are the aeolian or fluvial sands which are spatially connected to tidal sands. In this case, only observed to the North of the Aguada Pichana field, the diagenetic path diverges from what is usually observed in these facies and becomes similar to what is observed in the tidal series (open system diagenesis).
Hanor J. (2001): Reactive transport involving rock-buffered fluids of varying salinity. Geochimica et Cosmochimica Acta, Vol. 65, No. 21, 3721-3732.
AAPG Search and Discover Article #90100©2009 AAPG International Conference and Exhibition 15-18 November 2009, Rio de Janeiro, Brazil