Berthierine formation in reservoir rocks from the Siri oilfield (Danish North Sea) as result of fluid–rock interactions: Part II. Deciphering organic–inorganic processes by hydrogeochemical modeling

Abstract

Berthierine formation reduces the permeability of the glauconitic reservoir sandstones in the Siri oilfield (Danish North Sea), and is therefore a considerable production factor. A new hydrogeochemical batch model that is based on thermodynamics of chemical equilibrium was developed to identify the processes which activated and controlled berthierine formation. A good match has been achieved between the mineralogical alteration features observed in the Siri field and the results of the batch modeling. This match points to the validity of the modeling mechanisms for berthierine formation. Based on the identified mechanisms, the modeling enables to quantify the mass conversion of hydrogeochemical processes in glauconite-bearing and berthierine-forming water–rock–gas systems. The modeling results reveal that intense berthierine formation in the Siri field resulted from in situ glauconite alteration within a more or less closed system due to hydrocarbon degradation-driven water–rock–gas interactions. This process is accompanied by formation of secondary SiO2(s) and potassium mica. Precipitation of iron-bearing carbonates is prevented by berthierine formation, although the glauconitic sandstones are rich in iron and carbonate species. Moreover, such systems are brought to disequilibrium by the release of reducing and acidic agents originating from oil degradation, and undergo long-term, diffusion-dominated mass fluxes instead of advective mass fluxes.