Silicification and illitization are considered to be the main diagenetic causes of permeability reduction in many North Sea oil fields. Prediction of the extent and location of these phenomena is thus of economic importance. Reaction-transport modeling of important diagenetic reactions has recently become a useful tool to help determine field development strategies.
Definition of the "initial "state as input for a model is a crucial problem. Knowing that illitization and silicification are more developed in Dunbar than in North Alwyn, and that some places in Ellon and Dunbar have been "fossilized" due to early calcitisation, we examine the assumption that North Alwyn or Ellon may be considered as an "initial" state of equivalent facies at Dunbar. This leads us to consider the mass balance of elements, and thus to examine whether the system remained closed, or if advective transport is needed to explain the chemical reactions.
Correspondences between the depositional facies in these three fields have already been established by the sedimentologists. Samples were chosen from three facies, which occur in at least two of the three fields (i.e. upper shoreface of the Tarbert, tidal complex and channels in the Ness). About 50 samples have been analysed for major, minor and rare earth elements, as whole rock and as < 2 µm fractions. Based on petrographic, XRD and SEM observations, reconciliation of all the data has allowed translation of chemical analyses to quantitative mineralogical composition through stoichiometric formulas. The results have been displayed as a function of depth as well as of location.
It appears that besides differences in kaolinite, K-feldspars and illite abundance, there are marked differences in the K2O/Al2O3 ratio in equivalent facies from the different fields. Due to the low solubility of aluminium, the K2O/Al2O3 ratio can probably only evolve by moving potassium. However, this would necessitate large volumes of water if applied to the entire reservoir, or be less demanding if limited to decametric scales. The alternative hypothesis is that the chemical composition of the depositional facies were not equivalent over their entire extent.
Both hypotheses will be discussed. This will lead to different inputs to be tested in modeling, either in closed (i.e. including only chemical reactions) or open (i.e. also including fluid circulation) systems. Results of the modeling will provide constraints concerning the extent and the scale of closure of the system.