Potassium sources and illitization in Texas Gulf Coast shale diagenesis

The aim of this paper is to identify the controls on the rates of illitizat ion in Texas Gulf Coast shales under diagenetic conditions. Two different m ineral transformations are considered: the transformation of smectite to (n early) illite and the precipitation of discrete illite, i.e., the formation of pure illite, Analyses of potassium feldspar and kaolinite abundance, in addition to K2O, Al2O3, and TiO2 contents, in shales from three wells are used to supplemen t published data in order to identify the mineral reactions responsible for the smectite-to illite transformation in shales involving illite/smectite (I/S) mineral series. These data indicate that the first stage of the smect ite-to-illite reaction in shales (R = 0 structures, ca, 0-50% illite conten t) is controlled by the presence of feldspar and hence also by the dissolut ion kinetics of feldspar, The empirically derived activation energy for the first I/S transformation in shales is similar to that for K-feldspar disso lution, Kaolinite is not involved in this transformation in the shales inve stigated. The second stage of the reaction sequence (R = 1 structures, ca, 50-100% illite content) occurs after potassium feldspar has largely or comp letely disappeared from the shale system. This indicates that the feldspar no longer controls the second stage of the smectite-to-illite transformatio n, and suggests another, external, source of potassium for the reaction in the shales, This assumption is supported by the relative increase of potass ium content of the shales, at least in two of the three studied weds, Kaoli nite does not seem to be involved either in this second stage of the I/S re action sequence. The formation of discrete illite crystallites in shales is independent of t he I/S transformation reactions and takes place only after a very significa nt period of time has elapsed, The slow rate of this reaction reflects the low activity of potassium in solutions that are in equilibrium with shale, in comparison with sandstones, where a rapid precipitation of pure illite c an in some cases be observed. It is suggested that the different reactions in the smectite-to-illite tran sformation necessitate different sources of potassium either from within or outside an argillaceous whole-rock system. These different sources of pota ssium give rise to different rates for the I/S conversion reaction and the formation of illite.