KAOLINITE-TO-DICKITE REACTION IN SANDSTONE RESERVOIRS

The SEM, XRD, FTIR and DTA analyses of different size-fractions of cla y material from sandstone reservoirs which have experienced a large ra nge of burial conditions have been used to examine the different steps of the depth-related kaolinite-dickite reaction. Dickite progressivel y replaced kaolinite within a range of burial depths estimated between about 2500 m and 5000 m. The kaolinite-to-dickite reaction proceeds b y gradual structural changes concomitant to crystal coarsening and cha nge from booklet to blocky morphology. The crystallization of dickite proceeds by two distinct paths: (1) Accretion of new material from eit her dissolution of smaller unstable kaolinite crystals and/or detrital minerals (chiefly feldspars), on early-formed coarser metastable kaol inite crystals which exert extended morphological control on the growi ng crystals. (2) Neoformation of ordered dickite which will continue t o grow by a dissolution-crystallization process. The kaolinite-to-dick ite reaction is kinetically controlled and anomalies in the kaolinite/ dickite ratio observed in certain sandstone reservoirs may be used to assess the timing of invasion by hydrocarbons.