Reservoir description is achieved by the integration of geological, petrophysical and engineering data. Modelling of reservoir performance is made by creating a three-dimensional model of the reservoir volume, where the reservoir is built of cells and layered zones which are defined geologically. Although the scale of cell size is coarse compared to the scale of geological data, it is important that the geological input to define cells is as precise as possible. The texture of clay minerals and their composition are requisite for understanding their influence on reservoir characteristics. Wireline logs probably do not provide sufficient information about clay mineralogy to evaluate reservoir characteristics, but do allow the extrapolation of ‘point’ mineralogical data into a continuous reservoir description. Evaluation of porosity, permeability and saturation are described, and the possible influence of clay mineralogy on evaluation of these characteristics is discussed.

Study of sandstone diagenesis provides information about the origin, texture, distribution and composition of clay minerals, which in turn is used in reservoir description. Three examples of the use of clay mineralogy in reservoir description are given. (1) Kaolinite commonly forms pore-filling cements which are pervasive in specific sandstone intervals. It is shown that water-zone kaolinization homogenizes and lowers the porosity and permeability relative to the oil zone; in a reservoir model different Ø/K relationships are defined above and below the oil-water contact. (2) An occurrence of chlorite in dish-structured horizons is shown to increase horizontal permeability and decrease formation resistivity. The sensitivity of the neutron porosity log to the chlorite cement reduces the usefulness of the log for porosity evaluation. Uncritical application of wireline logs to define reservoir parameters can give pessimistic reservoir evaluation. (3) Sand production can be related to wettability, which in turn is strongly influenced by clay mineralogy. A perforation strategy to minimize sand production may then be based on knowledge of the clay mineralogy of a reservoir.