Formation, Distribution, and Prediction of Stylolites as Permeability Barriers in the Thamama Group, Abu Dhabi

ABSTRACT Stylolites and associated pressure dissolution phenomena are common features of most carbonate reservoirs but their effects on reduction of porosity and permeability are variable. In chalky limestones such as the Thamama Group of Abu Dhabi stylolites may cause a reduction in permeability sufficient to create a barrier to fluid movement. Most stylolites result from dissolution of carbonate as a result of overburden pressure. Dissolution occurs along pre-existing surfaces within the rock and in particular along bedding planes. The presence of even small amounts (<1%Rv) of clay material in the rock encourages stylolite development. Thus in a sequence of uniform, laterally persistent lithofacies such as occur in the Thamama II and IIIa zones, stylolites may be similarly distributed and form field-wide vertical permeability barriers. In view of the relationship between stylolites and depositional lithofacies, prediction of their distribution may be made by core and log study and depositional modelling. In an Offshore Abu Dhabi field, an attempt has been made to predict vertical permeability through stylolite zones in uncored wells. Comparison of the stylolite intensity index, calculated from FDC/CNL logs following the method of Johnson and Budd (1), with measurements from whole core and plug analyses showed that a statistically valid relationship exists between these two parameters. Separate relationships have been established for stylolites above and below the oil/water contact as values in the water zone tend to be further reduced by continued cementation. The establishment of these relationships has enabled the prediction of vertical permeabilities through stylolite zones in uncored wells. Vertical permeabilities of less than 1md equate to a stylolite intensity index of 50, and in the subject field such zones are likely to severely limit fluid movement. Such geological analysis can thus provide a useful model for aiding reservoir simulation and field development.