Variation of petrophysical properties along deformation bands and its implication in petroleum reservoirs

Microstructure and petrophysical properties of deformation bands from various sandstones deformed at different depths have been studied. This study has been done through analysis of both optical and BSE images of thin sections of deformed sandstones. By means of our developed image processing method, we were able to calculate porosity and permeability within deformation bands. We have found that the internal microstructure of many deformation bands varies along the bands at the cm or even mm scale. The variations can result in variations in petrophysical properties such as porosity and permeability. In the examples explored, porosity varies by up to 18% and permeability by up to two orders of magnitude within a single deformation bands. Such petrophysical variations are found in different types of deformation bands, but the range depends upon the deformation mechanisms, in particular on the degree of cataclasis and dissolution in cataclastic and dissolution bands, and on the phyllosilicate content in disaggregation bands. Moreover, the grain size distributions change along the bands with regard to the degree of cataclasis. The widely scattered distribution of previously reported porosity and permeability data for deformation bands can be explained by the explored variations in properties along the bands in this study. The rapid variation in properties along bands even on mm and cm scale, implies that deformation bands may not hold any significant pressure difference along them and probably do not contribute to the sealing capacity of faults. On the other hand, cataclastic bands can affect the petrophysical characteristics of the reservoirs in a production situation, although their influence on productivity strongly depends on the cumulative thickness of the bands as well as their internal reduction in permeability. Disaggregation bands are effective baffles to fluid flow only if the phyllosilicate content is high and evenly distributed throughout the host rock.