Petrophysical properties of deformation bands in high porous sandstones across fault zones in the Rio do Peixe Basin, Brazil

Abstract

In this contribution we show the variation of the uniaxial compressive strength (UCS) and petrophysical properties of different architectural elements of fault zones affecting poorly lithified coarse sandstone and conglomerates of the Rio do Peixe basin, NE Brazil. We worked on three distinct outcrops; (1) non-deformed (protolith); (2) single fault zone, presenting moderate strain; and (3) complex fault zone, presenting a high strain intensity, where multiple and well developed fault cores occurs. To characterize the structural domains, we performed scanlines and uniaxial geomechanical survey across deformation bands and clustering zones. We combined geomechanical data with porosity data acquired thru image analysis and gas expansion analysis. We acquired data on deformation band cores and zones between deformation bands (i.e., deformed zones that do not present deformation bands). Our results show that the complex fault zone exhibits a strong increase on the rock resistance, thus presenting UCS values three times higher than those obtained on the single fault zone outcrop and four times higher when compared with the non-deformed outcrop. It suggests that there is a direct relation between strain and rock resistance. The porosity is also strongly affected by the deformation, exhibiting an inverse relation with the strain intensity. Complex fault zone presents up to 75% of porosity reduction, while single fault zone, only about 10% of reduction. The petrophysical properties were calculated in the complex fault zone, where the resistance of the DBs was as high as twice that of the zone without bands, which had effects on the Young’s and the incompressibility’s moduli. The results suggest that even though DBs generate up to a 40% increase in the UCS and a 75% decrease in the porosity, the structural domains where the rock is located strongly controls the geomechanical and petrophysical properties, thus, directly affecting the anisotropy of a reservoir.