The effect of mechanical stratigraphy on natural fractures morphology and distribution in carbonate reservoirs

Abstract

High-resolution borehole images and other well logs are used to study the interrelationship between mechanical stratigraphy and natural fractures in the carbonates of the Dehram reservoirs in the Zagros area, Southwest of Iran. Natural fractures are characterized for their vertical distribution and morphology in six nearby anticlinal structures hosting one of the largest gas provinces in the world. Mechanical stratigraphy is evaluated using dipole sonic and density data from the same boreholes in these six gas fields. This study shows that the effect of mechanical stratigraphy on the fracture system in carbonate reservoirs can be evaluated in two scales. Large-scale mechanical stratigraphy, represented by rock strength variations within units with more than 1-m thickness, influences the overall fracture intensity. At the same time, the effect of small-scale mechanical stratigraphy is more pronounced on aperture and continuity as two elements of the morphology of natural fractures. This scale of rock mechanics is practically discernible using high-resolution borehole images. Both scales of mechanical stratigraphy were crucial when building predictive models of reservoir performance in naturally fractured carbonate reservoirs.