Structural Analysis of Fractured Carbonate Reservoirs: Implications for Exploration and Reservoir Modeling

Nolan M. Dehler¹, Délzio L. Machado Jr.¹, Raphael Hatushika¹, Rogério Gontijo¹, Carlos Beneduzzi¹, Leonardo Correa Gomes¹, Lúcia Dillon¹, and Fernanda Britto^2
1Departamento de Exploração e Produção, Petrobras, Rio de Janeiro, Brazil
²Centro de Pesquisa, Petrobras, Rio de Janeiro, Brazil

Fractured Aptian and Albian carbonate reservoirs, localized along the eastern Brazilian marginal basins, has proved to contain important oil and gas reserves. However, a better understanding of the structural behavior of those fractured systems is required. The purpose of this abstract is to discuss the results of a structural analysis carried out on selected cores, and their application for exploring and modeling fractured reservoirs.

Attributes of fractures and faults taken from sidewall cores, logs, and plug data, were used to estimate fracture density, aperture and stress regime, which were correlated with depositional sequences, facies, and other petrophysical properties, like porosity and granulation. Dynamic elastic constants (i.e. Young and shear modulii) were derived from density and sonic logs, and were used as input parameters to give a relative measure of the propensity of the material to fail under the same strain rate. We observe that the most deformed facies are mechanically soft, porous laminated mudstones. We believe that it happens because the prevalent criterion to fracture rocks in this particular case was the increase in strain rate near fault zones, as could be also constrained by seismic data.

We suggest that a clear, positive correlation between fracture density and some petrophysical parameters, like porosity, granulation, and rigidity, occurs when deformation results from wide wavelength perturbation. Nevertheless, this correlation is not so obvious when restricted fault-related deformation occurs. We suggest that in exploration, where well data are restricted, the identification of fault zones and highly curved structures (curvature analysis), together with an appropriated choice of an elastic parameter, may help the interpreter to find and drill the most important fractured suit spots.

AAPG International Conference and Exhibition, Cape Town, South Africa 2008 © AAPG Search and Discovery