Evaluation of the geomechanical parameters in low permeability reservoir analogous related to Reconcavo basin's syn-rift turbiditic systems in Bom Despacho, Itaparica Island, NE of Brazil

Abstract

The region of Itaparica Island, Bahia, located in the Todos os Santos Bay, to the East of Salvador City – Brazil, was chosen because it holds deposits of the Maracangalha Formation (MF). The MF has the most significant volume of gas stored in low permeability reservoirs in the Recôncavo basin (RB). Fault systems dominate the structural framework of the RB: the Maragogipe flexural border (N10o) to the West, the Salvador fault border (N30o) to the East, the Mata-Catu (N150o) to the center, the Barra (N090o) to South and Transversals (transfer and release) (N120o) distributed throughout RB. The locality of Bom Despacho, in the northeastern sector of the island, presents a lithostratigraphic arrangement of rocks from the Caruaçu and Pitanga members, which compose the Maracangalha Formation, and are represented by massive sandstones and shales with a dominance of the first over the second, in terms of proportion. This outcrop is a miniature of a larger system that combines turbiditic sandstones and shales from the Ilhas Group with potential importance as they represent large hydrocarbon deposits in unconventional reservoirs. This one exhibits a system of orthogonal fractures originating from paleotensors acting by tectonic efforts that contributed to the current configuration of the basin. Mechanical and structural parameters, printed on the massive sandstones of the Caruaçu Member, were used for methodological purposes of simulating the fracture behavior and recognizing the tectonic forces that originated the current fractal arrangement. Thus, five stations divide the study area, where 729 fractures were measured and classified according to two main trends of preferential orientation. The main orientations observed are two families close to N10o-30o, longer, and N100o-120o, shorter, related to the RB's Longitudinal and Transversal Fault Systems. The orientations and rare striations in the fracture planes indicate that they were formed mainly: (i) second a Tensile Failure Criterion (Griffith Criterion) in shallow crustal conditions, and (ii) by a rhombohedral-orthorhombic extension (Mode I fractures). This fracture pattern is usually associated with paleostress principal tensors magnitudes 1>2≈3, resulting in an orthorhombic extension close to the E-W direction. The treatments of structural data collected in the field, such as spacing, length, thickness, and fracture opening, were used to analyze numerical models and statistical distributions using the finite element method in COMSOL Multiphysics software. The results showed a close relationship to the mechanical layering of the rocks on tensile stress at the fracture tips. The behavior of fractures is influenced not only by the magnitude of the fluid pressure at the local level around the fracture tips but also by the distribution of fluid pressure gradients. Another aspect is the discussion of the fractal behavior in the light of the impacts on low permeability reservoirs. The present work expects to contribute to a better understanding of the formation and evolution of fracture families in turbidite formations, with a tectonic situation similar to the case study in terms of the potential for flow properties of low permeability reservoirs.