The properties of faults in the Brazil pre-salt: A reservoir characterization perspective

Subsurface characterization of faults and fault systems is a central piece to describe the subsurface permeability architecture of a carbonate reservoir during field development. Data integrity and quality are often compromised due to the challenges (mud losses and stuck pipe) that are posed when drilling into a fault zone. Core is frequently not recovered or in very poor condition (rubble) that make quantitative description of a fault zone very challenging. For these reasons wellbore trajectories tend to avoid fault zones leading to a sampling bias. As a result, many authors default to using field analogs to fill in the knowledge gap in fault properties that are required to build robust subsurface reservoir models. In this work, we select a subset of wells from the pre-Salt, offshore Brazil, where faults have been either intersected by a well or investigated by a well test. We use case studies to illustrate how observations made across a multidisciplinary dataset can be brought together to provide insights into likely ranges of fault properties. We discuss subsurface observations of fault zones in the pre-Salt in the context of analogs to propose end members of fault architecture that can have an impact on reservoir performance. Results suggest that these faults act as conduits for fluid flow. Their internal structure is characterized by a high density of fractures that often show evidence of dissolution enhancement. In the case of early syndepostional structures, early karst processes are responsible for enhancing fracture porosity and leading to the development of cavernous porosity. Faults developed during burial have a lesser degree of diagenetic overprinting, resulting in an overall lower pore volume. However, permeability (and its degree of anisotropy) in both cases is several orders of magnitude greater than that of the matrix. We have used numerical forward modeling of pressure derivatives to evaluate fault permeability values that yield 10's of darcy permeability associated with such features.