Topological analysis of fault network in naturally fractured reservoirs: A case study from the pre-salt section of the Santos Basin, Brazil

Abstract

The Santos Basin, located on the southeastern margin of Brazil, hosts giant hydrocarbon reserves of the pre-salt section that correspond to the carbonate sequences deposited during the syn-rift and transitional tectonic stages. The faults, which formed during the rift evolution, have proven to be crucial in controlling hydrocarbon accumulations and migration, not only as a pathway for fluid flow but as a factor controlling the deposition of sedimentary rocks. Thus, the topological analysis of fault networks is a key method to characterize the different fault sets, as it provides a way to describe the network arrangement of individual faults and their connectivity. In this work, the fault topology was analyzed in terms of nodes and branches for the main reservoir defined in the pre-salt section seismic units: Rift, Lower Barra Velha (Lower BVE), and Upper Barra Velha (Upper BVE). Four fault sets were characterized in these units with average orientations to NW-SE and E-W. The topology of this fault networks is characterized by single-phase rift network, featuring isolated systems with more than 50% of isolated branches and low connectivity (connections per branch being less than 0.5). The flow azimuth is NE-oriented, mostly oblique to the trend of the main faults, which suggests a low fault influence in the fluid flow. On the other hand, the paleotopographic highs developed during the Rift phase present the highest fault density and intensity, factors known to contribute to increased connectivity in reservoirs. A clear limitation of this type of study is the integration of the seismic-scale faults with high-frequency well log data, sparse but known to have great vertical resolution, and, in turn, assessing the vertical fluid flow, instead of lateral as is possible with this method. These limitations are to be known by one to apply this in other contexts. In this work, we seek to understand how the development of the rift fault system influences the generation of new fault sets, and fluid flow in a pre-salt carbonate reservoir.