ABSTRACT: Hydraulic and mechanical properties of poorly lithified sandstones can be significantly changed around fault zones. Understanding how architectural components of fault zones control the distribution of those properties is of primary relevance for reservoir engineering. Here we present the analysis of the distribution of the physical and the hydraulic properties of sandstones in both the footwall and the hanging wall of a normal fault. The rock in question is an arcosean sandstone deposited in alluvial fans and braided fluvial environments. The results point out to a clear difference in the properties measured in the hanging wall and in the footwall sandstones. A reduction on the grain size distribution of sandstone occurs at the hanging wall in sampling points nearest the fault core, where coarse grain size decreases and medium sand size particles predominate with simultaneous increasing in clay fraction in the rock matrix from around 15% in the protolith to 25% near the fault core. A similar variation in the particle size distribution curves is not observed in the footwall block where just a very discrete grain size reduction takes place. We concluded that the hydraulic properties of the sandstone were affected significantly at the hanging wall block.
1. INTRODUCTION
Both mechanical and hydraulic behavior of poorly lithified siliciclastic rocks are considerably affected by tectonic strain. The presence of structures like joints, faults and deformation or compaction bands may obliterate porous network or generate new pore connections, causing drastic variations in fluid flow through reservoirs.
Since the classic paper of Caine et al. (1996) dealing with different hydraulic compartments generated inside a fault zone (FZ), many other contributions related to developing of FZ in poorly lithified sandstones (e.g. Heynekamp et. al 1999; Rawling and Goodwing, 2003; Loveless et al., 2011) have been published in the last decades. Ballas et al (2015) presented an extensive compilation of works considering the theme related to the hydraulic behavior of poorly consolidated sandstones affected by FZ in extensional tectonic regimes and the associated presence of dilation bands.
Predicting the role played by tectonic structures in unconsolidated sandstone reservoirs is a complex task because deformation mechanisms may result in localized granular flow that forms pervasive structures in sub seismic scale (Kristensen et al, 2013). To overcome this issue, here we report results of a research carried out in poorly lithified sandstone considered analogue to Maastrichtian sandstone reservoirs in the offshore basins located on the Brazilian continental margin.
