**Abstract**

Shale rocks are an integral part of petroleum systems. Though, originally viewed primarily as source and seal rocks, introduction of horizontal drilling and hydraulic fracturing technologies have essentially redefined the role of shale rocks in unconventional reservoirs. In the geological setting, the deposition, formation and transformation of sedimentary rocks are characterised by interactions between their clay components and formation fluids at subsurface elevated temperatures and pressures. The main driving forces in evolution of any sedimentary rock formation are geochemistry (chemistry of solids and fluids) and geomechanics (earth stresses). During oil and gas production, clay minerals are exposed to engineered fluids, which initiate further reactions with significant implications. Application of hydraulic fracturing in shale formations also means exposure and reaction between shale clay minerals and hydraulic fracturing fluids. This chapter presents an overview of currently available published literature on interactions between formation clay minerals and fluids in the subsurface. The overview is particularly focused on the geochemical and geomechanical impacts of interactions between formation clays and hydraulic fracturing fluids, with the goal to identify knowledge gaps and new research questions on the subject.

**Keywords:** Shale geochemistry and geomechanics, Clay minerals, Hydraulic fracturing fluids, Shale formation fluids
