## 1 Introduction  
The influence of temperature on rock behaviour is studied in several geological domains, such as petroleum extraction, geothermal activity, or storage of radioactive waste, involving many rocks [1, 2, 3, 4]. The research studies show that high temperature changes the minerals and the void system of the rocks and the flow of the fluid in the rock may be enhanced or reduced [5, 6, 7, 8, 9]. The intensity of the modifications of the porous network is related to inherent stone characteristics such as mineralogy, texture, weathering degree [10], and thermal regimes involving extreme temperatures and repetition cycles. Chalk, sandstone, and granite are reservoir rocks, and their properties and behaviours facing different temperatures and pressure are often compared due to their different porous network, that is high microporosity for chalk, mesoporous distribution for sandstone, and low porosity of fissural type for granites. Chalk is an interesting reservoir rock characterised by a large microporosity, with around 40% of void volume or even up to 50% for the North Sea Chalk [11]. The main features are its very rich to almost pure calcite composition and its reactivity to water. Despite the high and anisotropic thermal extension of calcite crystals, the high microporosity usually accommodates the expansion. Some authors show that heating, even at very high temperatures, does not modify the elastic parameters of the chalk but rather causes an extension of the elastic phase [3]. In the case of a fluid flow, chalk crystals are prone to dissolution or mobilisation, which creates a modification in chalk microstructure [12]. Sandstone (5-30%) is considered, due to its porosity, as a good reservoir rock and forms aquifers exploited as geothermal systems. The submission to high temperatures can trigger effects such as crushing of sand grains, fracturing, thermal expansion, or mineral cohesion rupture [13; 14; 15], generating changes in permeability [13; 16; 17]. Granite is very sensitive to the effects of temperature due to its low porosity (from around 1% to a few% if weathered) and its mineral heterogeneity and its differential thermal expansion [18; 19; 20; 21]. That may lead to the generation of irreversible microcracks beyond a certain temperature threshold (e.g., [22; 23; 24; 25; 26]). In addition, for fresh and low porosity granites, textural features such as Quartz/Feldspar ratio, mineral orientation, or crystal size become determinants in its behaviour [27; 21; 28; 29]. When a stone is exposed to high temperatures, two questions about its behaviour arise. Which is the threshold of irreversible cracking? Secondly, when the heating-cooling cycles are repeated, what is the effect of thermal fatigue on the stones? Thus, the main aim of this study is to assess the microstructural variation of four rocks analogous to those found in hydrocarbons reservoirs or as geothermal energy sources submitted to different thermal regimes. For that purpose, two kinds of tests were performed: Thermal Threshold (TT)--progressive heating cycles from 90 \({}^{\circ}\)C to 130 \({}^{\circ}\)C to determine the stone crack threshold, and Thermal Fatigue (TF)--thermal fatigue with cycles of heating-cooling at 200 \({}^{\circ}\)C.  
The rock microstructure was evaluated before, during, and after the tests by several methods. A microscopical evaluation by scanning electronic microscopy was carried out to localise the new formed cracks. The variations of the bulk stone were carried out by mercury porosimetry. This technique gives information about the pore access distribution, which allows one to compare the size and frequency of the rock pore access ratio before and after heating [20; 30; 31; 32]. To ensure the evolution continuity, capillary water uptake was also used to control the rock microstructure. This technique is non-destructive and allows one to quantify indirectly the microcrack formation or variation in the same sample through the cycles [33; 34; 35].