**1. Introduction**

The need to discover new deposits of useful mineral substances led to the study of the physicochemical properties of the soil and subsoil constituents (geological layers).

From the beginning of the geophysical research of the subsoil, the knowledge of the temperature of rocks and constituent fluids was an absolutely necessary step to establish working conditions in tunnels and mine shafts (to prevent the formation of explosive mixtures and explosions) [1].

Subsequently, the geothermal phenomena (geothermal gradient and geothermal stage) were analyzed, in order to improve the exploitation of oil and gas deposits [2].

Knowing the geothermal flow of geological layers was useful for determining the temperature of the crust and the structure of the lithosphere and also for understanding how to form oil and gas deposits.

The exploitation of crude oil and gas from the Moesic Platform in Romania has created a database regarding the understanding of the thermals of geological strata and especially the formation of oil deposits in magmatic fields [3, 4].

Theoretical and practical aspects of the use of geothermal steps in the analysis of deposits of useful mineral substances were made by Dowle and Cobb [5].

The interest in determining the geothermal of the subsoil and in particular the geothermal of the oil and gas deposits were due to the following [6–8]:


The calculation of the thermal flow was performed by determining the thermal gradients and thermal conductivity in measurement points and subsequently by determining in the laboratory the physical properties of the rocks (cores) collected from geological research drilling [9].

The determined values had an estimative character (being often point values), but they were useful in determining the thermal structure of the geological layers [10].

Thus, following the measurements of the temperature of the oil fluid extraction wells, a low thermal flux was determined (45–57 mW/m<sup>2</sup> and 33–58 mW/m<sup>2</sup> ) in the areas rich in gas deposits and quite high in the areas with coal deposits (200 mW/m2 ).

But the most important physical property of rocks and the constituent fluids of oil and gas deposits, namely thermal conductivity, is very useful in establishing the tertiary oil recovery system and in determining the flow of fluids through rock pores.

That is why this property is determined in the laboratory, by analyzing the heat transfer that passes through the rocks collected from the oil fields.

Thermal conductivity (*k*) is the property (ability) of rocks and continuous media to transmit, to a greater or lesser degree, thermal energy (relation 1) [11].

$$k = \frac{Q}{(t\_2 - t\_1) \bullet \left(\frac{t}{2}\right) \bullet \tau} [\mathbf{w}/\mathbf{m}\ \mathbf{°C}] \tag{1}$$

In Eq. (1), *Q* is the amount of heat that passes through rock with cross section *s*, in a time *τ,* and with a length *l*.

Factors influencing rock conductivity to take into account the structural-textural peculiarities of rocks (composition, size and orientation of rock granules, porosity, and fluid content), the temperature measured at the faces of the analyzed rocks (*t2, t1*), and the pressure to which the structure is subjected: geological analysis [12].

The conductivity of the rocks decreases as the size of the rock granules decreases because the number of contacts between the granules and the heat flux increases in the flow of fluids through the rock pores.

Also, the orientation of the rock granules influences their thermal conductivity, schistosity, stratification, and fracturing, reducing their values.

The thermal conductivity of dry porous rocks is lower than that of compact rocks (air with a thermal conductivity value of 0.55 mcal/cm°C s).

At the same time, the flow of fluids or the fluid content of rocks changes the value of thermal conductivity, and rocks with water have higher conductivity than those containing oil or natural gas.

The pressure in the geological layers not influences the values of thermal conductivity.

But the increase of the pressure increasing the conductivity due to the friction of the rock particles.

The increase in temperature leads to a decrease in conductivity due to the increase in the interaction speed of the particles of the crystal lattice.

The determination of the thermal conductivity of the rocks is necessary for the evaluation of the thermal flow of the deposit and especially for the choice of the most useful technology in increasing the recovery factor of crude oil and petroleum gases.
