An apparatus for thermally treating an oil reservoir by separately introducing a first composition and a second composition into an oil well and contacting the first composition and second composition in a pay zone of the oil reservoir to initiate a chemical reaction producing heat and gases, the oil well being in fluid connection with at least one substantially vertical pair of horizontal wells, an upper horizontal well and a lower horizontal well, situated in the pay zone, the apparatus comprising: - a casing of the oil well provided with openings (4, 5) facing the horizontal wells (11, 12) to allow fluid exchange between the oil well and the horizontal wells (11, 12) of the oil reservoir, - a sealing device positioned in the casing of the oil well so that the opening in the casing facing the upper horizontal well is located above and the opening in the casing facing the lower horizontal well is located below the sealing device; - an inner tubing arranged inside the casing of the oil well and passing through the sealing device, the sealing device sealing the space between the inner tubing and the casing, the inner tubing provided with at least one opening in its distal end section located below the sealing device, - an outer tubing arranged coaxially with and outside the inner tubing and inside the casing, - the inside of the inner tubing defining a first fluid passage, - the inside of the casing of the oil well and outside of the outer tubing above the sealing device defining a second fluid passage, - the inside of the outer tubing and outside of the inner tubing defining a third fluid passage, - the outer tubing provided with openings connecting the third fluid passage with the second fluid passage, whereas the openings of the outer tubing are located close to the openings of the casing facing the upper horizontal well for transfer of the chemical reaction of the first composition and of the second composition to the pay zone of the oil reservoir. - a casing of the oil well provided with openings (4, 5) facing the horizontal wells (11, 12) to allow fluid exchange between the oil well and the horizontal wells (11, 12) of the oil reservoir, - a sealing device positioned in the casing of the oil well so that the opening in the casing facing the upper horizontal well is located above and the opening in the casing facing the lower horizontal well is located below the sealing device; - an inner tubing arranged inside the casing of the oil well and passing through the sealing device, the sealing device sealing the space between the inner tubing and the casing, the inner tubing provided with at least one opening in its distal end section located below the sealing device, - an outer tubing arranged coaxially with and outside the inner tubing and inside the casing, - the inside of the inner tubing defining a first fluid passage, - the inside of the casing of the oil well and outside of the outer tubing above the sealing device defining a second fluid passage, - the inside of the outer tubing and outside of the inner tubing defining a third fluid passage, - the outer tubing provided with openings connecting the third fluid passage with the second fluid passage, whereas