Reservoir in which the walls (111, 121) and the partitions (112, 122) are assembled by thermowelding the edges together with the exception of the edges (113a, 123a) of the well elements (113, 123), the edge (113a) of the upper element (113) and the edge (123a) of the lower element (123) of the well (104) have complementary shapes producing, by assembly under the pressure exerted by bringing together and welding the walls (111, 121) and the partitions (113, 123) of the reservoir (100), a fluid-tight joint between the edges of the two well elements, without generating projections that are deformed toward the interior of the well.The thermowelding hotplate (140) has a cut-out (143) along the line of the well elements (113, 123) so as to leave the edge (113a, 123a) of the well elements out of contact when the edge (111a, 121a) of the walls (111, 121) and that (112a, 122a) of the partitions (112, 122) of each of the reservoir parts (110, 120) is pressed against one face (141, 142) of the hotplate.