The carbonates of the oil shale calcite CaCO3 and dolomite CaMg(CO3)2 were the object of this study to determine their percentage inside the oil shales. Work has been carried out in the same context (A.Attaoui and H.Mokhlis:2022) for the Tarfaya shale layers Z0, Z1 and Z2. However, the Z3 and Z4 layers present a migration of matter: the light organic matter is found in the Z3 layer and the heavy one is present in the Z4 layer, and the heavy organic matter consumes more acid in the attack reaction than the light organic matter for the same mass, which is why it is necessary to follow the decomposition of this organic matter isothermally, especially at 500°C. Therefore, by combining the two reactions and by choosing well-defined isotherms (500°C, 600°C, 700°C and 800°C) we were able to approach the percentages of calcite and dolomite in these particular layers showing the phenomenon of in situ pyrolysis. The techniques used are isothermal combustion followed by HCl acid attacks and return dosing of the acid remaining after the attack. The objective of this study, which is done in return dosing using the Tacussel pH meter, is to establish a table of compositions in mass percentages of calcite and dolomite, which are the constituents of the carbonate of the Tarfaya oil shale (Morocco). Calcite CaCO3 is taken as a control; it was used under the same conditions as the oil shale, i.e. combustion in an isothermal regime, followed by acid attack.