The mastery of a number of chemical reactions can give us information on the constitution of compounds in the solid state. As an example, acid etching (HCl) is used for oil shale to separate the organic matrix by destroying the mineral matrix. (The use of the soxhlet). Similarly, by ashing at low temperatures (LTH) in combustion of the organic material, the mineral matrix can be kept to a minimum (A.Attaoui et al: 2022).Another way of using acid etching (HCl) at given concentrations can be carried out for oil shale in order to give the composition of the majority constituents of the latter, namely organic matter and carbonates (calcite and dolomite discerned). The objective of this study, which is carried out in a back-assay mode using the Tacussel pH meter, is to establish a table of compositions in mass percentages of the different constituents reacting to the acid attack of oil shale from the first three layers of the Tarfaya deposit (Morocco), which are Z0, Z1 and Z2.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.