Delvaux de fenffe, D. 1988. ‘Etude géochimique de la matière organique sédimentaire par pyrolyse. Caractérisation des roches à kérogène et des bitumes par pyrolyse comparative et analyse cinétique. Application au secteur pétrolier du Bas Zaïre – Angola’. 260 p.

Report (unpublished)

Pyrolysis is a valuable technique for the geochemical characterization of organic matter in sedimentary rocks. The Rock-Eval method (Espitalie & al., 1977), the main pyrolysis technique currently used in petroleum exploration, has many advantages. However, it has some disadvantages which limit the precision of the results and the extent of its application. In the present study, the Rock-Eve! method has been developed in an attempt to overcome these inadequacies and to allow maximum use of the experimental data. The three main improvements are (1) a new comparative pyrolysis method, (2) a technique for quantitative estimation of the rate of production, expulsion and accumulation of bitumen in source and reservoir rocks; and (3) a kinetic method for geochemical characterization of kerogen and resins + asphaltenes. The new comparative pyrolysis technique has been designed for the global and exhaustive analysis of insoluble and soluble fractions of sedimentary organic matter. It comprises a double analysis of each sample by classic Rock-Eva} pyrolysis. The first analysis is performed on whole rock sample while the second is carried out on a previously extracted sample with dichloromethane or chloroform as solvent. A third pyrolysis curve is computed by subtracting the pyrolysis curve of the extracted sample from that of whole rock sample. This new curve represents the soluble organic matter fraction as a whole (bitumen). It can be further subdivided into three fractions: S1 (volatilisation of C25- light hydrocarbons), S1' (volatilisation of C20+ heavy hydrocarbons) and S2' (hydrocarbons from the pyrolysis of resins + asphaltenes). The S2 fraction only contains hydrocarbons from kerogen decomposition. The S1 fraction has virtually the same composition as that obtained by the classic Rock-Eval method (Espital1e & al., 1977) and the sum (S1'+S2'+S2) corresponds to the classic S2 fraction. The total bitumen yield is expressed as the sum (S1+S1'+S2'). Comparative pyrolysis allows a better distinction between kerogen and bitumen in rock samples. Comparative pyrolysis also provides pyrolysis curves for resins + asphaltenes which are of great interest for their geochemical characterization. The relative yields of the four groups of components are expressed by new production indices. The quantitative estimation of bitumen generation in source rocks is performed by computation of an Estimated Production Index (IPE). This new index is obtained from the Rock-Eval parameters S2, IH and Tmax by estimating initial values of S2o and IHo in a IH-Tmax diagram. The quantity of bitumen generated in, expelled from or accumulated in each sample can thus be determined. By considering all samples from a given sedimentary formation, the primary migration can be determined. New parameters are proposed for the quantitative characterization and classification of source or reservoir formations: Rates of Expulsion, Accumulation, Immobilization and Internal Transfer. The kinetics of Rock-Eval pyrolysis has been investigated for a wide range of source and reservoir rocks. Pyrolysis records for kerogen (S2) and resins + asphaltenes (S2') were obtained by comparative pyrolysis. The kinetic parameters are computed by the freeman & Carroll method (1958), assuming a global reaction with a single apparent activation energy and a reaction order which is not necessarily equal to unity. The kinetic data where treated statistically to ensure results better reproducibility and representativeness. Results show that the global kinetic mechanism for the pyrolysis of kerogen and resins + asphaltenes is related to their chemical composition and structure. For kerogens, the values of activation energy E and reaction order n depend on type and maturity. Values of E are found to be 20 to 80 Kcal/mole. Values of n lie between 0.5 to 3.5. Using these data, references diagrams have been established for the kinetic characterization of kerogen type and maturity. The same procedure can be used for the study of resins + asphaltenes from bitumens of active source rocks. There is a close relationship between the kinetic parameters of kerogens and resins + asphaltenes. By establishing new references diagrams, resins + asphaltenes produced from type I, II or III kerogen can be differentiated. Maturity level can be evaluated by the temperature of maximum hydrocarbon generation during comparative pyrolysis. The combination of comparative pyrolysis and kinetic characterization is a new geochemical tool for the study of kerogens and resins + asphaltenes. These methods are particularly applicable to the study of bitumens of reservoir rocks, for the determination of the type of kerogen from which resins + asphaltenes were derived. Mathematical simulations of petroleum genesis in sediments have been performed using different kinetic models. These new geochemical tools have been successfully applied to basin analysis on the Bas Zaire Angola continental margin. They were used for the geochemical characterization of the main formations of source and reservoir rocks and for determining the probable origin of the oil recovered from the latter.