Zur Vorhersage der Erdöl- und Erdgaszusammensetzung durch die Integration von Labor- und Fallstudien

The distribution of oil and gas in reservoirs is ultimately controlled by the different pressure and temperature conditions in the petroleum system. Of initial importance is the timing of petroleum generation and the composition of the frrst-formed petroleum in source rocks. The latter is controlled by the source rock type and its maturity. Compositional predictions of frrst formed petroleum are of great interest because gas-oil ratio, aromaticity and wax content govem physical properties and phase behaviour of petroleum, including the segregation of oil and gas, along secondary migration routes. While natural petroleum rates are extremeley slow in human timeframe, artificial maturation experiments - using pyrolysis - make feasible the simulation of natural transformation processes in a reasonable time interval. The Posidonia-Shale (NW-Germany) and the Duvemay Formation (Canada), which were studied in this work, are both elements of weIl-known petroleum systems and form the basis of the research reported here. With a basinwide uniformity in the type of sedimentary organic matter these source rocks fulfIl the optimal requirements for simulating generation processes in the laboratory and its comparison to natural processes. The goals of the present study can be described as follows: a) predicting oil and gas generation in mature to overmature source rocks; b) analysing the feasibility and boundary conditions for making detailed compositional predictions; c) predicting the phase behaviour of frrst-formed petroleum as a function of maturity. The generation of liquid and gaseous hydrocarbons from the Posidonia Shale and the Duvemay Formation was simulated by heating immature kerogens in a c10sed system (MSSV pyrolysis) at three different heating rates (0.1, 0.7 and 5.0 Klmin). Applying a simple stoichiometric concept and complementary pyrolysis methods the primary transformation from kerogen to oil and gas as weIl as secondary oil to gas conversion processes could be simulated. After the calculation of kinetic parameters from the cumulative evolution curves of these different types of hydrocarbons, temperature predictions for a geological heating rate (5.3 Klma) were made. For both source rocks the onset of oil generation is predicted to occur at 90°C and the maximum oil formation rate at 140°C. The onset of primary and secondary gas occurs at 110°C/165°C and 150-160°c/180°C, for thePosidonia Shale and the Duvemay Formation respectively. These temperatures for secondary gas generation are 20-30°C lower than published temperatures for oil to gas cracking reactions under reservoir conditions. 1t has been deduced that these phenomenon is due to catalytic effects of macromolecular organic compounds in the source rocks. In the case of bulk liquid and gaseous hydrocarbons the above kinetic calculations can be considered valid because their maximum yields are independent of laboratory heating rates. In contrast, the contents of paraffms, aromatics and sulfur compounds show a pronounced heating rate dependence. Extrapolated to geological heating rates the compositional predictions are consistent with the bulk composition of natural products in the Duvemay - petroleum system. In addition to forward (kinetic) modelling, reverse (mass balance) modelling was performed using the natural maturity suite of the Duvemay Formation. Following a new method of mass balance calculations, the generation of hydrocarbons could be reconstructed on a molecular level. The direct comparison with artificial generated products from the MSSV -pyrolysis shows very good quantitative similarities. Most significant differences in product yields are controlled by secondary reactions, which result in the generation of additional compounds from the thermal cracking of unresolved compounds (hump) in pyrolysis-gas chromatograms at higher levels of kerogen transformation. For the simulation of the phase behaviour of frrst-formed petroleum, the molecular information from the MSSV -pyrolysis of immature kerogens from the Posidonia Shale and Duvemay Formation was used. In addition, physical properties of the gas chromatographically unresolved compounds were estimated and used for phase calculations. The calculated phase envelopes for artificially products of both source rocks show a development which is c10sely similar to the phase envelopes ca1culated for natural oils of different maturity. With the results of this study it was shown that the simulation of natural generation processes by laboratory heating experiments in a c10sed system delivers important compositional predictions for the generation of oil and gas in natural sediments