This article gives the main results of the geochemical survey made in the Paris Basin from cuttings and core samples coming from 100 petroleum or mining boreholes and from oil samples taken from Mesozoic reservoirs (Triassic, Upper Bathonian, Neocomian). The different geochemical methods used are pyrolysis, which is well suited for examining the numerous cuttings samples gathered, gas chromatography for the detailed analysis of oils and rock extracts, and mass spectrometry for examining biomarkers. The main times when oil and gas were formed were determined by computerized simulation using a mathematical model for the formation of hydrocarbons. Pyrolysis methods were used to compile geochemical logs of the boreholes investigated and to determine, by computing, the initial pyrolisis parameters of source rocks before their burial. In this way, within the area investigated, maps were compiled of the initial organic carbon, the total petroleum potentials and the amounts of migrated hydrocarbons. These results as a whole show that the source rocks for the great majority of the Mesozoic oils are situated in the Lias, ranging from the Toarcian to the Hettangian, and that the zones having the best petroleum potential in the Basin are located in areas of strong subsidence in Lias times. The detailed examination of the hydrocarbons and biomarkers did not distinguish different oils from each other, no matter what depth they came from, nor could these oils be attributed to a specific origin in the different levels of the Lias. The migration maps reveal lateral displacements of hydrocarbons in some source rocks (Lower Toarcian). They also show a considerable hydrocarbon deficit at the bottom of the Lias (Hettangian, Sinemurian) that appears to be a potential source rock for Triassic oils. With regard to these oils, their main migration paths were probably the faults that cut through the Triassic and the bottom of the Lias, which must have played an important role during the Tertiary (distensional faults). Lateral migration in reservoirs (porous limestone in the Dogger, sandstone in the Triassic) is considered. In the eastern part of the basin (south of Nancy), several gas and oil fields found in the Muschelkalk seem to be of Paleozoic origin (Saar-Lorraine Basin).
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