Faculté des sciences

Genèse des gisements d'asphalte des formations de la pierre jaune de Neuchâtel et des calcaires urgoniens du Jura (Jura neuchâtelois et nord vaudois, Suisse)

Zweidler, Daniel C. ; Remane, J. (Dir.)

Thèse de doctorat : Université de Neuchâtel, 1985 ; Th.1981.

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    In Switzerland, asphalt deposits and occurrences are prominent at the foothills and around the first synclines of the Jura Mountains in the cantons of Neuchâtel and Vaud. They are all closely associated with the Cretaceous, Hauterivian and Barremian, limestone formations. During most of the Hauterivian and Barremian, an important carbonate platform developed on the northern, passive margin of Tethys. This paper aims at providing a coherent lithostratigraphic and sedimentological framework (part I) enabling the study and elucidation of the origin of the asphalt deposits (part II). The lithostratigraphic framework study, based on a sequential analysis of the series, resulted in the revision of the classical formations defined in the region. A coherent subdivision for the region of the Jura of Neuchâtel and Northern Vaud is proposed. Two formations persist over the whole series, the "Pierre jaune de Neuchâtel", overlain by the "Calcaires urgoniens du Jura". Each formation representing a second order sequence separated by a major discontinuity, the "Discontinuité de La Russille". The sedimentological study further demonstrates the existence of three distinct carbonate depositional facies over the time period covering the two formations. Sandy oolithic and oo-bioclastic shoals, capped by the “Discontinuité De La Russille”, dominate the “Pierre jaune De Neuchâtel” paleogeography. Following the major discontinuity expressed by hard grounds, sedimentation resumed, with a calm transgressional facies constituted mainly of biostrome with large madrepores. This facies gradually morphs into a more typical urgonian facies: lagoon type deposits with rudists as well as white sandy bio-pelclastic shoal deposits. Both facies are part of the "Calcaires urgoniens du Jura" formation. The clastic facies from both formations offer good reservoir characteristic especially since they have for the most part undergone an early vadose diagenesis enhancing the primary porosity. The reservoir of the main and by far the largest asphalt deposit at La Presta (Val-de- Travers) is located in the "Calcaires urgoniens du Jura" formation. The detailed quantitative three dimensional structural and sedimentological study carried out at the asphalt mine of La Presta allows us to furthermore propose a multi-migration genetic model for the asphalt deposits. Hydrocarbon molecules expelled from the source rock by primary migration moved by secondary migration into stratigraphic and diagenetic traps formed by bio-pelclastic shoal deposits encased by pure biomicrite acting as seals, where they accumulated as oil. A tertiary migration took place during the folding of the Jura Mountains in the Tertiary, also bringing the oil-bearing reservoirs closer to surface, causing severe biodegradation of the oils into asphalts. Based on our observations at La Presta, we are now in the position to better understand the distribution of the asphalt deposits in general, controlled by the paleogeographic evolution of the Cretaceous carbonate platform and the resulting re-migration post dating the Tertiary folding of the Jura Mountains.