A Paleozoic-sourced oil play in the Jura Mountains of France and Switzerland

Abstract

Abstract The Jura region of France and Switzerland has significant hydrocarbon potential. This arcuate fold-and-thrust belt, located in front of the Western Alps, is composed of Mesozoic rocks, which overlie the North Swiss Basin Complex filled with Permo-Carboniferous sediments of Late Westphalian–Lower Permian (Autunian) age. The existence of three small producing gas fields, and the presence of recorded oil and gas shows in a number of petroleum wells, indicate the presence of an active petroleum system. Shows occur in stratigraphic horizons that lie beneath the regional Triassic evaporite seal. Very effective Permo-Carboniferous oil-prone source rocks include algal organically rich Autunian shale beds and Stephanian-aged bituminous shales. Carboniferous and Permian coals are also present, which would be excellent sources of gas. The primary reservoir target is the fluviatile Triassic Bunter Sandstone Formation, which is widely developed across the area and thickens to the NE. Reservoir quality is best developed in the cleaner channel sands. The La Chandelière oil field, discovered in 1989 by Exxon, is crucial evidence of the oil potential and trapping mechanism. Geochemical analysis demonstrates that the 41° API oil at La Chandelière is derived from Lower Permian Autunian shales. The trapping mechanism, interpreted from seismic and confirmed by structural modelling, is that of an unreactivated Mesozoic-aged structural high bounded by thrust faults, resulting in a large-scale rollover within the core of the structure. This combination of critical elements sets up the Paleozoic-sourced oil play within the Jura Mountains of France and Switzerland. The Jura is not generally regarded as a major petroleum province. It is a region of complex structuring and mountainous terrain. Switzerland, a country renowned for its beautiful mountain scenery, has no major oil and gas discoveries. However, this poorly explored region has active petroleum systems with good hydrocarbon shows and a number of small gas fields demonstrating hydrocarbon generation. Good-quality source rocks and excellent reservoir development have been proven by drilling, and seismic interpretation has demonstrated the existence of large closures at reservoir level. The La Chandelière oil discovery, in the French Jura, proves the presence of a Paleozoic-sourced oil play.