An Outcrop Analog of Kharaib & Shu'aiba Reservoirs: The Urgonian Platform of Southeastern France (Gard and Ardèche)

Abstract

Abstract The Barremo-Aptian Urgonian formation is widespread in southeastern France. This formation is a giant (20 000 km2) limestone platform deposited along the northern Tethyan margin during the lower Cretaceous, on the Vocontian basin margin. The Urgonian carbonates are mostly composed of rudist facies interupted by several Orbitolinids-rich level. This formation has an equivalent in time in the Middle-East series: the Kharaib, Hawar and the very beginning of the Shu’aiba formations. The quality and diversity of Urgonian outcrops in southeastern France lead that formation to be a good reference proxy of Middle-East reservoirs and to be used for the assessment of sedimentary internal architecture. A work program focused on the appraisal of geometrical and petrophysical heterogeneity in Urgonian has integrated the acquisition of multi-scales static and dynamic data: outcrop studies (facies, fractures, microfauna ...), well drilling, well tests, aerial photographs, springs and aquifer survey. A huge database has been constituted, it comprises several key sections and more than 60 vertical wells drilled in the Urgonian aquifer, 40 well tests and survey of 13 springs during one year. Three sequences have been described in the Upper Barremian (Ba3, Ba4 and Ba5) and two sequences in the Lower Aptian (A1 and A2). The existing biostratigraphic chart built for several years by Annie Arnaud-Vanneau for the northern region (Vercors) has been improved and completed including the new data from the western zone (Gard and Ardèche). Through a multidisciplinary approach integrating sedimentology, hydrogeology and structural geology, a new conceptual model for the architecture of the Urgonian deposits on the edge of the platform is proposed. In the study area, three N110° normal faults subdivide the domain into four tilted blocks. The differential subsidence related to this faults activity makes important variations in thickness and facies distribution according to the location in the blocks.