Methods for source rock identification on seismic data: An example from the Tanezzuft Formation (Tunisia)

Source rocks occurrence, extension and quality in the subsurface are usually characterized directly from well data. Seismic characterization is however becoming an emerging technique for extending source rock properties beyond well locations and an effective tool for mapping their lateral and vertical heterogeneities. The Tanezzuft Formation in Tunisia, an extremely organic-rich source rock, was used as a test for applying and comparing different seismic methods aimed at organic-rich sequences characterization. The data used consist of a 2D seismic line shot in the northern flank of the Ghadames Basin (Tunisia), pre-stack gathers extracted from an inline of a 3D survey and four wells with a complete set of wireline and petrophysical logs. Impedance inversion, AVO and seismic attributes were computed on the seismic lines and compared to well data. Impedance inversion together with a rock physics model of the formation generated a quantitative estimate of total organic carbon (TOC) content in the studied 2D seismic line. AVO analysis showed a Class IV and a Base Class IV response for the upper and lower reflectors delimiting the organic-rich interval respectively. Seismic attributes analyzed the formation by exploiting its high impedance contrast and intrinsic anelastic attenuation. The three methods here discussed showed coherent results both when compared each other and when compared with the well data and the regional geological model. In particular, AVO and seismic attribute analysis are methods that can be applied without the need of well information, and are then feasible methods for the investigation of source rock properties in undrilled basins.