Deterministic Seismic Inversion in a Tight Sandstone Reservoir and Integration in the Geological Model of the Irharen Gas Field (Timimoun Basin, SW Algeria)

Summary Extended abstract with main figures have been uploaded below: The Irharen field forms part of the Timimoun natural gas project, located in the Timimoun basin in southwest Algeria ( Figure 1 ). Dry gas is there produced from several tight reservoirs of Early Devonian age. The Irharen gas field, discovered in 1993, is currently producing from a very low poro-perm sandstone reservoir of Praguian age, which shows laterally better petrophysical properties more favorable for gas production Porosity prediction is the main challenge for the Praguian reservoir in the Irharen field. Good porosity areas are mainly controlled by local diagenesis and their distribution is very difficult to predict. This paper describes the process and findings of the deterministic inversion carried out on the reprocessed pre-stack Irharen 3D seismic and its input for reservoir geological modelling. Well log data showed a correlation between reservoir petrophysical parameters and rock elastic properties for the Praguian reservoir. Cross plotting between P-impedance and Vp/Vs ratio at log scale showed a correlation, which allowed discriminating gas saturated, and high porosity sandstones. Based on seismic partial stacks, wavelets and low frequency model, a model based pre-stack seismic inversion was carried-out. First result of the inversion was used to define a gradient volume obtained from the correlation between P impedance and VP/VS Ratio and is therefore related with porosity. Areas showing high gradient values correspond to low P-impedance and low VP/VS ratio zones, which may be associated to high porosity sand areas. Prior to modeling, a statistical analysis of the gradient volume versus well data was carried-out. Frequency diagrams by rock type enable to discretize the gradient cube in probability classes and optimized a minimum threshold for high productivity intervals identification. An improvement of the well to seismic relationship was achieved by crossing the gradient cube with a 3D Vclay model. These cubes were used to locally guide the porosity model giving satisfactory results in the history match and open up additional development well location in the Irharen structure.