Geomechanical study of the Tarfaya basin, West African coast, using 3D/2D static models and 2D evolutionary models

Abstract

[eng] This thesis uses different variants of geomechanical modelling approaches to investigate stress, strain and geometry distribution and evolution through time of the Tarfaya salt basin, located on the West African coast. This work has been conducted by geomechanically simulating a sector of the Tarfaya basin containing key features such as diapirs, faults and encasing sediments using 3D and 2D static models and 2D evolutionary models. The 3D and 2D static geomechanical models of the Tarfaya basin system allowed to predict the stresses and strains at present day. Both models are based on present-day basin geometries extracted from seismic data and use a poro-elastic description for the sediments based on calibrated log data and a visco-plastic description for the salt based on values from Avery Island. The models predict a significant horizontal stress reduction in the sediments located at the top of the principal salt structure, the Sandia diapir, consistent with wellbore data. However, the 2D static geomechanical model shows broader areas affected by the stress reduction compared to the 3D model and overestimates its magnitude by less than 1.5 MPa. These results highlight the possibility of using 2D static modelling as a valid approximation to the more complex and time-consuming 3D static models. A more in-depth study of the 2D static model using sensitivity analysis yielded a series of interesting observations: (1) the salt bodies and their geometry have the strongest impact on the final model results; (2) the elastic properties of the sediments do not impact the model results. In other words, the correct definition of the sediments with the highest material contrasts such as salt should be a priority when building static models. Such definition should be ranked ahead of the precise determination of the rheologic parameters for the sediments present in the basin. In this thesis, we also present the results of introducing an evolutionary geomechanical modelling approach to the Tarfaya basin. This study incorporates information of burial history, sea floor geometry and tectonic loads from a sequential kinematic restoration model to geologically constrain the 2D evolutionary geomechanical model. The sediments in the model follow a poro-elastoplastic description and the salt follows a visco-plastic description. The 2D evolutionary model predicts a similar Sandia diapir evolution when compared to the kinematic restoration. This proves this approach can offer a significant advance in the study of the basin, by not only providing the stress and strain distribution and salt geometry at present day, but also reproducing their evolution during the Tarfaya basin history. Sensitivity analysis on the evolutionary model indicates that temporal and spatial variation in sedimentation rate is a key control on the kinematic structural evolution of the salt system. The variation of sedimentation rates in the model controls whether the modelled salt body gets buried by Tertiary sediments (after a continuous growth during the Jurassic and

Cretaceous periods) or is able to remain active until the present day. Also, the imposed shortening affects the final stress distribution of the sediments at the present day. To conclude, the results obtained during this study allowed us to understand the formation and evolution of the diapirs in the Tarfaya basin using carefully built geomechanical models. The study demonstrates that carefully built 2D static models can provide information comparable to the 3D models, but without the time and computational power requirements of the 3D models. That makes the 2D approach very appropriate for the exploration stages of a particular prospect. If carefully built, such 2D models can approximate and yield useful information, even from complex 3D structures such as the Tarfaya basin salt structures. This thesis also concludes that incorporating kinematic restoration data into 2D evolutionary models provides insights into the key parameters controlling the evolution of the studied system. Furthermore, it enables more realistic geomechanical models, which, in turn, provide more insights into sediment stress and porosity.