Abstract

The reconstruction of deep geological structures is done by the integration of geophysical investigations and their geological interpretation. This latter is based on the geological principles together with geological cross section balancing techniques, that involve rock volume preservation and their possible layering. Processing of seismic signal allows its geological interpretation by the identification of reference surfaces and of their real geometry by depth-conversion algorithm. The final geological setting of studied structures (targets) results from their evolution through time. A deep geological section must therefore be compatible with its evolution though time, that is, to be admissible, should preserve, at each time-step, both balancing principles and continuity with the previous step settings. This is accomplished by modelling the structure during its whole evolution. L’assetto geologico finale delle strutture studiate è il risultato della loro evoluzione nel tempo. At the GeoQuTe Lab (Geodinamica Quantitativa e Telerilevamento) we developed a numerical method that allows to simulate the kinematic evolution of geological structures by the HCA Method (Hybrid Cellular Automata). This method combines he spatial characteristics of FEM/DEM (Finite Element Method/Discrete Element Method) with the computing speed of CA methods (Cellular Automata). The modeling is obtained through the simulation of the evolution of complex geological structures through successive steps in near-real time (not geologic!). Le time pace is selected in order to evolve though a series of successive equilibrium states (i.e. without stress accumulation). This allows to reduce to linearity the relations among neighbor cells and the influence of far cells is derived by the spatial diffusion though time of their properties and positions. At each step the new set of nearby cells is chosen among the previous and those close to them. This provides a resulting code with computing times linearly proportional to the number of cells in the mesh. In this way it is possible to replicate complex settings by using a huge number of cells (over 3 million in some simulations). The redundancy in the model that results from the abovementioned approximations is eliminated by the introduction in the mesh of a random noise at each step. The HCA modelling of geological structures simulate their kinematic evolution and allows to compute the stress induced though time, that is responsible for the minor deformation (e.g. fracturing in brittle behavior). This deformational history, rebuild during the full evolution of the structure, is cumulated to form the TSI (Time-Stress Integral) parameter that results proportional to the fracture intensity in brittle material and quantified by the mean H/S value (ratio between dimension and spacing of fractures). The linear relation with the TSI has been confirmed by the confrontation of field analogs with their HCA model. Specific modification in the HCA mesh during the step evolution allows to simulate the different rock physical properties as well as, among others, sinsedimentary, compaction, and erosion processes. This approach allows to predict the fracture distribution in deep targets and the related secondary permeability. The methodology allows the reconstruction of the 3D evolution(+ time, that is 4D modelling) of geological structures though the preparation of a series of 3D (2D + time) sections linked by transversal sections and include the prediction of fracture system attitudes developed during the geological history of the structures. These are then compared with indications of the present-day stress conditions to evaluate the impact to each fracture system on fluid circulation in the target. Typical application is based on data extracted from the geological models prepared within software packages used in the industry. Results are then directly implemented in the geological model as attribute (e.g. transmissibility in the three directions). The HCA methodology has been successfully applied to predict secondary permeability in the various geological contexts, independently form the rock age formation, as thrust-and-fold, extensional , inversion, salt, and strike-slip tectonics. Furthermore, it has been successfully used in the study of the tectonic evolution of collisional chains as well as to simulate the behavior of the ice sheet above the tectonic depressions that characterize the bedrock of East Antarctica. This methodology has applications either in the study of fluid reservoirs (water, oil, gas, CO2) and for the risk evaluation of waste sites. Applications as briefly described and include modelling of the evolution of geological structures in in Iran, South Atlantic, North Sea, Barents Sea, Algeria, Sardinia, Antarctica.

Francesco Salvini is full Professor at Roma Tre University, Science Dept., and Head of the quantitative geodynamics and remote sensing lab (GeoQuTe, Geodinamica Quantitativa e Telerilevamento). He has been the scientific coordinator of international research projects and projects funded by oil, gas, and geothermal industries. His topics include the geodynamics and the structural geology at the various scales based on a quantitative approach that integrates field analysis and numerical/analytical modelling. The developed methodologies include the modeling of the brittle deformations associated both to the evolution of fault zones and to the kinematics of complex geological structures in the various tectonic environments. These methodologies allow to predict the hydraulic behavior in fault-related structures.

Paola Cianfarra has a fellowship (formerly Researcher) at Roma Tre University, Science Dept., and works at the Quantitative Geodynamics and Remote Sensing Lab (GeoQuTe, Geodinamica Quantitativa e Telerilevamento). She participated in research projects and in projects funded by oil companies focused at understanding the role of fracturing in fault- and fold-related structures. Her topics are the structural geology and the geodynamics at the various scales and in different tectonic environments. The used methodologies integrate the numerical modeling of the kinematic evolution of complex geological structures with the remote sensing and the structural field analysis. The numerical modeling allows to predict the spatial fracture distribution by simulating of the evolution of geological structures characterized by thrusting, normal faulting, fault inversion, salt tectonics, syn-sedimentary tectonics, compaction, erosion.