Aquifer structure identification through geostatistical integration of geological parameters: case of the Triassic sandstone aquifer system (SE Tunisia)

Abstract

This paper presents an integrated modeling approach for constructing a geometric model of an Early to Middle Triassic sandstone aquifer system, commonly referred to as the Sahel El Ababsa aquifer, in southeastern Tunisia. The approach combines thorough interpretations derived from geological data along with various geostatistical methods to create accurate representations of the faulted bounding surfaces. The resulting architectural model was used to define the main aquifer system compartments and explain the nature of the hydraulic relations and communications within and beyond the aquifer system. A database was constructed based on multiple and various data sources, including water boreholes, exploration-petroleum wells, and seismic sections. These features were georeferenced and linked to the database used to build a stratigraphic framework of the area. The geological correlations based on wells information allowed a valuable observation on the structural trends and fault network as well as the lateral facies variation of the aquifer system. The derived interpreted faults were displayed on a georeferenced map which was later converted into a vertex database needed for the next modeling steps. A proper architectural modeling approach of the Early to Middle Triassic sandstone aquifer system was then performed using geostatistical methods that provide a variety of calculation techniques to model the reservoir bounding surfaces, through kriging methods including ordinary kriging and kriging with inequalities. The latter method was used to deal with the lack of data as it can retrieve information from the end of the boreholes that do not reach the surface to be calculated and integrate them in the modeling procedures. This enabled us to build the first architectural model with an accurate description of the bounding surfaces of the Early to Middle Triassic rocks. In fact, although it has been the center of geological and hydrogeological studies, the Upper Permian surface has not previously been the focus of an integrated geomodeling study. The generated models were analyzed, verified through kriging variance maps, then compared with the outcomes of the geological study. The results depict a new structural scheme of the Sahel El Ababsa area and provide an in-depth characterization of the complex compartmentalization affecting the aquifer system that allowed the definition and grouping of the hydrostratigraphic units. Therefore, this work is the first attempt through which an integrated architectural modeling approach is used to characterize and identify the Sahel El Ababsa aquifer system.