Geological Modelling of a Structurally Complex Reservoir

Abstract

This paper presents the technical problems faced and the solutions found in the construction of a suitable static model of Bu Attifel field. The re-interpretation of the seismic volume, the construction of a detailed sedimentological model and the accurate correlations within the reservoir allowed the identification of several tectonic phases and, particularly, the intra-Nubian tectonics affecting only the lower levels of the reservoir. The conclusions of the geological study led to the identification of a very complex structural model due to the number, slope and, often, synsedimentary characteristics of the faults crossing the reservoir. During simulation, it was not possible to apply the usual simplifications to the structural model since a dynamic model able to optimise future infill well locations and enhanced oil recovery processes was required. It was then decided to build a simulation grid reconstructing both fault trend on the horizontal plane and their slope as accurately as possible. To this purpose, a specific methodology was followed. It consisted of two main phases: the generation of a conventional model with vertical faults and the conversion into a model containing sloping faults. The final result was the construction of a geological model and a simulation grid representing the structural complexity of the reservoir. Introduction Bu Attifel oil field, located in the Libyan desert about 400 km SE of Benghazi, has been producing since 1972. The field, which is one of the best Libyan oil field, has produced 30% of the original oil in place. In 1992, it was decided to build a new reservoir model able to guide the final development phase. The problem was the optimisation of location and number of the new wells (water injectors, oil producers and, if necessary, gas injectors). As a consequence, a very detailed geological, and particularly, structural model proved to be necessary to support the simulation study. The previous reservoir model had been made in 1987. Considerable difficulties were encountered in predicting the well performance along the southern flank of the field due to the high geological uncertainty in that area. Thus, a detailed reinterpretation of the 3D seismic volume, integrated with the analysis of all the geological data available, was performed. The resulting picture of the field showed that the reservoir is structurally complex for the presence of many sloping faults related to different tectonic events. Particularly, these faults control the areal distribution of the volcanic body that caused the geological failure of the wells located near the southern flank of the field. P. 129