High resolution reservoir characterisation in the Troll West Gas Province

Abstract

Just like many other shallow marine fields, the Troll field is steadily increasing its recovery factor by performing an extensive infill drilling procedure. Well planning requires an ever increasing knowledge of the geological and sedimentary characteristics of the field, as higher drilling precision demands a more detailed reservoir model. The most important reservoir units of the Troll field consist of a stacking of shallow marine sand- and siltstones that were deposited in an asymmetric coastal spit system. The spit system consists of progradational surfaces including clinoform structures that contain heterogeneous and complex sediment accumulations which are difficult to map in the subsurface. A thorough literature investigation into the characteristics of asymmetric delta spit systems, clinoform deposition and the Troll geological history forms the basis of a new method to model the heterogeneous sediments in the field. Seismic interpretation and well log correlation between the many multilateral wells are used as input for the model. The model itself is built in the Compound Earth Simulator, a software package developed by Statoil’s research department. Using the Compound Earth Simulator, the sediments (logs and seismics) are brought back to their time of deposition with a backward time engine. The undeformed nature of the sediments at time of deposition makes it possible to find better correlation between the different wells, resulting in the construction of a high resolution sediment distribution. A forward time engine then brings the sediments forwards through time to their present day situation, in which all complexity of deformation is present again, but the high resolution sediment characteristics are still visible as well. The construction of synthetic seismics and virtual well logs show a quality check of the modelling procedure. The Compound Earth Simulator and the presented method offer a fast and easy routine to improve the modelling detail of a field by introducing a high resolution reservoir characterisation in problem areas. The results of the procedure show that the clinoform progradation direction in the Troll field is more heterogeneous than described in earlier work. For multiple parasequence in the specific area of interest within the field, the main progradation happens towards northwest or north, which is contradicting the earlier described west to southwest build-out direction that corresponds to the longshore current direction. Multiple possible explanations for this observation are presented. The concluded heterogeneities may be taken into account during future geomodel updates. The high resolution reservoir characterisation can moreover be used for well planning and drilling purposes. As the most important result, the method that is developed in this thesis may be used to create a high resolution reservoir characterisation in other parts of the Troll field or other fields as well.