GEOMECHANICS OF THE SOUTH FURIOUS FIELD - AN INTEGRATED APPROACH TOWARDS SOLVING COMPLEX STRUCTURAL GEOLOGICAL PROBLEMS, INCLUDING ANALOG AND FINITE-ELEMENT MODELING

The South Furious oil field, offshore Sabah (Malaisia), is an asymmetric anticline bounded on the steep southern flank by a series of steep E-W-striking reverse faults and affected by intense E-W crestal normal faulting immediately next to and contemporaneous with the reverse faults. In spite of the very good sand development and a high sand percentage, the reservoir performance is very poor. A study was undertaken to describe and understand the development and the mechanics of this type of structure and to explain their presence in almost undisturbed sediments around them. The regional tectonic development was studied to provide a basis for a geological model of the structure. A structural geological model was necessary to be able to predict the type of faults and fractures and their location in the structure. The proposed structural development was modelled with analogue sandbox experiments and numerically simulated with a finite-element program (GEOFEP). The results indicate that basement faulting related to the closing of the South China Sea, induced differential sediment loading, triggering asymmetric clay diapirism in the core of the anticline. The asymmetry resulted in contemporaneous formation of steep reverse faults at the leading edge of the migrating anticline and normal faults in a crestal collapse system. The reverse faults and associated shear fractures (as seen in cores) have a sealing cataclastic fabric. This explains the severe compartmentalisation of the reservoir with consequently poor production behaviour, in particular in the reverse faulted part of the field.