ABSTRACT: Detached inversion: a new structural style applied to the Mahakam Fold Belt, Kutai Basin, East Kalimantan, Indonesia

Ferguson, Angus¹, and Ken McClay²  
(1) Independent Consultant, Jakarta Selatan, Indonesia 
(2) Royal Holloway University of London, Egham, Surrey, United Kingdom

The development of a structural model for the Mahakam Fold Belt was completed in three stages. The first was a regional study incorporating 10,000 km of seismic and over 400 wells in the Sanga Sanga PSC. This determined the nature of the faults and the structures. The second stage was to complete balanced cross sections to determine the geometrical viability of the fault pattern. A third stage was to test the previous structural styles by scaled physical models in the lab to determine the mechanical reasonableness of each structural style. The results were compared to the observed Mahakam foldbelt structures. The final model, which is termed an inverted delta growth fault model, involves a structural style of detached inversion.

Basement inversion modelling used overpressure layers represented by a viscuous polymer to model the basement separated from the overlying sand prone deltaic sediments by a layer of overpressured shale. The resulting fault planes do not extend directly upward into the overlying sequences indicating that the fault pattern is detached from the basement.

The detached inversion structural style is mechanically, geometrically and kinematically viable to explain the formation of the Mahakam Fold Belt. The model is applicable universally, therefore in similar areas of thick sediment cover that had extension followed by contraction similar detached inversion structures are expected to occur.

AAPG Search and Discovery Article #90913©2000 AAPG International Conference and Exhibition, Bali, Indonesia