Abstract

Reinterpretation of 18 local and 7 regional seismic lines in northeast Java, numerous exploration wells and their integration with newly measured stratigraphic sections has enabled a new structural model to be developed for the Cepu oil fields. The generally shallow water limey-clastic sequence developed in a rifting back-arc basin with many northeast-southwest oriented basement faults.

Deformation in the early Middle Miocene caused reactivation of the basement faults in the Ngloboemanggi area with wrenching and the initial development of flower structures. This deformation caused areally restricted erosion of the main reservoir rocks in this area. Later Pliocene deformation accelerated the development of the flower structures in the Nglobo-Semanggi area which were reflected at the surface as a series of en echelon, hydrocarbon-bearing anticlines. The Tambakromo-Kawengan area underwent minor north over south thrusting along east-west oriented listric reverse faults with detachment at shallow depths and the development of hydrocarbon-bearing folds exist in the subsurface north of the Tambakromo-Kawengan structure. Such folds would be related to imbricate blind thrusts parallel to the Tambakromo-Kawengan thrust.

The wrench structures and detached compressional structures forming hydrocarbon-bearing folds in the Cepu Oil Fields are probably a result of the transference of stresses due to oblique subduction at the Java Trench during the Neogene to Pleistocene.