3D Seismic study of a shale expulsion syncline at the base of the Championdelta, offshore Brunei and its implications for the early structural evolution of large delta systems

An example of a Late Miocene expulsion rollover syncline related to mobile shale is described from 3D seismic data offshore Brunei. Prodelta shales ac cumulated in the subsiding syncline were altered after deep burial by overp ressure increase and hydraulic fracturing. Consequently seismic facies was modified from a continuous divergent reflection pattern to a low-amplitude, disrupted to chaotic seismic facies bounded by a high-amplitude event that cross-cuts original stratification. Without the resolution provided by 3D data, the high amplitude event would be mistaken for early growth faults an d the subdued country-rock reflections for mobile shale, typical features a t the base of large deltas. This study shows the existence of expulsion rol lovers related to mobile shale, which have not been described before, and d emonstrates that not all bright reflections at chaotic-coherent facies boun daries are actually fault planes. It also suggests that expulsion synclines , even during early phases of delta development may be compaction-related f eatures rather than ones that drive the shale mass out of the area of progr adation. (C) 2000 Elsevier Science Ltd. All rights reserved.