Origin of Pliocene Deep-Water Sedimentation in Salawati Basin, Eastern Indonesia: Deposition in Inverted Basin and Exploration Implication
AH Satyana*, I Setiawan*
* Exploration Pertamina MPS
INDONESIAN SEDIMENTOLOGISTS FORUM 2ND REGIONAL SEMINAR, Deep-Water Sedimentation of South East Asia, Jakarta 14 – 16 May 2001
Abstract
Salawati Basin is a foreland basin located at the frontal edge of the Indian-Australian continental plate. Sorong Fault, a major strike-slip fault in Eastern Indonesia and terminating the basin to the north, has inverted the basin’s polarity in the Late Pliocene by subsiding the whole northwestern part of the basin. Before this inversion, the Salawati Basin had a southern depocenter.
The newly formed northwestern depocenter has subsided rapidly since the inversion as an isostatic compensation to the southern and eastern uplifts. This condition resulted in the accommodation space for northwestern deep-water sedimentation. Sediments were eroded from the uplifted areas and deposited rapidly into the subsiding basin as debris flow deposits of Pliocene Klasaman sediments within bathyal depositional environment. The depocenter was increasingly subsided by tectonic loading of the contemporaneous Upper Klasaman multiple thrust sheets.
Thick deep Klasaman deposits became burial sediments for the Miocene source rocks once deposited in the lagoonal environment to attain a depth of oil window. Rapid Klasaman deposition triggered overpressuring and shale diapirism. The deposition was too fast for the sediments to compact and dewater in normal way. Overburden pressure and lack of permeable conduits caused the overpressuring. The Klasaman overpressuring presents a drilling hazard as undergone by all wells drilled in the area. Low densities of overpressured Klasaman shales caused the shales flowed upward as diapirs. Sorong Tectonism controlled these diapirs as shown by their parallel trends with the Sorong Fault. The Klasaman diapirism may relate with hydrocarbon traps of faulted domal structures, dragged beds below the diapirs’ overhang zones, faulted beds in the peripheral sinks, and turtle structures.
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