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New Perspective on Evolution of Northern Provinces of Offshore Sarawak Basin, Malaysia

Iyer, Subramania; Keong, Ong Swee; Nazihah, Fatma; Abdullah, Shahrul Amar B.
PETRONAS Exploration, PETRONAS Carigali Sdn Bhd, Kuala Lumpur, Malaysia.

The offshore Sarawak Basin is located towards the southwestern part of South China Sea marginal basin and hosts many oil and gas fields. The present paper summarizes the tectono-stratigraphic framework and a conceptual evolution model developed for the northern part of the Sarawak Basin and is based on a comprehensive study of seismic, gravity, and well data.

Basins in the study area initiated as intracratonic rifts on attenuated continental crust, on a foreland bulge part of offshore Sarawak, ensuing from Phase-I NW-SE to N-S extension, during Late Cretaceous (?) to Late Eocene. Gravity modeling results suggest attenuated continental crust, with a thickness range of 15 to 20 km in the study area. The general structural style during this stage is characterized by N-S and NE- SW trending half grabens, dipping to the east and southeast. Sequence-A/Pre Cycle-I filled the early formed half grabens. The phase of extension continued (Phase-II), with opening of South China Sea during Early Oligocene, and also during subsequent drift phase up to Early Mid Miocene. Unequal subsidence in the half grabens during this stage, accommodated variable thickness of infill sequences B and C, represented by Cycles I to III, showing diverse facies distribution as evidenced by the wells. Well results suggest non marine to transitional environment for the older section of Cycle I to Lower Cycle II, and outer neritic to bathyal setting for Cycle III, indicating progressive deepening up basin. Subsequent regional uplift in the area associated with plate convergence during Late Early Miocene to Middle Miocene, resulted in regional unconformity, designated Middle Miocene Unconformity (MMU). Cycle I sub cropping MMU towards western part of the study area substantiated by seismic sequence mapping suggests possible Early Miocene age for MMU, associated with uplift and erosion, also aided by major tilt to the east from East Natuna hinge zone. Age of MMU progressively becomes younger towards east. A major NW-SE transtension close to MMU time along SW Luconia fault zone transecting the study area resulted in the formation of West Luconia Trough, accommodating huge pile of post unconformity section. Northeasterly sag of the basin through Late Miocene to Recent lead to a deep water setting, ensuing in deposition of hemipelagics, overlain by mass transport dominated sequence. The present study paved the way for better understanding of the petroleum system elements.


AAPG Search and Discovery Article #90155©2012 AAPG International Conference & Exhibition, Singapore, 16-19 September 2012