ABSTRACT: The Mahakam Delta, East Kalimantan, Indonesia as a model for the Oficina Formation, East Venezuela Basin

Chambers, John L.C., and David Hadley , LASMO Venezuela BV, Caracas, Venezuela

Although differences in tectonic setting, geography and style of petroleum system far outweigh similarities between the Tertiary Kutei Basin and the Tertiary Eastern Venezuelan Basin, important sedimentological analogies do occur. Observations made on the Mahakam delta sediment distribution patterns in both the present delta and the Miocene outcrops and subsurface appear to be directly applicable to the style of sedimentation in the Eastern Venezuelan Basin. Both systems represent Lower to Middle Miocene equatorial depocenters where accommodation has matched extremely rapid sedimentation. Thick aggraded deltaic systems have developed with autocyclic lobe switching overriding the eustatic signature. Additionally, both systems appear to have deposited into marine environments with inferred small tides, low wave energy but large fluvial outflow.

Although mouth-bar sands are generally too thin to be imaged on seismic, their extensive lateral continuity and the high drilling density in the Dacion Block allows reasonable maps to be made. The geometries of mouthbar sand bodies in the subsurface appear to be lobe like sheets - supporting a point sourced fluvial origin rather than the more parallelic sand bars that would be expected from a wave dominant environment. Geometry of lateral sand bars within distributary channels can be imaged within modern 3D seismic data in the Oficina Formation illustrating patterns remarkably similar to that seen in the Modern Mahakam Delta.

Observations on the modern Mahakam Delta suggest that during delta progradation, distributary channels bifurcate around initial mouth bars. As the delta progrades, variable quality reservoirs in lobe like sheet sands are developed along with high permeability linear channel fill lateral bars. Individual episodes of progradation are capped with delta plane material and commonly coal. By studying wireline log patterns and recently acquired core material from the Dacion Block, an integrated sedimentological model for the Oficina Formation in the Dacion Block has been developed.

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