Michael H. Gardner¹, J. Sukanto², Chris Zahm¹, Donna S. Anderson¹, James M. Borer¹

(1) Colorado School of Mines, Golden, CO
(2) Repsol-YPF-Maxus, Jakarta, Indonesia

ABSTRACT: The Beheading of a Rift: Stratigraphic and structural evidence for oblique right lateral offset of the Asri and Sunda Basins, Sunda Shelf, Indonesia

The Asri and Sunda Basins are part of the Repsol-YPF-Maxus SES Block. These prolific hydrocarbon basins contain over 40 oil fields that have produced over 1 BBBl of oil from Oligocene to Miocene rift-fill. The rift fill of both basins records an upward change from: 1) coarse early basin-fill fluvial clastics; 2) deep lacustarine shales forming the principal source rock; and 3) a thick succession of nonmarine fluvial clastics and carbonates that record a northward-directed marine transgression. These basins have been interpreted as en-echelon asymmetric half-grabens. The offset of the two half-grabens occurs across a transform zone separating the two basins. In this view the present basin configuration controls rift-fill depositional patterns. Transverse fluvial systems would fill the rifts through a series of unrelated fluvial systems of different sizes that transport texturally immature sediment relatively short distances from rift flank highlands.

An alternative model suggests that the Asri and Sunda Basins represent one larger half-graben segmented by recent (5-7 Ma) oblique right-lateral, strike-slip offset. In this view the paleo-Asunda basin contained two axial fluvial systems fed by subordinate transverse systems. Texturally mature sandstone deposited from axial rivers form reservoirs in principal production fairways. Stratigraphic and structural evidence for post-depositional offset includes: 1) geometric fit of basins, 2) moment-tensor studies support oblique right-lateral offset, 3) northwest-southeast secondary shear faults, 4) two asymmetric half-grabens separated by a transfer zone should change polarity, however, this is not observed; 5) thick fluvial sandstones occur on structural highs in the southern Asri Basin, 6) similarity in size and trend of fluvial systems through both basins; 7) both basins contain basin-centered interfluves; 8) both basins show similar stratigraphy, and 9) both basins contain back-stepping fluvial systems recording a marine transgression.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado