ABSTRACT: Early Tertiary Rift Evolution and Its Relationship to Hydrocarbon Source, Reservoirs, and Seals in the Offshore Northwest Java Basins, Indonesia
ATKINSON, CHRISTOPHER D., and STEVEN W. SINCLAIR, ARCO Indonesia Inc., Jakarta, Indonesia
The Offshore Northwest Jave (ONWJ) Basins originated during late Eocene/early Oligocene by rifting of the southern margin of the Sunda Platform. North-south half-grabens fragment a low-grade schist and igneous terrane. The grabens show a pronounced fabric with the majority of the hanging-wall blocks dipping east. Opposing half-grabens are rare because the majority of the bounding faults are likely synthetic and merge with depth. Catchment basins evolve into 10-25 x 30-50 km sags with recurrent fault activity slightly altering the asymmetry.
Within most basins a lower to upper Oligocene syn-rift to rift-fill succession of largely nonmarine deposits is overlain by an upper Oligocene to lower Miocene post-rift succession of paralic to marine sediments. Internal to the syn-rift phase a major block rotation and truncation episode is noted in several of the basins. This truncation possibly represents the initial shift in subsidence via flexural processes. The syn-rift/rift-fill sediments range from lacustrine shales and drifted coals in the basin deeps to lake-edge/flood-plain shales and immature, basement-derived clastics at the rift margins. Isolated volcanism is recorded in several of the basins during this phase. By the early Miocene, active rifting generally had ceased and most basins were largely filled. The ONWJ area th s became an extensive, flat-lying coastal plain. Over this plain, northerly-derived, post-rift deltaic sediments comprising coals, delta-front siltstones, and mature distributary sandstones were deposited. With continued flexural subsidence and relative sea level rise, marine conditions advanced from the south transgressing the succession with shallow-water carbonates and shales.
The half-grabens form excellent hydrocarbon generation and accumulation systems. The syn-rift and rift-fill lacustrine succession contains both algal and sapropel-rich carbonaceous source rocks. Numerous, high-grade, reservoir sandstones occur in the overlying post-rift deltaic succession. Seal potential studies reveal that the post-rift marine deposits above these sandstones possess excellent sealing properties. Accumulations result from predominantly vertical migration from source to reservoir via the rift margin and associated faults.
AAPG Search and Discovery Article #91015©1992 AAPG International Conference, Sydney, N.S.W., Australia, August 2-5, 1992 (2009)