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AAPG Bulletin, V.
Linked lowstand delta to basin-floor fan deposition, offshore Indonesia: An analog for deep-water reservoir systems
1Unocal Corporation, Sugar Land, Texas; [email protected]
2Unocal Corporation, Sugar Land, Texas
3Unocal Indonesia, Balikpapan, Indonesia
4Unocal Indonesia, Jakarta, Indonesia
Art Saller currently works as a sedimentologist and stratigrapher for Unocal in Sugar Land, Texas. He received geology degrees from the University of Kansas (B.S., 1974–1978), Stanford University (M.S., 1980), and Louisiana State University (Ph.D., 1984). From 1984 to 1986, he worked for Cities Service Oil and Gas in Tulsa, Oklahoma, and joined Unocal in 1986.
Jesse Noah has been with Unocal for 22 years and is currently Unocal's chief geoscientist for the Deep-Water Gulf of Mexico. Initially, he worked on the United States mid-continent and Gulf of Mexico shelf. He has worked on deep-water exploration and development projects in the Gulf of Mexico and Indonesia since 1996. Jesse received his B.S. degree in exploration geophysics from the University of Oklahoma.
Alif Ruzuar graduated from the Bandung Institute of Technology, Indonesia, in 1998 with a B.S. degree in geology. He earned an M.Sc. degree in petroleum geoscience from the University of Brunei Darussalam (1999). Alif joined Unocal Indonesia in 2000 as a deep-water exploration geophysicist and is currently working as a development geophysicist for fields on the East Kalimantan shelf.
Rhys Schneider received his B.S. degree in geological engineering from the Colorado School of Mines in 1978. He joined Unocal in 1980 and has worked as an exploration and development geologist in the Anadarko basin, Permian basin, and many other basins throughout the world. He currently specializes in detailed, seismic-defined, reservoir characterization for deep-water systems of the Kutei Basin, Indonesia.
Many people helped us in this study and the assembly of this manuscript, including John Decker, Phil Teas, Tim Nicholson, Phil Johnston, Joel Alnes, Gary Christenson, Yusri, Jim Waugaman, Jim Friberg, Dave McGee, John Dunham, Tom Elliott, Mike DiMarco, Richard Armin, Garry Jones, and Art Trevena. We thank AAPG Bulletin reviewers Peter Webb and Gregory Nadon for their helpful comments. We thank WesternGeco (PT WesternGeco Indonesia) for permission to use data from their Makassar Straits multiclient 3-D survey in this publication. High-resolution biostratigraphic analysis of core samples (Table 1) was supplied by P. R. Thompson and W. H. Abbott. We thank Unocal Indonesia, ENI/Lasmo, and BP-Migas for permission to publish this study.
In offshore East Kalimantan, Indonesia, three-dimensional seismic reflectors can be traced downslope from a lowstand delta to a basin-floor fan, giving insight into depositional processes controlling the distribution of sands that serve as hydrocarbon reservoirs in many ancient deep-water settings. The studied interval includes the last three Pleistocene cycles (10–330 ka; each 110 k.y. in duration). Cycles on the shelf are dominated by progradational packages deposited during highstands and falling eustatic sea level. Progradational packages are separated by parallel reflectors and carbonate buildups of the transgressive systems tracts. During the last two lowstands of sea level (18 and 130 ka), coarse clastics were not deposited in deep-water environments because growth faults and regional subsidence prevented lowstand deltas from reaching the slope. During the lowstand of sea level that ended at about 240 ka, a delta prograded over the previous shelf edge, and sand-rich sediments spilled onto the slope.
Strata on the slope and basin floor show how a deep-water depositional system evolved during a single cycle of eustatic sea level. A canyon on the slope connects the 240-ka lowstand delta to a coeval basin-floor fan. The canyon has a sinuous, bipartite fill that consists of a lower, amalgamated channel complex and an upper channel-levee complex. The basin-floor fan formed at the toe of the slope also has two parts. The stratigraphically lower part of the basin-floor fan has broad lobes with relatively continuous reflectors. The stratigraphically higher part has a sinuous channel-levee complex that prograded over the lower fan and fed sheetlike lobes on the outermost fan. The amalgamated channel fills on the slope and sheetlike lobes on the basin-floor fan have moderate- to high-amplitude reflectors and are inferred to represent sand-rich, early lowstand deposits. The channel-levee complexes on the slope and basin floor are dominated by low-amplitude reflectors and are inferred to be mud-rich strata deposited during the late lowstand. Unlike classic sequence-stratigraphic models, these lowstand strata do not onlap the slope; instead, deep-water clastics extend from the last clinoforms of lowstand deltas. In this system, lowstand deltas determined when and where sand-rich sediments entered preexisting canyons on the slope to feed basin-floor fans.
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