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AAPG Bulletin, V.
Ancient shelf ridges-A potentially significant component of the transgressive systems tract: Case study from offshore northwest Java
Henry W. Posamentier1
1Anadarko Canada Corporation, 425-1st Street SW, Calgary, Alberta T2P 4V4, Canada; email: [email protected]
Henry W. Posamentier is the manager of geology for Anadarko Canada Corporation. Prior to joining Anadarko in 2001, he was with Veritas Exploration Services (2000-2001), the Atlantic Richfield Co. (1991-2000), Exxon Production Research Co. and Esso Resources Canada, Ltd. (1979-1991), and at Rider University as assistant professor of geology (1974-1979). Posamentier's research interests have been in the fields of sequence stratigraphy and depositional systems analysis, where he has published widely. He has employed an interdisciplinary approach using three-dimensional seismic data integrated with borehole data to interpret depositional systems and develop basin fill histories. Recently he has focused his efforts on deep-water depositional settings. In 1971-1972, Posamentier was a Fulbright Fellow to Austria. He has served as an AAPG Distinguished Lecturer to the United States (1991-1992), an AAPG Distinguished Lecturer to the former Soviet Union (1996-1997), and an AAPG Distinguished Lecturer to the Middle East (1998-1999).
I gratefully acknowledge the management of Atlantic Richfield Indonesia, Inc., for permission to publish this article. TotalFinaElf Indonesia is gratefully acknowledged for permission to show the seismic example in Figure 26. Thanks are due to Jim Howes, Caroline Whorlow, Terry Daw, Sugeng, Sena R., and Wayne Suyenaga for stimulating and constructive discussions. This article benefited greatly from insightful reviews by Jim Howes, Randi Martinsen, Simon Lang, and Ron Noble. Thanks are also due to Lilik Prasetyo, Saptoto Nugroho, and Serge Sauvagnac and his S
Detailed stratigraphic evaluation of three-dimensional (3-D) seis mic volumes calibrated with well-log and core data from the Mio cene section of the offshore northwest Java shelf reveals the exten sive presence of preserved shelf ridge deposits. These features are long linear bodies ranging from 0.3 to 2.0 km wide, more than 20 km long, and up to 17 m high. On close inspection, these features appear to be asymmetric, characteristically sharp-edged and thicker on one side and gradually thinning with an irregular edge on the other side. Possible sand waves, smaller in scale, are observed super imposed on these ridges and oriented oblique to the long axes of the ridges. The observed shelf ridge deposits tend to be sand prone and overlie ravinement surfaces. The ridges appear to be oriented parallel with the axes of broad paleoembayments associated with the structural fabric of the basin. In addition to shelf ridges, shelf ribbons, possibly less than 5 m thick and less than 100 m wide, are also imaged.
Sand ridges are common on modern shelves but significantly less commonly recognized in the subsurface or in outcrop. The features observed here represent examples of preserved ancient shelf ridges. These ridges are thought to have formed as a result of erosion and subsequent reworking of sand-prone deltaic and/or coastal-plain deposits by shelf tidal currents, which became active immediately after shoreline transgression. These deposits appear to have migrated across the ancient sea floor and represent an important component of the transgressive systems tract.
These transgressive systems tract deposits have significant exploration potential because they are commonly sand prone and tend to be encased in shelf mudstone seal facies. Depending on the degree to which sand is present in interridge locations, these linear sand bodies can comprise potential stratigraphic traps. (Begin page 76)
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