Gwang H. Lee

Geologic evolution of the Cuu Long and Nam Con Son basins, offshore southern Vietnam, South China Sea

Rifting and regional subsidence characterize the Cuu Long and Nam Con Son basins, offshore southern Vietnam. Initial rifting began in the Eocene-early Oligocene, followed by the uplift and rotation of the crustal blocks in the late Oligocene. The erosion of the uplifted blocks marked the transition from rifting to regional subsidence in the Cuu Long Basin. A second phase of rifting began in the Nam Con Son Basin, lasting until the late Miocene. Parts of the Nam Con Son Basin experienced inversion in the middle to late Miocene. The synrift and postrift units in the Cuu Long Basin consist of nonmarine deposits and paralic to shallow-marine sediments, respectively. The synrift deposits in the Nam Con Son Basin can be divided into the early synrift unit, corresponding to the initial rifting phase, and the late synrift unit, deposited during the second rifting phase. The early synrift and late synrift units consist of nonmarine sediments and nonmarine to shallow-marine sediments, respectively; the postrift unit is composed of shelf and deeper marine deposits. The data compiled from the published reports suggest that the Cuu Long Basin is oil-prone, with the oil reservoired mainly in fractured basement highs, whereas the Nam Con Son Basin is generally gas-prone, with the gas trapped in Miocene sands and late Miocene carbonates. These distinct trends may be attributed to differences in timing of trap formation and the disruption of trap integrity caused by prolonged rifting and inversion in the Nam Con Son Basin. Gwang H. Lee is associate professor at Kunsan National University, Korea. His research interests include application of seismic reflection to basin research and sequence stratigraphy. He received his B.S. and M.S. degrees in oceanography from Seoul National University, Korea, in 1981 and 1983, respectively, and a Ph.D. in geological/geophysical oceanography from Texas A&M University in 1990. From 1991 to 1994 he worked for Shell Offshore Inc. in New Orleans.Keumsuk Lee received a B.S. degree in mathematics (1994) and an M.S. degree in geological oceanography (1999) from Kunsan National University, Kunsan, Korea. He is currently attending Florida Atlantic University, Boca Raton, Florida. His main research interest is application of geophysical methods to various geological and environmental problems. Joel S. Watkins is the Earl F. Cook Professor, Department of Geology and Geophysics, Texas AM co-chief scientist on DSDP Leg 66 (Middle America Trench), and project director, manager, and vice president with Gulf Oil, 1977-1985. He is the author or co-author of more than 130 scientific publications and senior editor of AAPG Memoirs 29, 34, and 53. His interests are structure and stratigraphy of small ocean basins and seismic reservoir characterization.

Seismic sequence stratigraphy and hydrocarbon potential of the Phu Khanh Basin, offshore central Vietnam, South China Sea

The Phu Khanh Basin, offshore central Vietnam, is the only undrilled basin on the Vietnam margin of the South China Sea. Analysis of multichannel seismic data indicates that the Phu Khanh Basin follows a typical rift-margin development: faulted basement, synrift sedimentation, a breakup unconformity, and postrift sedimentation. Initial rifting probably began during the Late Cretaceous(?) or Paleogene. Rifting and uplift appear to have resumed or continued locally during the late Oligocene and early Miocene. The later part of basin development was dominated by subsidence. Postrift sedimentation evolved from a transgressive ramp phase to a regressive shelf-slope phase. During the transgressive phase, rising sea level provided favorable conditions for carbonate buildup development. The regressive interval contains a number of sequences composed of seismically resolvable lowstand, highstand, and rarely transgressive, systems tracts. These sequences are interpreted as third-order sequences superposed on second-order eustatic cycles. Principal potential source rocks are believed to be synrift lacustrine sediments. The synrift sediments may lie below the oil floor and probably are gas prone. In the postrift section, the transgressive interval and the starved, basinward portion of transgressive and highstand systems tracts within the regressive interval also may have source rock potential. Carbonate complexes, weathered basement, shallow-water sands, and basin-floor fans all have the potential to provide reservoirs. Potential traps include carbonate complexes, basement hills and associated anticlines, fault traps, and coastal sand traps. Hydrocarbon indicators, such as flat spots, amplitude anomalies, and gas chimneys with seeps, occur at a number of locations.

Bryant Canyon Fan System: An Unconfined, Large River-Sourced System in the Northwestern Gulf of Mexico

The high-relief sea floor of the northwestern Gulf of Mexico slope is pockmarked by salt-rimmed minibasins and dissected by channels and canyons. Analysis of multichannel seismic reflection data reveals that Bryant Canyon and its fan are an unconfined, fluvially sourced system associated with the ancestral Mississippi River. Bryant Canyon eroded an extensive bypassing cut on the slope and delivered sediments to the unconfined area of the base of slope where they formed a fan. The canyon incorporates several minibasins and shallow salt structures. The canyon cross sections are easily recognizable in the minibasins, but are partially or completely obliterated by salt tectonism where the canyon incised the shallow salt structures. Salt appears to have encroached upon the can on after abandonment and restored the sea floor to its precanyon, low-relief profile. We interpreted three depositional units in the Pleistocene section of the Bryant Canyon Fan. The lower units are characterized by erosional surfaces or channels and mass-transport deposits. The top unit consists of low- to moderate-amplitude reflections with moderate continuity, suggesting relatively uniform deposition. The widespread occurrence of erosional surfaces and channels in the lower units may suggest extensive canyon activity during sea level falls. Decreased depositional energy from lower units upward indicates abandonment of the canyon or eastward shift of the ancestral Mississippi River. The Bryant Canyon Fan system contains several potential reservoir facies, including turbidites, trapped in high-relief minibasins during periods of less extensive canyon activity, that may exhibit good reservoir quality and massive sand occurring in canyon fill. Channel deposits and distal, unchannelized turbidites of the fan also may contain reservoir-quality sand. Mass-transport deposits with moderate- to high-amplitude reflections and levee deposits close to channels probably contain sand.