1077Figure 13. Schematic illustrati

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Figure 13. Schematic illustration of the geologic evolution of the Cuu Long and Nam Con Son basins. (A) Proto-southern Vietnam margin was characterized by zones of weakness. (B) The zones of weakness provided the locations of rift initiation in the Paleogene. (Q In the Oligocene, rifting and extension created the Cuu Long and Nam Con Son basins on either side of the Con Son swell; this initial rifting phase was characterized by rapid subsidence and infilling. (Q A second phase of rifting began in the Nam Con Son Basin in the Miocene while the postrift subsidence continued in the Cuu Long Basin. (E) Inversion in the middle to late Miocene terminated the second phase of rifting in the Nam Con Son Basin. (F) Slowed subsidence and decreased sediment influx resulted in the shelf-slope transition in the eastern Nam Con Son Basin.

Recent drilling results in the area, however, were not very impressive ("Old guard,” 1997; Reid, 1997) and finding economically significant accumulations of hydrocarbons has become increasingly difficult. In the following discussion, we examine our findings and the data from published reports to determine some con¬straints to the petroleum geology of the area and thus to facilitate the evaluation of exploration opportunities and prospects.
Figure 14 provides a summary of the source rocks, hydrocarbon generation, potential traps, and trap potential for the Cuu Long and Nam Con Son basins. The information about the hydrocarbon types and reservoirs traps compiled from published data is summarized in Figure 15. Because the published data are mostly from industry magazines such as Oil Gas Journal and Offshore, information about exact locations of drilling sites and prospects within concession blocks

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Figure 6. Continued. (D) Iso- chron map of MS4 (late Pliocene and younger). Over much of the study area, MS4 is characterized by shelf environment; in the eastern part of the area, the complex clinoforms and the seaward shift of shelf edge that began in MS3 continued upward into MS4 and seaward beyond the eastern boundary of the study area.
the gentle hanging-wall slopes of half grabens and in other places; we interpret these low-angle clinoforms as deltas that prograded into shallow lakes. Relatively continuous reflections that gradually onlap topographic lows may suggest a progressive expansion of lacustrine deposition. Layers of short, high-amplitude, low-frequency complex reflections, seen near the basement in some places, are interpreted as volcanic flows and or sills associated with synrift volcanism.
MS2 (Early-Late Miocene)
MS2 (ca. 24 Ma-ca. 8 Ma) comprises the postrift unit in the Cuu Long Basin, representing the deposition during the thermal subsidence subsequent to the main phase of rifting. MS2 in the Nam Con Son Basin comprises the late synrift deposits corresponding to the second rifting phase; thickening of strata toward faults and differences in bed thickness on either side of faults indicate syndepositional faulting. MS2 occupied a larger