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Cretaceous Deposition in the Phuquoc-Kampot Som Basin On- and Offshore South Indochina

Fyhn, Michael B.1; Nielsen, Lars H.1; Nguyen, Nguyen D.4; Hong, Nguyen T.3; Nga, Le H.2; Giang, Tran C.2
1Geological Survey of Denmark and Greenland, Copenhagen K, Denmark.
2Vietnam Petroleum Institute, Hanoi, Viet Nam.
3Hanoi University of Science, Hanoi, Viet Nam.
4Hanoi University of Mining and Geology, Hanoi, Viet Nam.

The Phuquoc-Kampot Som Basin (PKSB) covers a large part of southern Indochina from Thailand in the west to Vietnam in the east. Seismic data indicate that the PKSB continues offshore in the Gulf of Thailand buried underneath Cainozoic strata making the basin one of the largest in Southeast Asia. The basin contains a kilometre thick U. Jurassic and Cretaceous succession that crops out on the Phu Quoc island offshore Vietnam. The depositional outline of the basin is poorly known in spite of its widespread distribution, but was recently suggested as a foreland basin associated with coeval arc formation to the east. Outcrop studies on Phu Quoc combined with the drilling of a 500 m fully cored stratigraphic well reveal a succession dominated by coarse grained, petrographically diverse, occasionally very pebbly sandstones. The sandstones mainly shows large scale planar cross-beds and parallel stratification primarily formed by braided rivers. The sandstones often contain coalyfied wood fragments; coal beds or rooted horizons were not observed. Better sorted marine shoreface sandstones with distinct burrows occur at a few stratigraphic levels. The sandstones are interbedded with variegated silty mudstones primarily formed in lakes. Sporadic occurrences of marine dinoflagellates indicate that mud deposition also occurred in lagoons at a few levels. The sandstones show mainly poor to fair reservoir properties, improving to excellent in a few intervals dependent on the amount of diagenetic cement.

A Cretaceous source rock interval informally termed the Jet Coals has been speculated to exist. However, we have not been able to confirm any source potential within this interlude. The interval seems to include scattered coalyfied wood fragments including large tree trunks occasional with roots. The coal fragments are dominated by inert kerogene and does not posses a source rock potential. The Jet Coal interval is suggested to represent an interlude of more humid conditions resulting in raised groundwater levels and forestation in the area.

The overall depositional outline of the PKSB resembles that of the Khorat Basin of Thailand to the north. This supports the notion that the two basins originally formed as a single basin that became segregated due to subsequent Early Cainozoic uplift and erosion. The presence of discrete marine interludes within the PKSB suggest a depositional setting situated closer to open seaways compared with that of the Khorat Basin.

 

AAPG Search and Discovery Article #90155©2012 AAPG International Conference & Exhibition, Singapore, 16-19 September 2012