Geochemical characteristics of oil seepages from Dam Thi Nai, Central Vietnam: Implications for hydrocarbon exploration in the offshore Phu Khanh Basin

Dam Thi Nai is a semi-enclosed embayment on the coast of central Vietnam, adjacent to the northern part of the offshore and largely unexplored Phu Khanh Basin. Seepages of oil have been known in Dam Thi Nai since the early part of the twentieth century. This paper presents organic geochemical data on a number of samples of seepage oil collected from Dam Thi Nai and discusses their implications for the prospectivity of the Phu Khanh Basin. The results indicate that the petroleum was generated from a Tertiary marine marl source rock. Seepage oils are found in varying degrees of biodegradation and modes of occurrence at different locations in the embayment. Thus, oil was observed to fill fractures in freshly quarried outcrops of Cretaceous granite, and also occurs in shallow pits dug in the beach sand and in shallow basins used for shrimp forming. The oils indicate active seepage from kitchen areas or leaking accumulations in the Phu Khanh Basin. Seismic data suggest the existence of both source rocks and kitchens, and indicate a possible migration route from the deep basin to the surface at the bay. A few samples show anomalous compositions, indicating the presence of two other oil types which have different sources. These occurrences cannot at present be explained. However, the results obtained are encouraging for future exploration in the Phu Khanh Basin.