Seepage in Vietnam — onshore and offshore examples

Abstract

Hydrocarbon seepages in Vietnam are generally linked to migration from Tertiary source rocks, and seeps are present both onshore and offshore. Residual oil can be observed at outcrop in breached traps within Tertiary sections and in exhumed ‘buried hill’ traps in pre-Tertiary rocks in the onshore Hanoi Basin. There is active oil seepage on the western margin of the offshore Phu Khanh Basin through fractured granites that outcrop at the basin margin. There are examples of onshore gas seepages in southern Vietnam. These are predominantly non-hydrocarbon, mantle-derived gases, sometimes mixed with biogenic methane, and are seeping via hot springs from fractured Mezosoic igneous rocks in active fault zones. Offshore, in the Nam Con Son Basin, sonar, seismic data and sea bed cores show a variety of seepage features, including shallow gas anomalies, mounds and pockmarks on the sea bed, and gas effects in the water column sometimes visible on sonar records. However, sea bed seepage features are not particularly common, and have a low preservation potential in the area due to powerful sea bed currents. Aircraft surveys of the sea surface have revealed several oil slicks that could be related to active oil seepages in the Nam Con Son Basin. Seepage appears to be occurring in Vietnam at two fundamentally different rates. ‘Background’ seepages flow at low rates, typically tens to thousands of cubic feet per day of gas, and up to tens of barrels per day of oil. These seeps flow episodically for extended periods of months or years, and leading to the formation of sea bed mounds. Higher flow rate eruptive seepages may have formed some of the pockmarks, and have a far shorter duration, probably only a few hours or days, with flow rates of millions of cubic feet per day of gas and any associated liquids.