Hydrocarbon Potential of Forearc Basins in Asia

Janssen, Paul; Frankowicz, Edyta; Steffen, Daniel
Upstream International Exploration, Shell International Exploration & Production, The Hague, Netherlands.

Forearc basins develop between the main volcanic arc and the accretionary prism of a subduction system, and are characterized by low heatflow, lithic-rich clastic fill and a complicated tectonic history. These basin characteristics in combination with, so far, limited exploration success result in the general perception of their relatively low hydrocarbon potential. There are, however, examples of prolific forearc basins (> 1bln bbl proven reserves) such as the Cook Inlet (Alaska) and the Talara basin (Peru).

Many of the Asian forearc basins are significantly underexplored, with the wells being limited to the shelfal areas. This together with the large amount of forearc basins present in the Asia region with hints of a working petroleum system (e.g. shows, seeps, slicks, DHI, etc) has led to the conclusion that there might be significant underestimated potential in some of these basins.

Based on a recent study on the Simeulue basin (NW Sumatra forearc basin) using multi-client 2D TGS seismic data, the following critical success factors for having significant hydrocarbon potential in forearc basins were formulated:

1) Presence of petroleum system (i.e. source rocks): When a prolific petroleum system exists in the associated back-arc basin and is related to a source rock pre-dating the formation of the subduction zone and forearc basin, the likelihood of a petroleum system in the forearc significantly increases.

2) Maturity: Due to the low heatflow in forearc basin significant burial, related to the amount of sediment input, is required to mature any source rock, which is mainly a function of the size of the hinterland and its geomorphology.

3) Clastic Reservoir quality: The surface geology of the hinterland has a significant impact on the quality of the potential clastic reservoirs, which tend to be lithic-rich along volcanic arcs.

4) Presence of carbonate buildups as separate play: Some forearc basins do experience periods of relative clastic influx quiescence allowing carbonate buildups to grow. These would be less risky exploration targets from a reservoir quality point of view.

In the Simeulue forearc basin indications of a working petroleum system (e.g. gas shows on shelf, thermal gas in dropcores and DHI) together with play diversity and multiple sizable structures were seen on the high quality 2D TGS seismic data, which indicates the hydrocarbon potential of this forearc basin.

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