Abstract

The majority of significant Indonesian oil provinces show a surface expression in the form of a belt of seeps situated on the basin edge. The spatial relationship between seeps and subsurface reserves is good on a basinal scale, but poor at sub-basinal and field scale. These trends can be explained by reviewing seepage in its structural and geochemical context.

The distribution of seeps in Western Indonesia is essentially controlled by the degree to which Plio-Pleistocene structural inversion extends over oil-bearing portions of the backarc basins. The less disturbed extensional fabric of the Central Sumatran Basin consequently shows less seepage than does the inverted fabric of the North and Central Sumatran basins, despite higher oil reserves. Flowing seeps frequently occur over active reverse faults and eroded steep anticlinal crests. The success rate of wells drilled near seeps is considerably higher than average but there is no relationship to field size. Most seeping fields are shallow and small. Within inverted areas, oil generation is usually no longer active and surface oil shows, therefore, represent a partial destruction of previously filled oil pools, often very small in size. Larger fields generally lie basinward of the seep belt in regions where a larger proportion of the oil system is likely to have been preserved.

Seeps are, therefore, of greatest value to the explorationist in Indonesia in indicating potential at a regional level and when evaluated in the light of the tectonic and geochemical history of the region concerned. Such integrated studies of seepage patterns are recommended for inclusion in frontier exploration programmes in areas such as Eastern Indonesia.