Hydrocarbon Potential of Volcanic Basins: Principles and Rules of Thumb

Max Rohrman
Shell Norway Exploration & Production, Tananger, Norway

Volcanic basins are defined as basins with significant flood basalt volcanism in their stratigraphic record and include basins on the North Atlantic, Western India, Karoo, Parana Basin (Brazil), Columbia Basin (Washington) and East Siberia. Hydrocarbon exploration has traditionally avoided basins dominated by volcanics and igneous intrusions, mainly due to the inability to seismically image below basalts and the perceived detrimental effect of volcanic activity on the petroleum system. Seismic problems are now being addressed using a lower frequency signal, to obtain a better sub-basalt image. The structural play is the main exploration target in sub-basalt prospecting. However, conspicuous sub-basalt 4-way-dip closures should be treated with caution as they might reveal a highly overmature sedimentary section, underlain by an igneous complex. Filtered gravity & magnetic data should be routinely integrated to avoid drilling igneous centers and lowgrade/highgrade acreage. Sub-basalt dykes and sills have limited influence on maturity, unless they occur in high numbers, when the intrusive/sediment ratio is higher than 40%. Furthermore, sill intrusions tend to be concentrated in stratigraphic layers that were within the upper few km's of the paleo-surface at the time of intrusion. Giant fields in East Siberia offer potential for reservoirs below sill affected strata. However, potentially the most prospective regions of volcanic basins are where flood basalts simply blanket sedimentary basins, without intrusives in the direct sub-surface, as evidenced in the Columbia Basin. Although an increase in volcanism in sedimentary basins generally tends to increase risks, some positive effects occur as well, such as the generation of sill-induced jack-up structures, forming traps and the sealing and reservoir capacity of igneous rocks.