Hydrocarbons Associated With Igneous Rocks (N. America & Worldwide)

Igneous rocks can also provide their own traps. Variations in porosity can produce a pseudostratigraphic trap; however, thick intrusive bodies (such as laccoliths and plugs) and volcanic cones can also provide trapping structures. Dikes can block hycrocarbon migration pathways. Syndepositional sedimentary and igneous facies may also provide traps, as well as post-emplacement draping over noncompacting igneous bodies.

While generally not considered to be source rocks, extrusive igneous terrains are often lake-prone, and thus may provide source rocks for subsequent igneous rocks.

Migration in igneous areas is predominantly normal, but both CO₂-dominated systems and those subject to supercritical steam may be present and they may have distinctive favorable peculiarities. Since they are nonpolar fluids, they are considerably more effective than water-dominated systems at mobilizing hydrocarbons, and the conventional “rules of thumb” for organic matter type, richness, and maturity may not apply.

Exploration methods need to be as variable as the igneous reservoirs. Some igneous rocks contain abundant iron, and have a strong magnetic signature. Others do not. Some igneous rocks are much denser than the surrounding rock; others are not. Notably, one of the earliest plays in Texas, the “serpentine” trend, was instrumental in developing many of the early geophysical techniques. Likewise, log results may be so variable as to be ambiguous. Some characteristics may be recognizable, but the nature of the igneous rocks and their relationships must be considered.

Due to the lack of “rules” and systematic study, currently the best approach to exploration in and around igneous rocks is to look for analogs. The search for analogs cannot be limited to one area or continent, but a global view is needed. Analogs from around the world have applicability to possible plays in North America. Thus, an extensive reference list of possible analogs should be considered. 

More broadly, the presence of hydrocarbons in or around igneous rocks in a basin may be a sign for an effective hydrocarbon system in that basin. There are still basins in the world where leaking hydrocarbons may indicate the potential of the basin. Also, large areas of productive sedimentary basins are covered by volcanics, both flood basalts and ash-flow tuffs. Exploration models can complement and enhance geophysical methods in such areas.

Useful References

DeJarnett, B. B., 2007, Oil in a basaltic reservoir? West Rozel Field, Box Elder County, Utah: Houston Geological Society Bulletin, v. 50, n. 3 (November), p. 35, 37.

Schutter, S. R., 2003, Hydrocarbon occurrence and exploration in and around igneous rocks: in Petford, N., and McCaffrey, K. J. W., eds., Hydrocarbons in crystalline rocks, Geological Society of London, Special Publication 214, p. 7-33.

            Discussion.

Schutter, S. R., 2003, Occurrences of hydrocarbons in and around igneous rocks: in Petford, N., and McCaffrey, K. J. W., eds., Hydrocarbons in crystalline rocks, Geological Society of London, Special Publication 214, p. 35-68.

            List of occurrences.