Evaluating Volcanic Reservoir

Hydrocarbons can be found in volcanic rock—sometimes in significant quantities. Petrophysical methods originally developed for sedimentary accumulations are being used to evaluate these unusual reservoirs.

In the early days of petroleum exploration, the discovery of hydrocarbons in anything other than sedimentary rock was largely accidental, and such accumulations were considered flukes. Serendipity is still part of exploration, but geologists now know that the presence of oil and gas in such rock is certainly no coincidence. Igneous rock—created by the solidification of magma—hosts petroleum reservoirs in many major hydro carbon provinces, sometimes predominating them.

In general, igneous rocks have been ignored and even avoided by the E&P industry. They have been ignored because of a perceived lack of reservoir quality. However, there are many ways in which igneous rocks can develop porosity and permeability. Far from inconsequential, igneous activity can influence every aspect of a petroleum system, providing source rock, affecting fluid maturation and creating migration pathways, traps, reservoirs and seals. Igneous rocks have been avoided for other reasons. They tend to be extremely hard, although improvements in bit technology are helping drillers cope with these tough lithologies. Because they typically prevent deep penetration of seismic energy, igneous layers are considered an impediment to evaluation of underlying sediments as well. New seismic methods are advancing solutions to this problem, but with their strong refractive qualities, igneous reservoirs remain difficult to characterize. Once hydrocarbons are found in igneous reservoirs, assessing hydrocarbon volumes and productivity presents several challenges.

Log interpretation in igneous reservoirs often requires adapting techniques designed for other environments. Logging tools and interpretation methods that succeed in sedimentary rock can give meaningful answers in igneous rock, but they often require artful application. Furthermore, because mineralogy varies greatly in these formations, methods that work in one volcanic province may fail in another. Usually, a combination of methods is required.