American Association of Petroleum Geologists (AAPG)
A 1 hour presentation. The distribution of oil and gas in subsurface hydrocarbon accumulations is in the first place strongly controlled by the types and maturities of source rocks that have generated the oil and gas. Sealing lithologies above the reservoirs (generally shales, tight carbonates or evaporites) prevent hydrocarbons from escaping to the surface. The role that these seals play in the distribution and relative quantities of trapped oil and gas is often understated. Seals are rarely perfect. Except for salt, most seals have some porosity and permeability allowing hydrocarbons to slowly leak out of the trap. Even at geological time-scales this leakage of hydrocarbons out of traps can be a very slow process. When the rate of leakage is less than the rate of charge, seals may appear effective. But there is a wide range of lithologies ranging from very good seals to non-seals. Ductile and fine-grained lithologies are the best seals. Sealing potential is less for lithologies that are more brittle, and/or more silty or sandy. Faults and fractures may be preferential leak paths, further compromising the effectiveness of seals. In areas of gas charge any (early) oil charge should normally be displaced by the lighter gas accumulating at the top of structures. The observation that in areas of abundant gas charge also oil may nevertheless be trapped, indicates that gas may leak out of traps preferentially – thus making room for oil. This notion should be seriously considered in predictions of the phase of trapped hydrocarbons in undrilled prospects that may have has access to both oil and gas charge. In the presentation examples will be shown of the dynamic nature of leakage and charge and of structures where oil has been trapped, despite abundant gas charge.