Geological Evidence for Fluid Overpressure in Mature Source Rocks within Foreland Basins of the Americas

Cobbold, Peter R.
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In the Americas, many foreland basins have produced oil or gas from conventional reservoirs. However, new techniques of hydraulic fracturing are facilitating production from unconventional reservoirs, especially in source rock. As well data accumulate, they are showing that overpressure is common within mature source rock. Here I will consider independent geological evidence for such overpressure, either at regional scale or at sample scale. Outcropping source rock is especially common at the edges of foreland basins, as a result of out-of-sequence thrusting and exhumation.

At regional scale, evidence for overpressure within source rock comes from thin-skinned flat-lying detachments, which have formed during thrusting. In many developing foreland basins, the deformation front coincides with the maturity front (entry into the oil window or gas window), at a depth of a few km. This suggests that overpressure develops as the source rock reaches maturity. In the past, many researchers have attributed such overpressure to mechanical compaction. Indeed, the volume increase, from kerogen to oil, is probably too small to cause overpressure by itself. On the other hand, according to recent physical modeling, chemical compaction and loss of strength of a solid framework cause the weight of overburden to transfer to pore fluids. Here I will review evidence for coincidence of the maturation front and the deformation front in the Magellan Basin (southern Patagonia), the Neuquén Basin (Argentina) the Chaco Basin (Bolivia and NW Argentina), various foreland basins in the U.S. and western Canada, and the North Slope (Alaska).

At sample scale, direct evidence for overpressure comes from bedding-parallel veins. By analogy with physical models, original hydraulic fractures have dilated vertically, against the force of gravity, as a result of overpressure gradients (seepage forces). The fractures progressively filled with material, becoming solid veins. The most typical infilling material is fibrous calcite (beef) or gypsum. Calcite beef is especially common in source rock of the Neuquén Basin (Argentina). A less common infilling material is asphaltite, which has solidified from oil by loss of volatile constituents. I will make the case for such asphaltite being the product of an abnormally high thermal gradient, due to magmatic intrusion, hydrothermalism, or a shallow Moho.

AAPG Search and Discovery Article #90166©2013 AAPG International Conference & Exhibition, Cartagena, Colombia, 8-11 September 2013