The underground occurrence of open extension fractures is important in petroleum exploration because the fractures provide plane conduits for the migration and storage of fluid. Extension fractures are considered to be natural hydraulic fractures that form like the artificial hydraulic fractures produced during well stimulation. Mechanics of natural hydraulic fracturing are discussed, so that the implications of the theory for the migration and accumulation of hydrocarbons are apparent.

Development of open extension fractures in the earth's crust is inhibited by gravitationally induced confining pressures; there should exist a depth above which extension fractures could form and below which faulting would be the dominant mode of failure. Mechanical considerations indicate that for the development of extension fractures on a regional scale this limiting depth is rather shallow--on the order of a few hundred meters to a few kilometers. Alternatively, mechanical considerations indicate that extension fractures could form locally in anomalously stressed regions at much greater depths if the ratio of pore fluid pressure to overburden weight approaches one. Thus, open extension fractures will be most likely to occur at depth in the earth's crust in places where the mean to al stress is anomalously low, and where the pore pressure is anomalously high.

Fracture porosity depends critically on fracture aperture, and on degree of infilling with secondary mineral matter. Observed apertures of fractures are usually one to three orders of magnitude larger than that predicted by theory, suggesting that pressure solution is important to the aperture of fractures.

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