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Mechanisms of natural fracture generation in a sedimentary basin may be studied by the use of physical properties of sedimentary rocks and of pore fluid pressure. The physical properties of rocks, such as Poisson's ratio and Young's modulus, under laboratory conditions are available, but only a few of these are reported under subsurface conditions.
In this paper, in-situ values of Poisson's ratio derived from analysis of artificial hydrofracturing data in the Gulf Coast are used for estimating horizontal stresses in both hydrostatically and abnormally pressured zones there. In a tectonically relaxed basin, such as the Gulf Coast, the horizontal stress can be a function of vertical (overburden) stress, fluid pressure, and Poisson's ratio. The horizontal stress increases with depth in the hydrostatic pressure zone, but is abnormally low in the abnormally pressured zone.
Fracture pressure (and its gradient) given as a sum of (least) horizontal stress and fluid pressure is, however, not abnormally low in the abnormally pressured zone because the fluid pressure there is abnormally high. This paper describes the in-situ fracture pressure gradient estimated for the Gulf Coast area. On the basis of models of vertical fluid charging through an aquifer and a fault system during continuous burial, the depth at which the charged fluid pressure gradient exceeds the fracture gradient (or the point of natural hydrofracturing) is estimated.
After an erosional event, the most significant decline of fracture pressure gradient is expected in the uppermost part of the original abnormal pressure zone. Hydrofracturing there could occur if an effective fluid is charging from deep to shallow through an aquifer or a fault system.
If the erosion continues to a point at which approximately two-thirds of the total sediment thickness is removed, the horizontal stress may become almost zero. If such a stress condition is developed, the sedimentary rocks become greatly relaxed, possibly causing fractures.
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