Abstract
Hydrocarbon production zones for source rocks in various basins throughout the world are at depths ranging from less than 3,000 m to greater than 5,000 m. Theoretical calculations completely independent of these observations indicate a comparable range of depths for the onset of microfractures in overpressured sedimentary beds undergoing burial. The basis for these calculations is a nonisothermal model of sedimentation which includes fluid flow as a pressure-dissipating mechanism, and water expansion and gravitational loading as pressure-producing mechanisms. A microfracture criterion is incorporated from experimental observations as a critical fluid pressure with respect to the least principal stress in a regional stress field. The theoretical results indicate that, for a constant rate of sediment burial, the depth to the onset of microfractures is fixed for a given geothermal gradient and regional stress distribution. The onset depth is increased when lateral expansion of a depositional basin is restricted by the pressure of the surrounding rock, and decreased when the degree of horizontal restraint is lessened. Once initiated, the average rate of propagation of the microfracture zone keeps pace with the rate of sediment burial, at least for significant parts of geologic time. For a given geothermal gradient and regional stress distribution, maturation possibilities vary widely prior to microfracture development, depending chiefly on the burial rate and kinetic factors.
