Abstract

Laboratory measurements of ultrasonic velocity and anisotropy in kerogen-rich black shales of varying maturity suggest that extensive, bedding-parallel microcracks exist in situ in most mature source rocks undergoing the major stage of hydrocarbon generation and migration. Given the normal faulting regime with the vertical stress being the maximum principal stress typical of most sedimentary basins, this microcrack alignment cannot be accounted for using simplified fracture mechanics concepts. This subhorizontal microcrack alignment is consistent with (1) a model of local principal stress rotation and deviatoric stress reduction within an overpressured formation undergoing hydrocarbon generation, and with (2) a strong mechanical strength anisotropy of kerogen-rich shales caused by bedding-parallel alignment of kerogen microlayers. Microcracks originate within kerogen or at kerogen-illite interfaces when pore pressure exceeds the bedding-normal total stress by only a few MPa due to the extremely low-fracture toughness of organic matter. P-wave and, especially, S-wave anisotropy of the most mature black shales, measured as a function of confining pressure, indicate the effective closure pressure of these microcracks in the range from 10 to 25 MPa. Estimates of pore pressure cycles in the matrix of the active hydrocarbon-generating/expelling part of the source rock formation show that microcracks can be maintained open over the sequence of these cycles and hence be detectable via high-resolution in-situ sonic/seismic studies.

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