HYDROCARBON-GENERATION - INDUCED MICROCRACKING OF SOURCE ROCKS

Laboratory measurements of ultrasonic velocity and anisotropy in kerog en-rich black shales of varying maturity suggest that extensive, beddi ng-parallel microcracks exist in situ in most mature source rocks unde rgoing 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 a lignment cannot be accounted for using simplified fracture mechanics c oncepts. This subhorizontal microcrack alignment is consistent with (1 ) a model of local principal stress rotation and deviatoric stress red uction within an overpressured formation undergoing hydrocarbon genera tion, and with (2) a strong mechanical strength anisotropy of kerogen- rich shales caused by bedding-parallel alignment of kerogen microlayer s. Microcracks originate within kerogen or at kerogen-illite interface s 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 s hales, measured as a function of confining pressure, indicate the effe ctive closure pressure of these microcracks in the range from 10 to 25 MPa. Estimates of pore pressure cycles in the matrix of the active hy drocarbon-generating/expelling part of the source rock formation show that microcracks can be maintained open over the sequence of these cyc les and hence be detectable via high-resolution in-situ sonic/seismic studies.