Hydrocarbon-generation-induced microcracking of source rocks

Journal Article · Fri Apr 01 04:00:00 EST 1994 · Geophysics; (United States)

DOI:https://doi.org/10.1190/1.1443616· OSTI ID:7115883

Vernik, L ^[1]

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1. Stanford Univ., Stanford, CA (United States). Dept. of Geophysics

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.

• OSTI ID: 7115883
• Journal Information: Geophysics; (United States), Journal Name: Geophysics; (United States) Vol. 59:4; ISSN GPYSA7; ISSN 0016-8033
• Country of Publication: United States
• Language: English

Related Subjects

02 PETROLEUM
020200* -- Petroleum-- Reserves
Geology
& Exploration
58 GEOSCIENCES
580000 -- Geosciences
CRACK PROPAGATION
FRACTURE MECHANICS
GEOLOGIC DEPOSITS
GEOLOGIC FRACTURES
GEOLOGIC STRUCTURES
GEOMETRY
MATHEMATICS
MATURATION
MECHANICS
MICROSTRUCTURE
MINERAL RESOURCES
ORIENTATION
ORIGIN
PETROLEUM DEPOSITS
RESOURCES
SOURCE ROCKS