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Generation and Expulsion of Oil as Determined by Hydrous Pyrolysis

LEWAN, M. D., Amoco Production Co., Tulsa, OK

Hydrous pyrolysis is a laboratory technique used to simulate oil generation within and expulsion from thermally immature source rocks. This technique maintains a liquid-water phase in contact with the source rock as it is heated at subcritical water temperatures (<374 degrees C). If the proper time and temperature conditions are employed, a free-flowing liquid oil accumulates on the water surface. Oil formation by hydrous pyrolysis may be divided into four distinct stages: (1) pre-oil, (2) incipient-oil, (3) primary-oil, and (4) post-oil. Pre-oil generation represents a thermally immature stage in which the organic matter occurs predominantly as a solid insoluble kerogen dispersed within the rock matrix. Incipient-oil generation begins when thermal stress is sufficient to initiate he decomposition of kerogen into a tarry soluble bitumen. The net volume increase generated by this overall reaction results in bitumen expansion along the planar bedding fabric of the rock matrix to form a continuous bitumen network. As thermal stress increases, primary-oil generation begins with the bitumen partially decomposing into a liquid oil. The net volume increase generated by this overall reaction and the lack of pore volume in the bitumen-impregnated rock matrix results in expulsion of the generated oil. This suggests that oil expulsion from a rock is a consequence of oil generation within a rock, and generation and expulsion of oil may be considered as one collective process. Post-oil generation begins when oil generation ceases. As thermal stress continues to increase, the r maining bitumen generates gas and pyrobitumen.


AAPG Search and Discovery Article #91004 © 1991 AAPG Annual Convention Dallas, Texas, April 7-10, 1991 (2009)