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ABSTRACT: Possible Significance of Rock Fabric in Hydrocarbon Migration from Organic Shales

N. R. O'Brien, R. M. Slatt, J. T. Senftle

Attempts to understand hydrocarbon migration from organic shales have traditionally focused on chemical processes. Fabric analysis by x-radiography, thin section, and scanning electron microscopy of the Woodford, Kimmeridge, Monterey, Rundle, and Green River formations suggest physical fabric may also have played an important role in hydrocarbon migration from these and other important source rocks. All except the latter (carbonate rock) are black to dark gray shales or mudstones with high organic content (TOC >9%). All formed under anaerobic conditions which favored organic accumulation and preservation, and all contain hydrogen-rich kerogen.

These rocks exhibit an open network of micropores (micrometers in scale) within an organic hash matrix. This fabric consists of fine (<5 µm) discrete organic particles plus platy minerals which are moderately to highly preferentially oriented. This open fabric suggests deposition and subsequent compaction of organic-rich flocculated clay sediment but preservation of the original flocculated fabric. This preserved open pore network could have facilitated hydrocarbon migration upon burial. Commonly associated fine-grained, thin silt laminae could also have provided additional migration pathways. A preliminary model is presented which illustrates the role that rock fabric plays in providing clues to understanding hydrocarbon migration from organic shales.

AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990