Abstract: "Micro-Channels" in Organic-Rich Shales and Mudrocks as Primary Hydrocarbon Migration Pathways: Possible Example from the Monterey Formation, California

Neal R. O'Brien, Geoffrey D. Thyne, Roger M. Slatt

Previous work (O'Brien and Slatt, 1990) has shown that organic-rich shales may retain an open pore network upon burial, thus providing possible pathways for primary migration of hydrocarbons. To further evaluate this, a sample of the Miocene Monterey formation was artificially matured by hydrous pyrolysis for 72 hours at temperatures of 260-360°C. Liquid and gaseous hydrocarbons and CO₂ were generated during maturation.

Thin section examination of unheated and heated sample chips indicates heating produces a much more ordered structure to the shale fabric. Organic laminae appear to be more closely-spaced and assume a honeycomb appearance in two dimensions, probably due to heating-induced shrinkage of the inorganic laminae, which are comprised mainly of opal-CT and quartz. Heating produces a crinkly appearance to the organic laminae, probably also due to shrinkage.

Scanning electron microscope analysis reveals the presence of numerous tube-shaped, linear "micro-channels" that are lined with authigenic minerals. Some "micro-channels" contain globules, presumably of hydrocarbons, since the presence of opal-CT indicates sufficient burial to begin hydrocarbon generation. Some globules appear to have been preserved in a state of migration through the "micro-channels."

Based upon the above observations, a working hypothesis is that shrinkage, resulting from dewatering and compaction during burial, produces "micro-channels" which may become primary migration pathways for hydrocarbons.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994