ABSTRACT: Petrography and Reflectance of Vitrinite-Like Particles in Precambrian Rocks, U.S.A. and U.S.S.R.

PAWLEWICZ, MARK, and JAMES G. PALACAS, U.S. Geological Survey, Lakewood, CO

Vitrinite-like particles were isolated from three widely scattered Precambrian terranes: the late Proterozoic Chuar Group, Grand Canyon, Arizona, U.S.A; the middle Proterozoic, Keweenawan Group (Nonesuch Shale equivalent), Midcontinent Rift System (MRS), western Iowa, U.S.A.; and the Vendian-Riphean rocks in the Lena-Tunguska Province, eastern Siberia, U.S.S.R. Samples that were processed to concentrate the organic matter (OM) were extracted with chloroform to remove any soluble bituminous substances. Some of the remaining material, examined in reflected light, is indistinguishable from post-Silurian vitrinite-group macerals. The particles of organic matter are angular pieces and have a weakly expressed internal cellular structure. In addition, in the analysis of whole rock samples, t e distribution of the OM was as isolated particles, disassociated from any bedding feature or mineral fabric, suggesting a nonbitumen macromolecular entity.

With few exceptions, reflectance (Rm) values of the vitrinite-like particles agree with Rock-Eval pyrolysis Tmax values, providing some validity to the reflectance measurements. These two measurements showed a range of maturity values (with respect to liquid hydrocarbon generation) in the studied samples: Those from eastern Siberia are immature or marginally mature (average Rm = 0.42%, Tmax = 432 degrees C), those from the Chuar Group (Kwagunt Formation) are mature (average Rm = 0.73%, Tmax ~ 437 degrees C), and the MRS (Keweenawan) rocks are overmature (average Rm = 2.2%, Tmax = 503 degrees C).

These results demonstrate that the reflectance technique can be extended to Precambrian rocks for determination of thermal maturation levels.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)