Precambrian vein pyrobitumen: evidence for petroleum generation and migration 2 Ga ago

Abstract

Abstract Isotopic, optical reflectance, and chemical analyses of large pure samples of Lower Proterozoic vein pyrobitumens (anthraxolite and thucholite of older reports) collected from five widely separated localities in the Great Lakes region of the United States and Canada suggest that in the Lower Proterozoic oil was generated from kerogen and migrated in much the same way as during the Phanerozoic. Furthermore, maturation and organic metamorphism followed the same successive stages that mark the generation and destruction of Phanerozoic petroleum. Kerogens derived from algae and bacteria were deposited with subaqueous muds that ultimately became black shales or slates. Some of the kerogen, however, yielded petroleum, which migrated into fractures and subsequently became pyrobitumen. The δ13C values average −32.6%. (PDB). This value compares favorably with those from Phanerozoic petroleum and asphaltites and is consistent with the proposition that the carbon isotopes were fractionated by biological activity. Reflectance values of the pyrobitumens range from 0.9% for Elliot Lake, Ontario, to 9.5% for Iron River, Michigan, and H/C ratios range from 0.6 for Elliot Lake to 0.1 for Sudbury, Ontario. When compared with the stages of petroleum maturation, these values indicate maturation which ranges from the catagenesis stage through the metagenesis stage of petroleum generation and metamorphism.