Abstract Potential source rocks were selected from some 300 Silurian core samples by measurements of total organic carbon. Characterization of 60 Michigan basin oils by use of infrared-absorption spectra and carbon-isotope data indicates that the group of oils produced from Niagaran reefs are unique and distinct from other oils in the Michigan basin. Correlation of the oils with potential source-rock kerogen by use of carbon-isotope data indicates that carbonaceous, laminated, interreef Salina A-1 carbonates are the principal source rocks, with the upper algal or "Brown Niagaran" facies of the reef itself constituting a subordinate source. Both facies were deposited in a mesosaline environment, i.e., in waters more saline than normal marine but insufficiently concentrated to precipitate calcium sulfate.