Organic geochemistry of oils and hydrocarbon source rocks, Illinois basin

Organic geochemical analyses of 82 Illinois basin oils demonstrate three distinct oil groups. Gas chromatographic characteristics of group 1 oils (n = 12) include predominance of odd-carbon-numbered n-alkanes between n-C/sub 11/ and n-C/sub 19/ (CPI = 1.3-1.5), relatively small amounts of branched and cyclic alkanes (including a virtual absence of isoprenoids), and relatively small amounts of alkanes with carbon numbers greater than n-C/sub 19/. Group 2 oils (n = 4) are characterized by a predominance of odd-carbon-numbered n-alkanes that extend from n-C/sub 11/ to n-C/sub 21/ (CPI, n-C/sub 11/ to n-C/sub 19/ = 1.2-1.3) and by full n-alkane and isoprenoid profiles. Group 3 oils (n = 66) exhibit no n-alkane odd predominance and have full n-alkane and isoprenoid profiles. Pristane/phytane ratios range from 1.3 to 1.4 for group 2 oils and from 1.6 to 2.0 for group 3 oils. delta/sup 13/C/sub sat/ (delta/sup 13/C/sub arom/) range from -29.0 to -29.7 (-28.8 to -29.9) per thousand (PDB) for the group 1 oils, -29.8 to -31.8 (-29.4 to -31.1) per thousand for group 2 oils, and -29.1 to -30.0 (-28.0 to -28.9) per thousand for group 3 oils. Tricyclic terpane distributions (m/z = 191) for the oil groups are distinctive. Group 3more » oils are characterized by a maximum at C/sub 23/ and relatively abundant extended tricyclic terpanes, whereas group 1 and group 2 oils are characterized by a maximum at C/sub 19/ and a relatively abundant C/sub 24/ tetracyclic terpane. Group 1 oils occur primarily in late Champlainian reservoirs; group 2 oils, late Champlainian and Silurian reservoirs along the northern edge of the basin; and group 3 oils, Silurian, Devonian, Mississippian, and Pennsylvanian reservoirs.« less