Organic Geochemistry of Oils and Hydrocarbon Source Rocks, Illinois Basin
Joseph R. Hatch, J. Bruno Risatti, J. David King, Donald R. Dickerson
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-C11 and n-C19 (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-C19. Group 2 oils (n = 4) are characterized by a predominance of odd-carbon-numbered n-alkanes that extend from n-C11 to n-C21 (CPI, n-C11 to n-C19 = 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 -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. ^dgr13Csat (^dgr13Carom) range from -29.0 to -29.7 (-28.8 to -29.9) ^pmil (PDB) for the group 1 oils, -29.8 to -31.8 (-29.4 to -31.3) ^pmil for group 2 oils, and -29.1 to -30.0 (-28.0 to -28.9) ^pmil for group 3 oils. Tricyclic terpane distributions (m/z = 191) for the oil groups are distinctive. Group 3 oils are characterized by a maximum at C23 and relatively abundant extended tricyclic terpanes, whereas group 1 and group 2 oils are characterized by a maximum at C19 and a relatively abundant C24 tetracyclic terpane. Group 1 oils occur primarily in late Champlainian reservoirs; g oup 2 oils, late Champlainian and Silurian reservoirs along the northern edge of the basin; and group 3 oils, Silurian, Devonian, Mississippian, and Pennsylvanian reservoirs.
Correlation of oils with CHCl3 extracts of 40 representative rock samples from within the basin and from the stable shelf to the northwest show that source rocks for group 1 and group 2 oils are two distinct Champlainian organic-matter facies; for group 3 oils, source rocks are the Upper Devonian-Kinderhookian New Albany Shale. Extracts of organic-matter-rich samples from the Upper Ordovician Maquoketa Shale and the Middle Pennsylvanian marine black shales do not correlate with any oil, suggesting limited source rock potential for these units. Biomarker thermal maturation indicators (Tm/Ts, C29 sterane isomerization ratios, and triaromatic/monoaromatic ratios), API gravities, and vanadium contents in the group 1 oils are related to reservoir depths, suggesting limited vertical oil migration. In contrast, these parameters are unrelated to depth of the group 3 oils, suggesting significant vertical migration and mixing. Comparison of maturation indicators in group 3 oils and New Albany Shale e tracts shows the oils were generated from rocks with present-day burial depths between 3,500 and 5,000 ft (1,050-1,500 m). Migration distances of about 150 mi (240 km) are indicated for group 3 oils presently produced from Silurian reservoirs in Brown and Adams Counties, Illinois.
AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.