ABSTRACT: Geochemical Characterization of Dissimilar-Sourced Oils from Upper Knox and Middle Devonian Rocks, Wayne County, Illinois

SALMON, G. L., J. B. RISATTI, and S. T. WHITAKER, Illinois State Geological Survey, Champaign, IL

Drill-stem tests at the UNOCAL #1 Cisne well in Wayne County, Illinois, recovered oils from depths of 4,950, 5,065, and 7,876 ft. The two shallow oils were found in separate horizons within the same Middle Devonian carbonate formation, but are geochemically similar. The oil recovered from 7,876 ft is from the upper Knox and is significantly different geochemically from the shallower two, indicating a different source. The saturate fractions of the shallow oils are characterized by abundant n-alkanes with a predominance of odd n-alkanes between n-C[9] and n-C[23], and smoothly decreasing amounts of n-alkanes between n-C[23] and n-C[40]. The deepest oil shows no odd-even predominance within the n-C[9] to n-C[40] range, few cyclo-alkanes (1.5%) and almost no C[27]-C[29], 5a(H),14a(H),17a H) steranes (<0.1%). The shallow oils contain relatively large amounts of cyclo-alkanes (19%) and C[27]-C[29], 5a(H),14a(H),17a(H) steranes (2%). The aromatic fractions of all the oils exhibit a full range of single and fused-ring polyaromatic compounds and the isomaric shifts of the methylated polyaromatics suggest low to moderate thermal maturity. The shallow oils are poor in sulfur-substituted heteroatomic analogs, in contrast to the deeper oil.

The sources of these oils are enigmatic. Based on n-alkane, saturate and aromatic biomarker distributions, and spatial distributions, the shallow oils appear to be sourced from Upper Devonian or Lower Mississippian rocks other than the New Albany Shale, the accepted primary source for most Illinois oils. The oil from the upper Knox carbonates appears to be sourced from Upper Cambrian or Lower Ordovician strata and is the deepest known oil occurrence in the Illinois basin.

AAPG Search and Discovery Article #91013©1992 AAPG Eastern Section Meeting, Champaign, Illinois, September 20-22, 1992 (2009)