Organic geochemical analyses of 104 crude oils and 190 core samples of dark-colored shales from the greater Anadarko basin show three major oil types which generally correlate with reservoir age and source-rock age. Analyses include C3-C30 whole-oil gas chromatography, C10+ saturated-hydrocarbon-fraction gas chromatography, and carbon stable isotopes (ppt relative to PDB) of saturated (sat) and aromatic (arom) hydrocarbon fractions. Three samples from Middle Ordovician Simpson Group reservoirs are "typical" Ordovician oils (type 1), having strong odd-carbon predominance in the C13 to C19 n-alkanes, containing little or no acyclic isoprenoids, an δ13C values of -33.9 ppt (sat) and -33.7 ppt (arom). Oils from Silurian to Devonian and Mississippian reservoirs (type 2) show little or no odd-carbon predominance in the n-alkanes, a regular decrease in abundance of n-alkanes with increasing carbon number, pristane/phytane ratios (pr/ph) of 1.1 to 1.5, and δ13C values of -30.6 ppt (sat) and -30.1 ppt (arom). Oils in Pennsylvanian reservoirs (type 3) have the greatest amounts of C15+ hydrocarbons, are isotopically heavy (-27.5 ppt [sat] and -26.4 ppt [arom]), have methyl-cyclohexane as the most abundant hydrocarbon, and have pr/ph values from 2.0 to 0.9. Oils from the Kansas shelf area of the Anadarko basin are similar to the Anadarko oil types except that they have only traces of toluene and no detectable benzene. The relative abundance of toluene in the C7 hydrocarbons systematically decreases with distance from the depocenter of the basin. The aromatic compounds are removed by water-washing, and hence could have been lost by contact with progressively greater amounts of formation water during long-distance migration. The lack of thermally mature source rocks in southern and central Kansas supports this hypothesis.
Additional Publication Details
Geochemistry of oils and hydrocarbon source rocks, greater Anadarko Basin: evidence for multiple sources of oils and long-distance oil migration