Regional Variations in Crude Oil Geochemistry, Anadarko Basin, Oklahoma, Texas, and Kansas--Evidence for Multiple Sources, Mixing, and Migration Distances: ABSTRACT

Geochemical analyses of 96 crude oil and condensate samples from the deep Anardarko basin and adjacent shelf areas show three major oil types, which generally correlate with reservoir age. Analyses include C/sub 3/-C/sub 30/ whole oil gas chromatography, C/sub 10/ + saturated hydrocarbon fraction gas chromatography, carbon stable isotopes (ppt relative to PDB) of saturated (sat) and aromatic (arom) hydrocarbon fractions, and computerized GC/MS of selected samples. Three samples from Ordovician Simpson Group reservoirs are typical Ordovician oils (type 1), having strong odd-carbon predominance in the C/sub 13/ to Cr/sub 19/ n-alkanes, containing little or no acyclic isoprenoids, and delta/sup 13/C of -33.9 ppt (sat) and -33.7 ppt (arom). Oils from Devonian and Mississippian reservoirs (type 2) show little or no odd-carbon predominance in the n-alkanes, an exponential decrease in abundance with increasing carbon number, pristane/phytane ratios (pr/ph) of 1.1 to 1.5, and delta/sup 13/C of -30.6 (sat) and -30.1 ppt (arom). Oils in Pennsylvanian reservoirs (type 3) have the greatest amounts of C/sub 15/ + hydrocarbons of all the oils, 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 1.8 to 0.9 Type 3 oils withmore » pr/ph < 1 form a subgroup in the Texas and Oklahoma panhandles. Condensates correlate with the three oil types based on carbon isotopic and gasoline-range compositions. Oils of type 2 composition occur in rocks of Ordovician to Pennsylvanian age in complex structural traps near the Arbuckle Mountains and in subcrop plays where Pennsylvanian reservoirs directly overlie Devonian and older rocks. Such traps also contain oils that could be mixtures of types 2 and 3 and types 1 and 2. Oils from the Kansas shelf are similar to the Anadarko oil types except that they have only traces of toluene and no detectable benzene.« less