Regional Variations in Crude Oil Geochemistry, Anadarko Basin, Oklahoma, Texas, and Kansas--Evidence for Multiple Sources, Mixing, and Migration Distances
R. C. Burruss, J. R. Hatch
Geochemical analyses of 96 crude oil and condensate samples from the deep Anadarko basin and adjacent shelf areas show three major oil types, which generally correlate with reservoir age. Analyses include C3-C30 whole oil gas chromatography, C10+ 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 C13 to Cr19 n-alkanes, containing little or no acyclic isoprenoids, and ^dgr13C of -33.9 ppt (sat) and -33.7 ppt (arom). Oils f om 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 ^dgr13C of -30.6 ppt (sat) and -30.1 ppt (arom). Oils in Pennsylvanian reservoirs (type 3) have the greatest amounts of C15+ 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 with 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. These compounds are removed by water washing and, hence, could have been lost by contact with formation water during long-distance migration. The lack of mature source rocks in southern and central Kansas and the loss of benzene and toluene is consistent with oil migration from the central Anadarko basin.
AAPG Search and Discovery Article #91038©1987 AAPG Annual Convention, Los Angeles, California, June 7-10, 1987.