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The Source and Fate of Oils in the Lawton Oilfield, Southwestern Oklahoma

Abstract

The Lawton oilfield is located in the east of the Wichita Mountain Uplift in the Anadarko Basin, Southwestern Oklahoma. 18 oil samples were collected and characterized by a variety of geochemical techniques, including bulk composition, biomarkers and stable isotopes. Despite the fate that all of these oils were recovered from reservoirs between 200 and 900ft, none of them appeared to be biodegraded as manifested by the abundance of the complete range of n-alkanes. The presence of the 25-norhopanes in all 18 samples suggests that these oils are in all probability mixtures of degraded and non-degraded oils. A cross-plot of dibenzothiophene/phenanthrene(DBT/PHEN) and pristane/phytane(Pr/Ph) shows the source rock for the Lawton oils is marine shale. Furthermore, the cross-plot of d13C values for the saturate and aromatic fractions shows that the source rock is probably Woodford Shale. The different 25-norhopane/C30 hopane ratio and dimethylnaphthalene/C2820S triaromatic steroid ratio suggests differing levels of biodegradation across the field and differing mixes of degraded and non-degraded oils. The isoheptane and heptane ratio cross plot, C29sterane 20S/ (20S+20R) and C29sterane ßß/ (ßß+aa) predicts the thermal maturity of Lawton oil is 1.0-1.3% Ro. This is clearly incorrect and suggests that these commonly used maturity parameters cannot be used for mixtures of degraded and nondegraded oils to predicted maturity level at which the oil was generated. Finally, a hydrocarbon accumulation model has been proposed for this field: there were two reservoir charging periods in Lawton oilfield. At first, the crude oil charged the reservoir and was biodegraded. The second charging period was after the uplift of Wichita Mountains and the subsidence of the southern part of the Anadarko Basin leading to deeper burial of the source rock and production of higher maturity oil. The second charge of light and nondegraded oil is then mixed with the in-situ biodegraded oils. Whilst the Lawton oil field is an old field with a long production history this study provides the opportunity to further our understanding on reservoir charging history when the oils have undergone secondary alteration.