Abstract

Unusual carbon and oxygen isotopic compositions characterize outcropping dolomitic sandstone of Permian age over the Davenport oil field, Oklahoma. The δC13PDB ranges from −5.1 to −11.3/mil and the δO18SMOW ranges from +28.8 to +48.8/mil. Areal distribution of both isotopic ranges is systematic and closely reflects the distribution of petroleum in the producing interval at a depth of 1,000 m (3,300 ft). The 20/mil positive oxygen range in these surface rocks suggests evaporative fractionation of ground water controlled by the near-surface expansion of vertically migrating dry natural gas from the underlying deposit. The rather light, though unexceptional, carbon values, unlike those reported for other hydrocarbon-derived carbonate rocks, indicate only partial chemical or biochemical oxidation of the gas, suggesting further that the leakage rate was relatively rapid.

At the Cement oil field, Oklahoma, similar relations in surface rocks occur, but the carbon isotopic values are exceptionally light (−5.5 to −39.2/ml relative to PDB), indicating significant near-surface oxidation of slowly seeping hydrocarbons of low molecular weight.

The occurrence of carbonate rocks derived from the chemical oxidation or bacterial conversion of hydrocarbons is well known (Thode and others, 1953; S. R. Silverman and others, unpub. data; Russell and others, 1967; Hathaway and Degens, 1968; Mamchur, 1969; Davis and Kirkland, 1970). Such carbonates typically have δC13PDB values more negative than about −25/mil. Similarly, unusual isotopic signatures have been documented in carbonate-cemented sandstone and calcitized gypsum cropping out over the Cement oil field in the southeastern part of the Anadarko basin of Oklahoma (Donovan, 1974). There, the rock cements display carbon isotopic ratios that are increasingly light and oxygen ratios that are increasingly heavy toward the crest of the anticlinal structure. These trends are limited areally and are systematic in their patterns, reflecting the subsurface distribution of petroleum. Both liquid and gaseous hydrocarbons have leaked from the Cement reservoirs; much of the hydrocarbon was oxidized and the carbon incorporated into the interstitial carbonate cements of the overlying rocks. Rapid vertical leakage of gas, expansive under depressurization, resulted in preferential evaporation of H2O16, compared with H2O18 from near-surface ground waters. This evapo-escape to the atmosphere. Rayleigh distillation proceeds if escape rates cause evaporation of ground water at rates faster than the replacement or recharge rate of meteoric water. Rocks whose isotopic compositions reflect an intermediate mix of these contrasting processes have not yet been found but presumably exist. The existence of these kinds of isotopically anomalous carbonate cements would prove useful in petroleum exploration independently of other kinds of geochemical indicators.

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