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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 57 (1973)

Issue: 4. (April)

First Page: 776

Last Page: 776

Title: Empirical Evidence for Petroleum Migration in Solution and Detection of Altered Crude Oils: ABSTRACT

Author(s): Terrence J. Donovan

Article Type: Meeting abstract


The volume and composition of light-liquid fractions in unaltered petroleum deposits are influenced by the salinity of associated formation waters. The implication is that low-molecular-weight hydrocarbons (boiling less than 200°C) are in a "dynamic equilibrium" with their accompanying waters. Hence, any significant variations in salinity could affect their relative solubilities and control their accumulation and escape.

The specific gravity of the light fractions in an unaltered crude is an approximate measure of the composition and relative abundance of the hydrocarbon components. A factor which normalizes the volume of light liquid fractions for variations in composition (specific gravity) is devised by dividing the volume (%) total plus naphtha, 10 times the specific gravity. The volume percent of the light fraction and the specific gravity are taken directly from U.S. Bureau of Mines Routine Crude-Oil Analyses.

If the volume of light fractions is salinity dependent, as zero-salt concentration is approached, the ratio of ^lgr (%) to salinity (%) similarly should approach some limit. A log-log plot of ^lgr divided by salinity versus salinity for 83 petroleum deposits of wide geographic distribution and geologic age shows a marked relation.

Evaporation, inorganic oxidation, through-put of relatively fresh water (water-washing), and microbial activity are known to affect appreciably the low-molecular-weight fractions of crudes, commonly resulting in oils that are in a disequilibrium with their formation waters (altered crudes). The low-carbon-number n-paraffins appear to be particularly biodegradable; biodegradation of a petroleum has been interpreted to be evidenced by a greatly reduced n-paraffin content, increased nitrogen content, and increased optical rotation of the remaining dense fraction. Detection of these effects requires relatively complicated laboratory procedures. Altered deposits show no systematic relation between ^lgr divided by salinity and salinity, and this permits their ready identification. Laborator analysis of a variety of oils corroborates the interpretation.

The nature of the data for unaltered crudes and theoretical considerations suggest an equilibrium relation in the form log ^lgr equals 1 plus A times the square root of c, minus B times c, where c is the total concentration of solids in the associated formation water and A and B are empirically determined constants. A semilog plot of ^lgr minus one divided by the square root of c, versus the square root of c, yields a straight line, suggesting that the volume and composition of the light liquid fraction in a petroleum are indeed salinity dependent.

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Copyright 1997 American Association of Petroleum Geologists