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AAPG Bulletin

Abstract


Volume: 57 (1973)

Issue: 9. (September)

First Page: 1618

Last Page: 1643

Title: Colloidal Soap as Proposed Primary Migration Medium for Hydrocarbons

Author(s): Robert J. Cordell (2)

Abstract:

Aqueous solution of hydrocarbons appears necessary to explain primary migration of liquid hydrocarbons in argillaceous source beds. However, relatively high solubility within the depth ranges of origin and primary migration (5,000-10,000 ft or more) is necessary to account for the known crude oil reserves.

Colloidal soap greatly enhances hydrocarbon solubility, but its role has been questioned for several reasons: (1) lack of observations of micelles in the sedimentary section; (2) requirement of many times more soap than hydrocarbons for hydrocarbon solubilization; (3) problem of explaining the fate of the large quantities of soap required; (4) adsorption of soap on host rocks in specific cases; (5) doubt that soap concentrations in fluids at depth are sufficiently high for micelle formation; (6) conclusion by some authors that the more acidic, potentially soap-forming bitumen remains in source beds; (7) the extremely small interstitial pore spaces of fine-grained sediments, which constitute a mechanical barrier to micellar migration; and (8) an electrical barrier to migration due to n gative charges on both the ionic micelles and clay minerals.

It is concluded that (1) search for micelles in presumed source beds has not yet been attempted; (2) a potential source of large amounts of soap exists in source beds; (3) after entering the reservoir, soap could be transformed to intermediate- and high-molecular-weight hydrocarbons and the asphaltenes and resins of crude oil; (4) relatively high temperature and electrolytic character of source-bed fluids at depth should promote solubility of soap; (5) soap concentrations in source-bed fluid should be sufficiently high for micelle formation; and (6) patterns with depth of organic elements (C, H, N, S, O, etc.), hydrocarbons, and acidic bitumen are compatible with soap formation.

The most perplexing problems are the extremely small pore spaces and the electrical barrier in clayey source beds. Pore sizes are so minute that even the small ionic micelles and large hydrocarbon molecules would encounter difficulty in migrating during late compaction. The semi-permeable membrane effect on electrolytic fluids during late compaction apparently is caused by negative charges on closely spaced clay-mineral grains. Hence pore-to-pore migration of the negatively charged ionic micelles would be resisted. Clay mineral transformations, accompanied or followed by microfracture formation, perhaps could overcome these barriers. This postulate needs further testing by experimental and subsurface investigations.

The large amounts of bitumen generated from kerogen contain high percentages of acid-enriched asphaltenes and resins. Known depth patterns of increasing amounts of asphaltenes and resins in organic matter, coupled with decreasing O+N+S content, could reflect, in part, a transfer of acids to interstitial waters by saponification.

The higher the soap concentration, the larger would be the hydrocarbon solubilization capacity of each mole of soap present. Moderately high temperatures and release of bound water from clays should increase true solution of hydrocarbons which further enhances soap micelle formation. Transformation with depth of montmorillonite to increasing proportions of illite in mixed-layer clays should release additional saponifiable organic matter. Experiments demonstrate an adequate degree of hydrocarbon solubilization in concentrated soap solutions.

Increases in salinity to above normal marine values progressively enhance micelle generation and hydrocarbon solubilization. Variations from brackish to well above normal marine salinities apparently exist at depth in marine source bed section. The accompanying moderately high temperature would promote transformation of the organic component, desorption of organic matter and water for migration, and decrease in micellar size.

Eh, pH, and cation content of fluids in the major hydrocarbon migration depth intervals should be suitable for soluble soap formation. The low Eh is related to fine-grained character, clay content, and amounts and types of organic matter. In downdip Gulf Coast areas of Louisiana, pH is about 8.0, or between 8.0 and 9.0, at depths of several thousand feet and more. Calcium and magnesium cation concentrations are quite low and pore waters are of the sodium bicarbonate type.

After source-bed fluids enter a reservoir system, several mechanisms could be responsible for breaking the soap colloid and freeing hydrocarbons. The most probable mechanisms are dilution of soap, decrease in pH, and increase in alkali-earth cation concentrations. If micelles persist until the moving reservoir fluid reaches a trap, the predominant mechanisms, triggered by water leakage into the caprock, would be increase in salinity, increase in calcium-ion concentration, and decrease in pH.

Future research should assess the simultaneous effects of temperature, Previous HitpressureTop, salinity, and other factors on micelle formation, hydrocarbon solubilization, and primary migration. Modeling should simulate subsurface conditions as closely as possible.

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