Fixed-ammonium in clays associated with crude oils

Lynda B. Williams, Ray E. Ferrell, Elizabeth W. Chinn, Roger Sassen

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64 Scopus citations


The association of ammonium (NH4+) silicates with organic-rich sedimentary environments has stimulated interest in the chemical cycle of N, and its possible application as an indicator of in situ organic maturation reactions or crude oil migration. Fixed-NH4 in clay minerals was determined from three hydrocarbon occurrences of similar depositional environment but different ages, depth and thermal maturity, to determine whether anomalously high NH4-substitution occurs near mature hydrocarbons. Results show higher fixed-NH4 concentrations in marginally mature mudstones than in immature sediments. The highest fixed-NH4 concentrations were found in clays from sandstone reservoirs containing migrated crude oil. Fixed-NH4 in clays from Holocene oil seep sediments in the Gulf of Mexico continental slope, offshore Louisiana, averages 0.08 wt % and increases with depth in shallow cores (420 cm), reflecting an early diagenetic trend that is apparently not influenced by migrating crude oil. Programmed pyrolysis shows that the sediments are thermally immature (av. Tmax = 419°C). High Hydrogen Index values (av. = 359 mg/g) are the result of biodegraded crude oil, and a high Oxygen Index (av. = 182 mg/g) reflects the presence of authigenic carbonate. Fixed-NH4 averages 0.16 wt % in Wilcox Group (Eocene) mudstones enclosing two sandstone reservoirs at Fordoche Field, onshore Louisiana. In comparison to these mudstones, anomalously high NH4-fixation appears to occur in reservoir clay minerals. Pyrolysis shows that the sediments are marginally mature for crude oil generation (av. Tmax = 432°C). Average Hydrogen Index (187 mg/g) and Oxygen Index values (75 mg/g) are consistent with oil-prone Type II and Type III kerogen. Increased pyrolysis Production Index values and solvent extraction shows the presence of migrated crude oil. This suggests that a reaction which releases NH3 during crude oil generation or migration is recorded by NH4+ substitution in clays. Fixed-NH4 and total organic carbon (TOC) at Fordoche Field show no statistically significant correlation, suggesting that NH4+ substitution in clay minerals is not simply related to the amount of organic matter in the section, but is also influenced by the presence of crude oil. Once NH4+ has been fixed in clays, it is a more stable hydrocarbon proximity indicator than pore fluid tracers, because it is less influenced by later chemical or geological changes.

Original languageEnglish (US)
Pages (from-to)605-616
Number of pages12
JournalApplied Geochemistry
Issue number6
StatePublished - 1989
Externally publishedYes

ASJC Scopus subject areas

  • Environmental Chemistry
  • Pollution
  • Geochemistry and Petrology


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