Inorganic fixed-ammonium (NH4) contents as high as 0.28 wt% were measured in organicrich, quartz-grade siliceous rocks of the Miocene Monterey Formation from the Santa Maria and San Joaquin basins, California. The greatest amount of fixed-NH4 was found in rocks associated with hydrocarbons in the Point Arguello and Lost Hills oil fields, where the NH4 (NH4 + K) molar ratio of bulk samples ranges from 0.17-0.35. The formation of NH4-illite is suggested by the parallel increase in the percent of illite in the mixed-layered illite/smectite ( I S) and in the NH4 (NH4 + K) molar ratio of the clay-sized fraction with increasing burial depth. Mineralization of NH4 appears to be promoted by the coincident timing of the smectite-to-illite clay mineral transformation and the release of NH4 during catagenesis. NH4-feldspar may form at shallow burial depths in rocks from the Point Arguello field that contain a greater amount of detrital K-feldspar and in which the I/S contains only 10-20% illite. Quartzgrade siliceous Monterey rocks from coastal outcrops in the Lions Head area lack significant amounts of hydrocarbons and have NH4 (NH4 + K) molar ratios of 0.14-0.21. Rocks from the Lions Head area show a strong positive correlation between diagenetic illite and fixed-NH4 contents, with NH4 constituting 18-21 Mol% of the fixed interlayer cations in the I/S. Low grade, opal-A and opal-CT siliceous Miocene rocks from coastal outcrops in the Pt. Pedemales area have low fixed-NH4, contents (≤0.01 wt%) because these rocks contain minor amounts of diagenetic K-bearing minerals (I/S contains ≤ 10% illite) and they lack significant amounts of generated or migrated hydrocarbons. The increase in the organic ( C N) ratio with burial depth most likely reflects the preferential release of N compared to C and suggests that NH4 release is greatest during shallow burial bacterial degradation and deep burial catagenesis (oil generation). The results of this study support the suggestion of Williams et al. (1989) that high fixed-NH4 contents may provide a long-term geologic record of low-temperature (<150°C) NH4 mineralization associated with hydrocarbon generation and migration.
ASJC Scopus subject areas
- Geochemistry and Petrology