Mineralization of organogenic ammonium in the Monterey Formation, Santa Maria and San Joaquin basins, California, USA

Abstract

Inorganic fixed-ammonium (NH 4) 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-NH 4 was found in rocks associated with hydrocarbons in the Point Arguello and Lost Hills oil fields, where the NH 4 (NH 4 + K) molar ratio of bulk samples ranges from 0.17–0.35. The formation of NH 4-illite is suggested by the parallel increase in the percent of illite in the mixed-layered illite/smectite ( I S ) and in the NH 4 (NH 4 + K) molar ratio of the clay-sized fraction with increasing burial depth. Mineralization of NH 4 appears to be promoted by the coincident timing of the smectite-to-illite clay mineral transformation and the release of NH 4 during catagenesis. NH 4-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 NH 4 (NH 4 + K) molar ratios of 0.14–0.21. Rocks from the Lions Head area show a strong positive correlation between diagenetic illite and fixed-NH 4 contents, with NH 4 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-NH 4, 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 NH 4 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-NH 4 contents may provide a long-term geologic record of low-temperature (<150°C) NH 4 mineralization associated with hydrocarbon generation and migration.