MINERALIZATION OF ORGANOGENIC AMMONIUM IN THE MONTEREY FORMATION, SANTA-MARIA BASIN AND SAN-JOAQUIN BASIN, CALIFORNIA, USA

Inorganic fixed-ammonium (NH4) contents as high as 0.28 wt% were measured in organic-rich, 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. 1 7-0.35. 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. Quartz-grade 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. Pedernales area have low fixed-NH4 contents (less-than-or-equal-to 0.01 wt%) because these rocks contain minor amounts of diagenetic K-bearing minerals (I/S contains less-than-or-equal-to 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-degrees-C) NH4 mineralization associated with hydrocarbon generation and migration.