In arkosic sandstones of the San Joaquin and Los Angeles Basins presently at temperatures between 35 degrees C and 174 degrees C, trace amounts of authigenic K-NH 4 -feldspar are present as microfracture fillings and overgrowths on detrital K-feldspar. Microchemical analyses of this authigenic phase reveal up to 80 mole % buddingtonite. The largest ammonium concentrations are observed in a sandy interval of the Antelope shale (80 mole %) and in the Stevens Sands ( nearly equal 50 mole %) of the San Joaquin Basin. This latter unit was deposited as a turbidite in the organic-rich Fruitvale Shale, an equivalent of the Antelope Shale. The lowest ammonium contents (0-16 mole %) are present in the shallow-marine Vedder Sands and the marginal marine San Joaquin Formation in the same basin. Petrographic, delta 18 O, delta 13 C, and 87 Sr/ 86 Sr analyses of a dolomite cement that postdates authigenic K-NH 4 -feldspar indicate that the feldspar precipitated below 28 degrees C in the zone of methanogenesis, from pore waters with the same Sr signature as sea water at the time of sedimentation. Authigenic K-NH 4 -feldspar is thus an early-diagenetic phase that crystallized prior to oil migration, under anoxic conditions when organic matter releases ammonium. The source of ammonium is bacterial decay of organic matter in the sandstones themselves and/or in contemporaneous shales.