AUTHIGENIC K-NH4-FELDSPAR IN SANDSTONES - A FINGERPRINT OF THE DIAGENESIS OF ORGANIC-MATTER

In arkosic sandstones of the San Joaquin and Los Angeles Basins presen tly at temperatures between 35 degrees C and 174 degrees C, trace amou nts of authigenic K-NH4-feldspar are present as microfracture fillings and overgrowths on detrital K-feldspar. Microchemical analyses of thi s authigenic phase reveal up to 80 mole % buddingtonite. The largest a mmonium concentrations are observed in a sandy interval of the Antelop e shale (80 mole %) and in the Stevens Sands (approximate to 50 mole % ) of the San Joaquin Basin. This latter unit was deposited as a turbid ite 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 Format ion in the same basin. Petrographic, delta(18)O, delta(13)C, and Sr-87 /Sr-86 analyses of a dolomite cement that postdates authigenic K-NH4-f eldspar indicate that the feldspar precipitated below 28 degrees C in the zone of methanogenesis, from pore waters with the same Sr signatur e as sea water at the time of sedimentation. Authigenic K-NH4-feldspar is thus an early-diagenetic phase that crystallized prior to oil migr ation, under anoxic conditions when organic matter releases ammonium. The source of ammonium is bacterial decay of organic matter in the san dstones themselves and/or in contemporaneous shales.