Lynda B. Williams
As organic matter matures, nitrogen (NH3) is released within the oil window at the same temperatures that authigenic illite forms. Because of the coincidence of these reactions, NH4+ is available for substitution into illite. The substitution of NH4+ for K+ in clays increases with organic maturity. This study suggests that continued release of N from hydrocarbons during migration, and its retention in authigenic illite, provides a method for tracing migration paths.
Fixed-NH4 concentrations of clays were examined in mudstones and sandstones of the Wilcox and Midway groups. Samples were taken from a dry hole near Hurricane Creek field, Allen Parish; Louisiana, a crude-oil-producing well in Fordoche field, Pointe Coupee Parish, Louisiana; and an abandoned gas well at the edge of Fordoche field. Analyses included Kjeldahl distillation, Fourier transform infrared spectroscopy, x-ray diffraction, and programmed pyrolysis.
Fixed-NH4 concentrations average 0.191 wt.% in sandstones from the dry hole but increase to 0.226 wt.% in the producing Wilcox sandstones and 0.381 wt. % in the gas well. Fixed-NH4 concentrations in mudstones from both oil fields average 0.160 wt.%. All mudstones contain mature types II and III kerogen. Fixed NH4 increases with depth within the Wilcox but remains constant (0.183 wt. %) in the Midway shales. Infrared spectroscopy demonstrates that the NH4+ is in illite. The fixed-NH4 concentrations are not related to organic carbon content of the sample but are apparently mostly influenced by level of organic maturation (degradation). Among the sandstones examined there are no significant mineralogical differences; therefore, it ap ears that fixed-NH4 concentrations reflect the extent of hydrocarbon migration and degradation.
AAPG Search and Discovery Article #91003©1990 AAPG Annual Convention, San Francisco, California, June 3-6, 1990