A METHOD FOR THE DETERMINATION OF NITROGEN IN CLAYS, WITH APPLICATIONTO THE BURIAL DIAGENESIS OF SHALES

A new operationally defined method for the measurement of fixed inorga nic nitrogen (N(fix)) associated with clays has been tested and applie d to a sequence of mixed-layer illite/smectite (I/S) (ranging from 40 to 68% illite in I/S) in shales from a drill site in Terrebonne Parish , Louisiana. This new method has distinct advantages over other method s in that it is simple and can accommodate small sample sizes (10 mg). N(fix) is measured, using a carbon-nitrogen analyzer, on a sample tha t has been ashed for 8 hr at 450-degrees-C to remove organic and non-f ixed inorganic nitrogen. Total nitrogen is measured on an separate unt reated sample. Content of organic carbon and nitrogen can be assessed by the weight-percent difference between untreated and ashed samples. Fourier-transform infrared spectroscopy confirmed the presence of nitr ogen as ammonium in untreated and heat-created samples. N(fix) content of the < 0.2 mum fraction of the shale sequence correlates positively (R2 = 0.91) with the percent of illite in the mixed-layer I/S. Corres ponding with the increase in percent of illite in I/S with depth, an i ncrease in N(fix) content is also seen with increasing depth of burial . Nitrogen from thermal breakdown of organic-matter during later diage nesis probably provides the N(fix) found in I/S. In the range of diage nesis studied here, N(fix) concentration appears to depend on the avai lable fixing capacity of illite. Because nitrogen content varies with organic-matter type, however, caution should be used when assuming thi s relationship for other diagenetic environments.