Determination of the content and distribution of fixed ammonium in illite-smectite using a modified X-ray diffraction technique: Application to oil source rocks of western Greenland

摘要

The X-ray diffraction method previously developed for the determination of the amount and distribution of fixed NH4+ in illite-smectite has been modified to include the effects of layer thickness fluctuations of K-saturated and heated smectite and the effects of mean thickness of coherent scattering domains (CSDs). X-ray diffraction patterns and 002 and 005 reflection profiles are calculated for K-saturated and dehydrated NH4+-bearing I-S representing models having different distributions of K and NH4+ over mica-like interlayers, different proportions of interlayer types, different mean thicknesses of CSDs, and different degrees of thickness fluctuations for K-saturated and heated smectite layers. The diffraction criteria for identification of these models are discussed. The amount of fixed NH4+ can be determined accurately from the position of the 005 reflection. Diffraction methods have low sensitivity to different distributions of fixed K and NH4+ over mica-like interlayers in NH4-bearing I-S containing a high amount of expandable interlayers and a low amount of fixed NH4. However, the interstratified nature of NH4-bearing illites or I-S can be determined unambiguously in two limited cases: first, when the structures have a low (<20%) content of expandable layers (W-S) and, secondly, when NH4/(NH4 + K) greater than or equal to 0.20 in the mica-like interlayers, even for W-S > 0.20. The method is applied to I-S from western Greenland Cretaceous oil source rocks heated by intrusions. The samples contain I-T-S consisting of 13-33% tobelite layers. Two groups of samples are identified. One includes I-T-S structures in which dehydrated K-smectite layers have no significant thickness fluctuations. For these samples different broadening of 002 and 005 reflections is due only to interstratification of the 9.98 and 10.33 Angstrom layers. In the other group a satisfactory agreement between the experimental and calculated positions and profiles of the 002 and 005 reflections is achieved only when thickness fluctuations for the K-smectite layers are taken into account.