Fabric development and the smectite to illite transition in Gulf of Mexico mudstones: an image analysis approach

Geostatistical image analysis of backscattered electron microscope (BSEM) images has been used to determine the degree of alignment (anisotropy) of clay minerals in late Tertiary mudstone samples from a range of depths from the Gulf of Mexico. This has been tied to mineral chemical analysis using secondary X-rays to determine whether fabric development is related to mineral diagenesis. Although compacted, mudstones at similar to 5100 m have isotropic clay fabrics. Thousands of meters of overburden and the resulting stress have not induced the development of aligned fabric in mudstones. Mudstones adopt a progressively more anisotropic fabric with increasing depth of burial to similar to 5600 m. Smectite is progressively replaced by illite with increasing depth over the same interval so there is a close correspondence between the illitization of smectite and fabric development. The fabric seems to be a consequence of mineral diagenesis. The kinetically controlled illitization of smectite has facilitated the development of fabric which is thus likely to be due to a dissolution and reprecipitation process rather than a solid-state process. Fabric development, with its consequences for porosity and permeability, is thus more likely to be a function of temperature and the thermal history rather than just depth and effective stress. (C) 2003 Elsevier Science B.V. All rights reserved.