Reorientation Mechanisms of Phyllosilicate Minerals in Mudstones: Gulf of Mexico, North Sea, and Podhale Basin

Day-Stirrat, Ruarri J.¹, Andrew C. Aplin², Ben E. Van der Pluijm³, Jan Srodon⁴ (1) University of Texas, Austin, TX (2) Newcastle University, Newcastle, United Kingdom (3) University of Michigan, Ann Arbor, MI (4) Polish Academy of Sciences, Kraków, Poland

High-resolution X-ray texture goniometry (HRXTG) has been used to quantify the orientation of phyllosilicates in mudrocks from the Gulf of Mexico, the Podhale Basin (Poland), and the Northern North Sea. Samples represent a range of temperature and pore pressure regimes. The Tertiary, Cretaceous, and Jurassic of the Northern North Sea, all distinct depositional facies, have exerted some initial control on phyllosilicate orientation.

Mechanical compaction to porosities around 15% does not markedly enhance the alignment of illite/smectite, kaolinite, or chlorite. BSE micrographs document clay-rich aggregates persisting to 3,000 m in the Northern North Sea and 5,000 m in the Gulf of Mexico, corresponding to effective stresses of 20 to 30 MPa. Grain size, or the ratio of phyllosilicates to more equidimensional nonphyllosilicates, exerts some control on orientation, with samples of higher clay contents having a higher degree of preferred orientation. A key driver of reorientation appears to be illitization of smectite and mixed-layer illite-smectite. The bulk of reorientation occurs as %S in I/S changes from 60 to 20%, at temperatures around 75 to 125^o C, documenting what we infer to be a dissolution-precipitation mechanism of mineral change. In the Podhale Basin, burial for an additional 2 km beyond the main phase of illitization leads to only a very minor increase in alignment. We speculate that the major changes in fabric associated with clay-mineral recrystallization will have a significant impact, not only on the anisotropy of mudstones, but also on their mechanical, fluid-flow properties and potentially seismic response.