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

Ruarri J. Day-Stirrat, Bureau of Economic Geology, University of Texas, John A. and Katherine G. Jackson School of Geosciences, University Station, Box X, Austin, TX 78713, phone: 512-471-7313, Ruarri.Day-Stirrat@beg.utexas.edu, Andrew C. Aplin, School of Civil Engineering and Geosciences, Newcastle University, NRG, Newcastle, NE1 7RU, United Kingdom, Ben E. Van der Pluijm, Department of Geological Sciences, University of Michigan, C.C. Little Building, 425 E. University Ave, Ann Arbor, MI 48109, and Jan Srodon, Institute of Geological Sciences, Polish Academy of Sciences, Senacka 1, 31-002, 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.

Mudrock Facies Models: Sources, Seals and Reservoirs (SEPM/AAPG)
2007 AAPG Annual Convention and Exhibition (April 1 - 4, 2007) Technical Program