Pressure Solution and Mica in Quartzose Sandstone: Observations and Experiments

James R. Boles, Jacob Israelachvili, Emily Meyer, Wren Greene, and Margaret Pataki
University of California, Santa Barbara, CA

Observations and experiments show that feldspar and quartz pressure solution is greatly accelerated in the presence of micaceous clays. Silicate grains typically flatten against detrital muscovite, but enhanced dissolution is not a general effect with all clays (cf., kaolinite, chlorite). In a North Sea quartzose oil reservoir, quartz dissolution causes micaceous organic lamina to deform into irregular stylolites in which the wavelength and amplitude is several times the magnitude of the mean grain diameter. Modeling of the formation of these stylolites shows their development is more complicated than simply random differences in solubility of grains. Pressure solution is well developed in sedimentary rocks older than about 100 m.y. compared with rocks of younger age. However, incipient pressure solution occurs in sedimentary rocks as young as 6 m.y at burial conditions as low as 40-50°C and lithostatic pressures of <22 MPa (<1 km burial depth). Using a Surface Forces Apparatus (SFA), we have performed SFA experiments with mica sheets in contact with quartz surfaces and between silica surfaces at very low pressure ( 0.2 MPa ). Initial dissolution rates for the quartz adjacent to mica were as high as 10 to 40 Ǻ/min, before settling into a steady state rate of about 0.1 Ǻ/min. Silica-silica surfaces at the same conditions do not show evidence of dissolution. Given the angstrom resolution of the optical interference technique used in the SFA, we can monitor typical geologic dissolution rates in hours and in situ, rather than over months or years.