Print this pageThe Significance of Co-Existing Micro- and Meso-Crystalline Silica Cements in the Upper Jurassic Brora Sandstone, Inner Moray Firth, Scotland
Richard H. Worden1, Sasha C. Haddad1, Craig Smalley2, and David Prior1
1Earth and Ocean Sciences, University of Liverpool, Liverpool, United Kingdom
2BP Exploration, Sunbury on Thames, United Kingdom
The relationship between micro- and meso-crystalline silica cements in sandstones is highly significant because microcrystalline quartz cement is often cited as a mechanism for inhibiting subsequent mesoquartz cement, preserving porosity in oil and gas reservoirs. The Brora Sandstone (Upper Jurassic, Inner Moray Firth, NE Scotland) provides an excellent opportunity to study this relationship, because microcrystalline cements co-exist with low-temperature mesoquartz cement; in some cases, even on the same grains. This study combined the use of SEM techniques including CL and EBSD in order to address the occurrence, form and crystallography of the silica cements and detrital sponge spicules. The study confirmed that dissolution of silica from sponge spicules provided a silica source internal to the sandstone. Spicule remnants are now composed of various forms of microcrystalline silica. The precipitation of microcrystalline quartz on detrital quartz sand grains occurred via two mechanisms: (1) some microcrystals are perfectly crystallographically orientated with the host quartz and probably grew on defect-free grain surfaces, while (2) other microcrystals grew initially with the same orientation as the host grain but became progressively misoriented relative to the host during growth. The second case represents bent crystals with the maximum misorientation being 4 degrees. It is likely that the bent crystals nucleated on screw dislocations at the surface of detrital grains. Subsequent mesoquartz cementation occurred only where microquartz was not misoriented with the substrate; where microquartz was misoriented, mesoquartz cement growth was inhibited. This study suggests that not all microquartz cement is capable of inhibiting mesoquartz cement and that inhibition results from small scale misorientation effects rather than supersaturation effects.
AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas