AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
How Clay Grain Coats Inhibit Quartz Cement and Preserve Deep Sandstone Porosity: Some Experimental Observations
(1) Technology, ExxonMobil International Limited, Leatherhead, United Kingdom.
(2) Consultant, Durango, CO.
Early clay grain coats are thought to preserve deep porosity by inhibiting later quartz-cement nucleation. Experiments were performed on naturally occurring sand grains to test this hypothesis and evaluate the mechanisms of cement inhibition.
Clay coats were removed from deeply buried sandstone samples using mild solvents, and underlying grain surfaces inspected in SEM. Cretaceous Tuscaloosa channel sands from 7800 ft (2377 m), originally coated with diagenetic chlorite, had no to very small (<1um) quartz cement crystallites on grain surfaces, though relatively large (100um) euhedral quartz prisms occurred along grain fractures. However, in similarly chlorite-coated Tuscaloosa samples from >16,000 ft (4877m), cleaned grain surfaces were observed to be covered with abundant, mainly anhedral quartz crystallites <10 um in size. Permian and Jurassic eolian sand grains from >10,000 ft (3048m), cleaned of infiltrated and/or diagenetic clay coats, were found to be covered with anhedral (<5um) and euhedral (<30um) quartz crystallites.
Cleaned and uncleaned eolian sample splits were subjected to quartz-cementing conditions in a reactor, and changes in quartz crystallites on grain surfaces assessed by SEM inspection and weight measurements. During reactor experiments with cleaned samples, anhedral quartz crystallites coalesced and grew significantly, engulfing euhedral crystallites, which did not grow visibly. In uncleaned samples, quartz cement grew through tangential illite coats, but not through chlorite coats. Weight gain was systematically greater in cleaned vs uncleaned sample splits.
These results suggest that 1) clay coats slow but do not stop quartz-cement nucleation on grain surfaces; 2) at higher temperatures, clay coats inhibit quartz cement by forming physical barriers to the coalescence of early overgrowths; and 3) different clay coats may inhibit quartz cement to different levels of thermal exposure, with diagenetic chlorite coats more persistent than tangential coats.