from Mud to Shale: the Role of Micro-Quartz Cementation
The important mud to shale transformation is well known in sedimentary basins and is identified by changes in physical rock properties observed in well log velocity and density measurements. The transformation processes is however poorly understood. New discoveries of fine-grained micro-pore filling quartz cement found in Late Cretaceous mudstones offshore Norway containing reactive silica releasing phases like opal-CT and smectite reveal the importance of micro-quartz cementation and its impact on petrophysical properties. Based on direct petrographic evidence of micro-quartz crystals with CL-responses indicating authigenic origin a micro-quartz cementation process that may explain how mudstones originally containing smectite stiffen to shale is proposed. The fine-grained quartz released in the clay mineral reaction smectite to illite within the micro-pores of the shale precipitate as 1-3 µm sub-spherical discrete grains, short chains and small clusters interpreted to be parts of larger inter-connected micro-quartz networks and interlocking aggregates of several micro-quartz and authigenic clay (illite-smectite and illite) crystals. A significant increase in the velocity is recorded at a burial depth around 2500 m /80-85 C reflecting formation of a pervasive micro-quartz cement network at this depth in smectite rich mudstones. The smectite to illite reaction will commence between 60 and 70 C in mudstones indicating that the temperature (80-85C) where the velocity increase takes place reflects formation of a critical amount of interlocking complexes of inter-connected micro-quartz networks and aggregates stiffening and strengthening the mudstones. The sluggish nature of the illitization process in mudstones reflected by the wide temperature range (60-100C) smectite is found in mudstones result in a progressive formation of micro-quartz crystals. This will in most mudstones with less smectite than the ones studied herein most probably only result in a slow continous progressive stiffening of the mudstone framework. This may explain why this important cementing process in smectite containing mudstones has been overlooked in the past.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009