Sedimentologic and Petrographic Comparison of Some Deep and Shallow Marine Shales and Their Sealing Character
Structure-less mudstones and their laminated equivalent Shales, together with other fine grained rocks comprise roughly 70% of the sedimentary record; however their sedimentological and petrological character is still poorly understood. Shale facies and seal character vary systematically and exhibit a strong correlation with sequence stratigraphic position, suggesting that at least some depositional parameters influence sealing capacity. Petrographic and petrologic studies on outcrop samples of clay rich samples of the Paleozoic (Jackfork Group), carbonate rich of the Mesozoic (Indidura Fm) and previously published Gulf of Mexico well data reveal that original fabric and geochemical composition are intrinsically related to their sealing potential. A detailed analysis of these two parameters demonstrates that textural features (original + diagenesis modified fabric) are of a primary importance in determining the sealing character. In particular the presence of organic matter in well-developed parallel laminae and well developed grain sorting seem to play a critical role on enhancing seal quality. In contrast, increased percentages of silt-sized detrital grains (> 20%) can degrade sealing capacities. Furthermore, the stress regime, burial history and geothermal gradient have a key role as well because they promote re-organization of clay particles, diagenetic changes in clay minerals and, carbonate and quartz diagenesis, should be considered. Here we demonstrate that standard petrographic analyses can proxy as a first pass as to determine the sealing character of a rock in the absence of MICP, XRD and SEM studies and furthermore should be of primary use in the delimitation of sampling strategies. Because of variations in fabric and texture, these shale types have different compaction (depth/porosity) trends.
AAPG Search and Discover Article #90078©2008 AAPG Annual Convention, San Antonio, Texas