Petrographic Comparison of Some Deep Marine Shales and Their Sealing Character
The use of geologic analogs is of primary importance in petroleum exploration. Outcrop and well samples of mudstone or shale can be segregated into distinct facies that demonstrate sealing character changes in a systematic way controlled by the facies in both vertical and horizontal profiles. These variations suggest that, primary depositional parameters exert a primary control on the sealing character and hence on the ability of deep-water mudstones to prevent/retard the migration of subsurface fluids. Here, we use MICP, XRD and SEM analysis to compare the record of Paleozoic deep-water mudstones of the Jackfork Group (Arkansas) with Miocene deep-water mudstones of West Africa in order to exemplify how these changes occur. Petrographic and petrologic studies of outcrop samples of clay rich samples of Paleozoic and Cenozoic of Western Africa well data reveal that original fabric and geochemical composition are intrinsically related to their sealing potential. Out of these parameters, the ones that seem to have the most control on the sealing character are the presence of organic matter in well-developed parallel laminae, and the abundance of “well-sorted” fine-grained clay particles. The presence of silt-size detrital grains (>20%) have the greatest dilatory effect. Furthermore, fabric observations show that primary textural and diagenetic modifications of primary importance when determining the sealing character of finely laminated sediments. Hence, understanding the stress regime and burial history is very important because they can promote or slow down the re-organization of clay particles and re-crystallization of both carbonate and quartz cements.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009