Clay diagenesis and overpressure development in Upper Cretaceous and Tertiary shales of South Texas

Abstract

We studied the origins of overpressure, primarily using cuttings and wireline logs, in a nearly 4 km-thick succession of Tertiary and Upper Cretaceous shales and sandstones in the northwestern Onshore Gulf of Mexico. Our goals were: a) to specifically look for the microtextural changes in shale that are often inferred, by pore-pressure specialists, to develop in overpressured shale, and b) to test the utility of cuttings for that purpose. Overpressures begin approximately 2.5 km below the present ground surface and are moderate at the base of our study interval. Much of that section has velocity and density properties that are diagnostic of unloading, excess pore pressures generated after compaction has occurred. Our cuttings-based analyses included XRD and Rock-Eval measurements. Those datasets allowed us to define the changes in mineralogy and organic content (maturity and amount) with depth. We rule out organic maturation as a significant source of overpressure and instead identify clay diagenesis (smectite-to-illite transition) as the major source of excess pore pressure. Finally, we use SEM imaging of ion-milled samples to examine and document the nano-to micron-scale textural and mineralogic changes that occurred during diagenesis. Diagenetic quartz, present as overgrowths on quartz silt grains and microquartz intergrown with illite, is associated with the changes in clay mineralogy. Some nano-to micro-porosity can be preserved locally when detrital silt or rigid diagenetic minerals (mainly quartz and pyrite in our samples) prevent physical compaction. The stratigraphic offset between the onset of overpressure and the onset of clay diagenesis is probably due to the combined effects of the broad-scale stratigraphy (the section becomes sandier at shallower depths), and the temporal changes associated with overpressure generation and dissipation. Our experiment with cuttings demonstrated the utility and limitations of their use. Finally, our results challenge some of the assumptions about shale diagenesis that can be used during pore-pressure studies.