Abstract

Development of a predictive model to estimate the distribution, sealing capacity, and petrophysical properties of shale seals and flow barriers will significantly reduce the risks associated with hydrocarbon exploration and exploitation. Such a predictive model, based on sequence stratigraphy, must be grounded in outcrop and field analogs. Our previous research efforts concentrated on Cretaceous shales from the Denver basin. Here, we examined the sealing capacity, petrophysical properties, and distribution of Upper Cretaceous Lewis marine shales in two wells from south-central Wyoming. The sealing capacity of these shales, as determined by mercury injection capillary pressure analysis, varies with textural and compositional factors that appear to be controlled by sequence stratigraphic position.

The Lewis Shale depositional sequence is divided into six argillaceous microfacies, each showing distinctive compositional and petrophysical properties. These microfacies occupy well-defined sequence stratigraphic positions including transgressive, highstand, and condensed section deposits. Furthermore, each microfacies has characteristic seal and seismic properties. These microfacies, in order of seal capacity as measured by mercury injection capillary pressure, are phosphatic shales, pyritic fissile shales, silty shales, silty calcareous shales, silty calcareous mudstones, and bioturbated argillaceous siltstones. The most promising seals, the phosphatic and pyritic shales, belong to the condensed section and uppermost transgressive systems tract. The phosphatic shale displays the most promising sealing capacity and is characterized by the highest content of both total organic carbon (TOC) and authigenic minerals. Interestingly, neither of these two high sealing capacity microfacies show more detrital clay than other microfacies. The microfacies with lower sealing capacities belong to the highstand systems tract and are generally poorer in iron-rich minerals than the better sealing microfacies. The best sealing microfacies also possess petrophysical characteristics that distinguish them from the highstand systems tract microfacies, including high bulk density, shear velocity, Young s modulus, and shear modulus. This correspondence between sealing capacity and petrophysical properties suggests that seismic data may have good potential as a tool for seal evaluation.