Variability of rock mechanical properties in the sequence stratigraphic context of the Upper Devonian New Albany Shale, Illinois Basin

Rock mechanical properties of tight shale reservoirs are important parameters in the exploration and development of unconventional shale oil and gas. Understanding the stratigraphic variability of rock mechanical properties in black shale successions is important for identifying promising intervals for hydraulic stimulation. High-resolution hardness tests (6 cm spacing) with an Equotip Bambino 2 hardness tester were conducted on a New Albany Shale core to study the stratigraphic variability of rock mechanical properties. Combined with high-resolution geochemical analyses (8 cm spacing) acquired with a portable X-ray fluorescence spectrometer, it is possible to relate hardness variability to shale composition. The results of our study suggest an average quartz content of about 28% for the entire succession. Approximately half of that amount (13%) is interpreted to be of biogenic origin. Biogenic quartz, mainly derived from dissolution of radiolaria, occurs as authigenic quartz precipitated in Tasmanites cysts, and as micro crystalline quartz precipitated in the shale matrix. Petrographic observations indicate that the hardness of the New Albany Shale is critically controlled by biogenic quartz content, not by total quartz content as suggested in multiple prior studies. Biogenic quartz forms an interconnected stiff framework which increases the hardness of shales. In contrast, dispersed hard grains (e.g., detrital quartz, feldspar, and dolomite) do not contribute to brittle behavior. When examined under a scanning electron microscope, hard shales are rich in biogenic quartz and contrast visibly with soft shales that are characterized by high contents of clay minerals. Negative correlation between hardness and Al2O3 content, as well as differential compaction of clay minerals around hard grains, indicate that clay minerals are an important ductile component in the New Albany Shale. Although the general softness of kerogen suggests that there should be an influence of organic matter on hardness, this effect is counteracted by biogenic quartz cementation which has a comparable stratigraphic distribution pattern. We propose a new mineral-composition-based brittleness index on the basis of biogenic quartz content to characterize the brittleness of tight shale reservoirs. Within a sequence stratigraphic context, the hardness of the New Albany Shale increases in transgressive systems tracts, reaches a maximum at maximum flooding surfaces, and decreases in highstand systems tracts with the exception of the Blocher Member. Maximum flooding surface intervals have the highest potential to develop brittle fractures and should be potential targets for hydraulic stimulation.