Compositional and Diagenetic Controls of Hardness in Siliceous Mudstones of the Monterey Formation, Belridge Oil Field, CA: Implications for Fracture Developmen

Rock hardness, as a proxy for geomechanical properties of brittleness and unconfined compressive strength, is useful as a high-resolution tool for fracture prediction. This study examines the compositional and diagenetic influences on rebound hardness of upper Monterey Formation mudstones in the San Joaquin Basin of California. The hardness of highly siliceous mudstones evolves through multiple stages of silica diagenesis (opal-A to opal-CT to quartz). Silica diagenesis occurs in two steps that dramatically change porosity from about 60% to 40% to 20% at about 2,000 feet and 5,500 feet of burial depth, respectively. Each step creates a more crystalline and connected silica framework that is increasingly prone to brittle failure. Micro-rebound hardness (HLD) and X-ray fluorescence scanning data show that proportion of diagenetic silica relative to clay-rich detritus is the primary influence on rock hardness within any single diagenetic phase. In general, rocks with higher silica contents are harder. Silica diagenesis increases mean hardness by 69% from opal-A to opal-CT but only 10% from opal-CT to quartz. In rocks buried to 12,500 feet, hardness increases by 24% occurs with no additional silica-phase change but through compaction and cementation during illitization and catagenesis. Opal-A mudstones failed to show a clear trend of hardness to most physical properties. In opal-CT and 6000-foot quartz phase mudstones hardness trends converge at greater than 70% diagenetic silica. Failure by brittle jointing is likely to prevail at >775 HLD in 12,000-foot quartz phase mudstones. The Monterey Formation is consistently harder and potentially more heterogeneous than the Marcellus, Niobrara, Eagle Ford, Horn River, and Woodford shale formations. This study clearly demonstrates an evolution of mechanical stratigraphy due to silica diagenesis; a process that may be under-regarded in the timing of natural fractures of other shales with siliceous components.