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Brittleness in Siliceous Mudrocks: Part 1 - Origins

Abstract

One of the critical attributes of successful shale plays is brittleness. A specific degree of brittleness must exist in order for a shale formation to be susceptible to hydraulic fracturing and commercial productivity. Several authors have published formulas for calculating a brittleness index using mineralogy as the primary input; however, after analyzing a wide range of mudrock formations from a wide range of basins around the world it has become apparent that mineralogy alone does not, in every case, accurately reflect the hardness or “brittleness” of a given formation. Mudrocks of similar XRD composition have widely varying Young's Modulii and Poisson's Ratios, and the variance is controlled by: 1) the initial, or starting, composition of the formation; and 2) the burial history. In the original sediment detrital quartz may have constituted the bulk of the total quartz as grains that were mostly encased in a soft, clay-rich matrix. As such, this granular quartz would not contribute to brittleness. Some original sediments also contained biogenic silica (opal-A) in the form of opaline tests of micro-organisms, but opal is amorphous and not generally detectable by X-Ray diffraction. Several diagenetic processes take place during burial that generate authigenic quartz, which is very efficient in “hardening” the rock, and creating a formation that can by hydraulically stimulated. The greatest volume of authigenic quartz in most mudrocks is generated through the illitization of detrital smectite. This process releases silica that can be exported to associated sandstones where it can be precipitated as quartz overgrowths, and may also harden the host sediment. What is often the most effective quartz cement in some mudrocks is “biogenous” quartz derived from the conversion of biogenic silica. These three forms of quartz (detrital, authigenic, and biogenous) are present in different combinations and proportions in nearly all basinal mudrocks, and all are detected as quartz in X-ray diffractometry, albeit with sometimes diagnostic crystallinity indices. Each mudrock formation will follow its own path to brittleness based on the original, depositional composition and burial history.