--> Haynesville-Bossier Shale: Diagenetic Development and Reservoir Quality Implications

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Haynesville-Bossier Shale: Diagenetic Development and Reservoir Quality Implications

Abstract

Recent research on the Haynesville-Bossier Shale has focused on sedimentological and stratigraphic variability; but diagenetic processes play a major role in the development of shale attributes on a range of scales. Here we document data from petrographic and mineralogical observations for three Haynesville-Bossier Shale core datasets, including organic-, carbonate- and silica-rich sections, and discusses process-controls and implications for reservoir quality. The most common microfacies present are silt-rich, silt-bearing and clay-rich mudstones. Commonly occurring diagenetic features in the mudstone microfacies are quartz and calcite overgrowths, partial to nearly complete albite replacement of large pyrite- and kaolinite-filled bioclasts, and extensive pyrite framboid development. Iron-rich chlorite is also present, sometimes as a kaolinite pseudomorph. Associated with the mudstone microfacies are generally thin (<50cm), cement-dominated mudstones and siltstones comprising variably of apatite-cored pyrite, calcite, dolomite, and silica cements. Within these cementation is pervasive, either completely destroying the primary fine-grained matrix or limiting fines to the interstices of grain overgrowths. Clay mineral inter-particle pores (<1μm) are the most common type of pore, while intra-particle and intra-crystalline pores (<10 μm) are present. Organic matter pores are normally small (<1μm) and rare. Dolomite-dominated and phosphate-cored pyrite cements are early diagenetic features caused by organic matter oxidation close to the sediment surface. This commonly results from low sedimentation rates during a marine transgression. Quartz overgrowths in mudstones commonly develop due to the dissolution of biologically-sourced opaline silica. Calcite overgrowths and cements probably developed from the local dissolution of bioclasts. Uncompacted bioclasts became filled with early diagenetic pyrite and kaolinite, while albite replacement of calcite requires saline waters as a source of sodium. Aluminium for albite formation could be sourced from the chloritisation of kaolinite, a process which itself would require a source of mobile iron. As in conventional reservoirs, understanding processes of dissolution and precipitation of diagenetic minerals is essential to predicting porosity and permeability; however in shales, it also enables the recognition of reservoir intervals with good fracture potential.