2014 Rocky Mountain Section AAPG Annual Meeting

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Diagenesis of Organic-Rich Shale: Views from Foraminifera Penetralia, Eagle Ford Formation, Maverick Basin, Texas


Diagenetic studies for conventional reservoirs is well-established by the petroleum industry to better understand the geologic controls of reservoir quality, designing appropriate completion fluids, and evaluating potential rock-fluid changes during primary and enhanced recovery. Until recently, shales were not considered as important petroleum reservoirs, and industry interest in shale diagenesis has been much more limited, mainly concentrated on clay diagenesis, fluid expulsion and the genesis of overpressure. Recent advances in sample preparation and scanning electron microscope (SEM) techniques have allowed researches to adequately image clay-sized gains and to peer into nanometer scale pores that lie beyond the capabilities of optical microscopy. This study presents new methods and observations using color-enhanced backscattered electron SEM images to interpret mineral cement and organic matter paragenesis to develop a diagenetic model for Eagle Ford organic-rich shale and reservoir quality evolution. Planktonic foraminifera tests are abundant and widespread in the Eagle Ford shale of south Texas. The initially hollow internal chambers of these minute foraminifera tests (commonly less than 100 mm long) provide sturdy and stable miniature crucibles that offer a unique opportunity to study the evolution of diagenetic products resulting from changing thermo-chemical reactions during sediment burial. Cement mineralogy and paragenesis within a single SEM specimen differs between foraminifera chambers the interparticle pores found within the surrounding calcareous and siliceous matrix. These differences may reflect fluid flow circulation and chemistry differences between the relatively isolated and restricted foraminifera chambers and the more open matrix pore network.