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Comparison of Pore Evolution in the Barnett, Eagle Ford (Boquillas), and Woodford Shale With Regard to Thermal Maturation by Laboratory Pyrolysis

Abstract

It is now well known that pore development in organic-rich mudrocks is associated with organic matter (OM) thermal maturation. Organic-rich mudrocks usually contain mixed types of kerogen. Therefore, routinely-used vitrinite reflectance measurements cannot define exact OM transformation stages. Understanding the evolution of OM-hosted pores and mineral pores to well-defined oil and gas generation stage is essential to characterize mudrock reservoirs. Immature Barnett (quartz and clay mineral-rich), Woodford chert and mudstone (quartz and clay mineral-rich), and low-maturity Boquillas (carbonate-rich) core and outcrop samples were heated anhydrously in gold tubes to study the evolution of OM and OM pores during maturation. Geochemical characterization such as oil and gas yields, Rock-Eval, and Leco TOC analyses were used to characterize kerogen type and OM transformation stages. Samples were also prepared using Ar-ion milling to investigate pore development with field-emission scanning electron microscopy (FE-SEM). The OM in these immature and low-maturity mudrocks can be dominantly kerogen (Barnett) or bitumen (Boquillas) or a mixture. The difference between kerogen (insoluble in-situ OM) and bitumen (soluble migrated OM) significantly affects OM pore shape and size with increasing thermal maturation because kerogen, which initially thermally cracks to bitumen, contains more inert (dead) carbon than bitumen. Mudrocks with the same total organic carbon (TOC) content but different proportions of kerogen and bitumen show different styles of OM pore evolution. In the Woodford chert and Bouquillas Fm, micron-sized interface pores and OM pores are dominant during bitumen and oil generation, while during gas generation, nm-sized equant OM-hosted pores are dominant. The nanometer-sized equant OM-hosted pores observed during wet gas and dry gas window are interpreted to be related to gas generation. In the Barnett and Woodford mudstone, as maturation begins, OM first shrinks, forming artificial shrinkage pores. Later, the volume of OM significantly decreases with the formation of OM pores. These pores continue to develop into the gas generation stage.