Diagenetic History of the Upper Jurassic Smackover Formation, Southwest Alabama: Evaluation of the Dolomitization Models and Implications for Dolomitizing Fluids

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In this research, this history of important diagenetic phases of the Smackover Formation is investigated the history of using petrography, stable isotope geochemistry, cathodoluminescence, and fluid inclusion microthermometry. The focus is a study of three Smackover fields in the East Gulf Coastal Plain of the United States, namely the Appleton, Little Cedar Creek and Big Escambia Creek Fields in Alabama. Each of these represents a unique depositional or structural environment and each had materials, such as rock cores and chips and geophysical well logs available through the Geological Survey of Alabama and the State Oil and Gas Board of Alabama. A paragenetic sequence is established for each field from observations of core, thin section and/or cathodoluminescence, and interpretations of the diagenetic history are supported using geochemistry and fluid inclusion microthermometry. Early dolomitization was widespread and began as hypersaline brines that developed during the Buckner anhydrite formation descended into the Smackover. Later, during a marine transgression, the platform was flooded and this dilution created leached grains and moldic pores that are commonly observed. Following this, extensive periods of burial and recrystallization creates the mottled textures and zoned crystals evidenced in cathodoluminescence and the wide spread of temperature ranges reflected by the isotope and fluid inclusion data. Data from over 500 measurements of microthermometry analysis from two-phase fluid inclusions are presented from the three fields and shows minimum temperatures for the dolomitizing fluid begin at around 50°C, with salinities for the system that range from 13 to 27 wt% NaCl. Eutectic melting temperatures from fluid inclusions are used to characterize the composition of the brine in more detail and indicate complex and highly saline dolomitizing fluids with initial melting temperatures (mean -31 °C) occurring far below a pure H2O-NaCl brine system, signifying the influence of MgCl2, and other salts such as CaCl2 and Kl.

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