Early Diagenetic Controls on Porosity Distribution in a Carbonate Mound

Russell, Mark I.¹, Tracy D. Frank², Miriam B. Andres³ (1) University of Nebraska- Lincoln, Lincoln, NE (2) University of Nebraska - Lincoln, Lincoln (3) University of Miami, Miami, FL

Diagenetic processes that occur prior to lithification and deep burial may profoundly affect later patterns of diagenesis and the distribution of porosity in carbonate reservoirs. Elucidating the effects of these processes is difficult in ancient systems due to overprinting by late-stage diagenetic alteration. Integrated Ocean Drilling Program Expedition 307 drilled Challenger Mound: a 155 m high, deep-water carbonate mound in the Porcupine Seabight, SW of Ireland, which provides an opportunity to study early diagenetic processes in structures without the complications of burial diagenesis. Challenger mound is unlithified, comprising a floatstone texture, and muddy, coccolith-rich matrix. Mound biota is dominated by the azooxanthellate coral Lophelia pertusa, whose aragonitic skeleton imparts a high diagenetic potential to the mound sediments. Saturation indices for aragonite, calcite, and dolomite derived from pore water analysis show that all three phases remain undersaturated throughout much of the mound. Sulfate reduction is occurring in the upper 50 m and below the mound base, where both calcite and dolomite are oversaturated. These patterns of dissolution and precipitation are also recognized in petrographic data. Aragonite dissolution is generally expected to promote diagenetic calcite precipitation. However, organic matter decomposition and iron hydrolosis appear to keep this system undersaturated with respect to calcite. This pattern of preferential aragonite dissolution is expected to produce moldic porosity, lower the diagenetic potential of the mound, and delay lithification until deep burial. The clay content of the mound may limit the extent to which pressure-solution processes can lead to cement precipitation and porosity occlusion. Studying the controls on porosity distribution in modern mounds enables us to better predict areas of higher reservoir quality in these important carbonate systems.