ANCIENT METHANE COLD-SEEP STRUCTURE, MINERALOGY, AND APPARENT FLUID FLOW RATES

The approximate fluid flow rate of ancient methane cold-seeps can be determined by an examination of the seep structure and mineralogy of seep cements. Modern methane cold-seeps contain differing mineralogies and conduit complexity with varying rates of continuous fluid flow. Previous investigations of ancient methane seeps have also shown shifts in mineralogy and seep structure depending on the ‘stage of development’ often referred to as early and late stages of maturity. Within the Pierre Shale, South Dakota, cold-seeps contain an even greater disparity in structure complexity and mineralogical assemblages than those already documented and do not necessarily reflect a natural progression of stages. A total of 10 seep deposits were measured in height and width and conduit intricacy and 25 seep cements were analyzed using electron dispersive spectrometry to determine their mineral assemblages. There are three distinct mineralogies present: clay-rich (smectite or kaolinite), calcite-rich, and Mg-calcite-rich. These mineralogies correlate with increases in structural complexity and a shift from a taller, singular conduit system to a more intricate and wider based seep system with the shift from a soft, clay-based substratum to a hard substratum which reflects changes in apparent fluid flow. Through this investigation we have determined that a seep which has a greater high height to width ratio, displaying few to no secondary conduits and is clay-dominated is reflective of high fluid flow, seeps with nearly equal height to width ratios, with few secondary conduits and containing calcite-rich cements are reflective of moderate fluid flow, and seeps with a high width to height ratio with numerous secondary conduits containing Mg-calcite-rich cements is reflective of low fluid flow. No seeps included in this investigation display multiple structure systems or mineralogies, which suggests that the structure and complexity of seeps is more contingent on the fluid flow rate rather than the duration of emission.