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Authigenic Minerals Formation and Detrital Minerals Accumulation Associated with Tasmanites Cysts, and Initial Depositional Porosity in the New Albany Shale, Illinois Basin


Alginite derived from Tasmanites algae is an important contributor to the organic matter in the New Albany Shale (NAS) of the Illinois Basin. Tasmanites cysts play a significant role in the formation of authigenic minerals in the NAS, providing a comparatively large and partially walled off pore space with potential for hosting diagenetic microenvironments that facilitate mineral precipitation. Detailed study of the diagenesis of the NAS from a core drilled in Daviess county, Indiana, was conducted by optical microscope and a field emission scanning electron microscope with an attached energy dispersive spectrometer. Authigenic minerals precipitated in these cysts include quartz, pyrite, marcasite, and calcite. Quartz is the most common cement and can occur as monocrystalline and microcrystalline quartz (chalcedony and chert). The quartz enclosed in Tasmanites cysts can be distinguished from detrital quartz grains on the basis of embayments, as well as on the basis of SEM-CL characteristics. Differential compaction around quartz-filled Tasmanites cysts grains indicate that they were formed before the onset of compaction. Tasmanites cysts can also be filled with detrital particles (quartz, K-feldspar, clay and mica) and recycled dolomite when reworked during lowstand intervals, and these detrital infills are readily distinguishable from chemically precipitated infills. Algal cysts filled with detrital minerals are always associated with quartz silt lamina and conodont fragments. During reworking and transport, algal cysts can be separated from their infills. If of detrital origin, these internal aggregates may be recognized via the presence of K-feldspar, mica and clay minerals that are mingled with silica/quartz and dolomite. Sandy and silty lags (1-2 cm thick) largely composed of authigenic quartz and pyrite (0.02-0.3 mm in size) could be formed by intermittent reworking and erosion of underlying muddy sediments by intermittent storms. Intervals that show abundant silica infill of Tasmanites cysts also contain abundant finely dispersed matrix quartz that together with cyst fills provides a compaction retarding 3D network of silica “bridges” that on one hand enhance brittleness of shale and the possibilities for reservoir stimulation, and on the other hand help to preserve early diagenetic porosity (intraparticle porosity in clay and organic matter and interparticle porosity) and favorable reservoir characteristics.