47th Annual AAPG-SPE Eastern Section Joint Meeting

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Carbonate concretions in the Middle and Upper Devonian shales of the Appalachian Basin: Insights into origin, formation, and their effects on drilling efficiency


Limestone “stringers” encountered while drilling horizontal Marcellus wells negatively impact drilling operations by reducing drilling rates, damaging bits, and requiring excessive steering corrections to penetrate or extricate the bit from the horizon. Analysis of cuttings, cores, and well logs reveals that the limestone “stringers” are often carbonate concretions. Observations of surface exposures of the Middle and Upper Devonian shale succession, including randomly tilted concretions and differential compaction of host sediment laminae around concretions, suggest early diagenetic formation in unconsolidated sediment. Estimates of pre-cementation host sediment porosity based on carbonate cement volume (74-93%), a consistency in center to edge laminae thickness, and the preservation of a card-house clay fabric within concretions is consistent with shallow, rapid diagenetic growth. Carbon isotopic composition of Rhinestreet and Marcellus concretions range from -16.33 ‰ to +1.7 ‰ indicating a carbon mix of depleted and isotopically heavy sources. These observations suggest that carbonate concretions form by the anaerobic oxidation of methane in a narrow zone perhaps just a few meters below the seafloor. Crucial to this mechanism is a slowing or pause in sedimentation rate that would have held the zone of carbonate precipitation at a fixed depth long enough for concretions to grow. A resumption in sedimentation and burial of the would have brought concretion growth to an end. Using this model, we attempt to predict the size and location of concretions to avoid encountering them while drilling. The size of Marcellus concretions was predicted using measurements of their vertical thickness in core compared to the relationship of the vertical and horizontal length of Rhinestreet Shale concretions. These measurements suggest that concretions up to three feet in length are common in the Marcellus. The potential for encountering concretion horizons was predicted using uranium to organic carbon ratios to identify hiatuses in sedimentation. Since the attachment of uranium to organic carbon macerals occurs across the sediment-water interface, an increase in the abundance of uranium per unit organic carbon indicates a cessation in sedimentation and the potential for concretion growth. Indeed, when comparing well log response to core, uranium to organic carbon excursions predicted the location of two concretion horizons in the Marcellus Shale of southwestern PA