CALCULATION OF THE FLUID RETENTION BURIAL DEPTH FOR THE UPPER DEVONIAN RHINESTREET SHALE, WESTERN NEW YORK STATE

Under normal conditions, burial-related compaction of fine-grained sediment results in an exponential loss of porosity. Specifically, a rapid drop in porosity over the first several hundred meters (<500 m) is followed by a markedly reduced rate of porosity occlusion at progressively greater depths. If, however, fluid expulsion is arrested at some point during burial, the reduction of porosity is halted even as burial continues and the sediment is referred to as underconsolidated. The depth at which compaction stops, the fluid retention depth, is observed in porosity-depth profiles from various basins and typically reflects disequilibrium compaction. We used measurements of differential compaction of shale around more than 100 carbonate concretions in the Upper Devonian Rhinestreet shale of western New York State to estimate the porosity of the sediment at the time burial compaction stopped. The Rhinestreet concretions are especially suited to this work because (1) they formed very early in the diagenetic history of these deposits, perhaps within a meter of the sediment-water interface, and (2) the total carbonate comprising the concretions provides a minimum estimate of the porosity of the sediment at the time the concretions formed (75%). The calculated burial depth of the Rhinestreet shale when mechanical compaction was arrested, based on published compaction trends for clay-rich sediment, is 515 m. Cessation of mechanical compaction of the Rhinestreet shale at a depth well shy of its modeled maximum burial depth (~3 km) probably reflects the effects of disequilibrium compaction probably induced by an increase in sedimentation rate related to progradation of the Catskill Delta and younger Carboniferous deposits.