Secondary Porosity and Late Diagenetic Cements of the Upper Knox Group, Central Tennessee Region: A Temporal and Spatial History of Fluid Flow Conduit Development Within the Knox Regional Aquifer

An integrated stratigraphic petrographic and petrophysical study of closely spaced cores of the Lower Ordovician Upper Knox Group from the greater central Tennessee region documents stratigraphic trends in the distribution of early and late diagenetic replacement dolomites secondary porosity and MVT mineralization that correlate with the distribution of depositional facies and cycle stacking patterns and thelong accommodation history they record Thin to thick 5 to 30 m intervals of nonporous to porous late diagenetic replacement dolomites occur with limestones in transgressive facies of large depositional sequences that formed during long increases in accommodation space In contrast all facies within regressive cycles of large depositional sequences are near completely replaced by nonporous early diagenetic dolomites minimal limestone or late diagenetic dolomite occurs within regressive cycles Early diagenetic dolomites formed syndepositionally during tidalprogradation and exposure governed by short sea falls and were extensively developed in regressive cycles that formed during overall longer decreases in accommodation space Shortened periods of progradation and exposure during short sea falls superimposed on longer increases in accommodation space resulted in partial dolomitization of transgressive cycles and retention of variable amounts of host limestone Early diagenetic dolomites have minimal present porosity avg of2 range of 0to8porosity and permeability avg of0 md range of0 to0 md but are interpreted to have been porous and permeable during deposition and early burial Continued early diagenetic dolomitization coupled with varying degrees of dia genetic modification by burial dolomitizing fluids transformed porous and permeable early diagenetic dolomites into tightly interlocking nonporous dolomite mosaics prior to maximum burial Transgressive cycles contain minor10 to abundant50 volumes of present nonporous to porous avg of8 range of 2to 15 porosity and impermeable to permeable avg of 70md range of0 to 1030 md late diagenetic dolomites Late diagenetic dolomite replacement of early diagenetic dolomites and limestones is interpreted to have occurred between latest Devonian to Carboniferous time at intermediate to deep burial depths Limestones were extensively neomorphosed and cemented by Middle Ordovician meteoric waters and early burial fluids and have negligible present porosity avg of1 and permeability avg of 0 md Moderate to extensive intercrystal line moldic vug and channel porosity developed within late diagenetic dolomites during advanced stages of dolomitization whereas limestones and early diagenetic dolomites were unaffected by burial dissolution Intervals of laterally continuous porous late diagenetic replacement dolomites were established as hydrologically well conduits in transgressive facies by intermediate to deep burial of Knox carbonates moderate to thick intervals of early diagenetic replacement dolomites in regressive facies and stratigraphically isolated limestone intervals served as aquitards between conduits Multiple generations of late diagenetic carbonate cements and MVT ore minerals and hydrocarbons are preferentially developed within late diagenetic replacement dolomites which coupled with isotopic ages of 260 40 to 39 Ma for some late diagenetic minerals indicate that dolomite conduits focused late diagenetic and mineralizing brines through the Knox regional aquifer for 100of million years The systematic distribution of diagenetic facies with respect to depositional facies and cycle stacking patterns suggests a more complex internal architecture for the Knox regional aquifer than that implied by schematic models and hydrogeologic simulations of basinwide fluid migration The three distribution of this vertically stratified network of multiple yet laterally continuous fluid flow conduits and potential reservoirs shows a close relationship to the sequence stratigraphic framework and was ultimately governed by thelong accommodation history of these cyclic carbonates