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Abstract

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 the long-term 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-scale depositional sequences that formed during long-term increases in accommodation space. In contrast, all facies within regressive cycles of large-scale 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 tidal-flat progradation and exposure governed by short-term sea-level falls and were extensively developed in regressive cycles that formed during overall longer-term decreases in accommodation space. Shortened periods of progradation and exposure during short-term sea-level falls superimposed on longer-term 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-day porosity (avg. of 2.4%; range of 0.5 to 8.5% porosity) and permeability (avg. of 0.02 md; range of 0.00 to 0.14 md) but are interpreted to have been porous and permeable during deposition and early burial. Continued early diagenetic dolomitization coupled with varying degrees of diagenetic 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 minor (< 10%) to abundant (>50%) volumes of present-day, nonporous to porous (avg. of 8.2%; range of 2.2 to 15.6% porosity) and impermeable to permeable (avg. of 70.2 md; range of 0.02 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-day porosity (avg. of 1.1%) and permeability (avg. of 0.00 md). Moderate to extensive intercrystalline, 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-related dissolution.

Intervals of laterally continuous, porous late diagenetic replacement dolomites were established as hydrologically well-connected 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 100’s of 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-dimensional 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 the long-term accommodation history of these cyclic carbonates.

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