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A petrographic and stratigraphic study of the Middle Mississippian Greenbrier Group ("Big Lime") and associated units of southwestern West Virginia was undertaken by using 879 thin sections from 15 wells, 633 drillers' logs, and 66 mechanical logs. The "Big Lime" of southwestern West Virginia was deposited in a shallow- shelf, shoaling environment following slow transgression of the subtropical Greenbrier seas over a Pocono-Maccrady, deltaic-tidalflat complex. Regressive-transgressive fluctuations caused shoaling during lower to middle Union time, producing halo and crestal type oolite accumulations over Maccrady topographic highs. Biosparites were intimately associated with the shoal areas. Deepening of waters upon continued transgression produced micritic-type carbonates, which capped the shoal reservoirs. Early to late replacement-type dolomitization created intercrystalline porosity in some units and probably plugged original interparticle porosity in others. At least two, and possibly three, separate periods of dolomitization are represented. Each period may have had several sources of magnesium-bearing fluids. Producing reservoirs generally consist of oosparites, biosparites, and dolomites, but excellent shows have been observed in more micritic carbonate units* Newly found capillary and microfracture-type porosities, of large vertical extent, make these micritic-type carbonates potential reservoirs, if different well completion techniques are used. Excellent reservoir potential for oil and gas exists in the "Big Injun" deltaic complex and gas reservoirs are possible in the "Big Lime" particularly where microfracturing occurs. Less potential is seen for "Big Lime" oil reservoirs, though fields will probably be found along the shelf trend.