The "Big Injun" oil and gas reservoir of Early and middle Mississippian ages had produced 200 million bbl of oil by 1962. In north-central West Virginia the "Big Injun" usually is in the lower 20-60 ft of the Greenbrier Formation of middle Mississippian age.

Study of the "Big Injun" reservoir rock in both subsurface cores and outcrops established a uniform shallow-water, high-energy depositional environment in a large area. Such conditions are favorable for reef growth, and at least one oil-saturated fossil reef has been found.

The reservoir rocks in the Vadis and Spruce Creek pools and the New Milton field are very much alike in the three cores studied. In these three districts alone, proved reserves as large as 88 million bbl of oil have been discovered since 1959. Hydraulic fracturing made the production of oil profitable in these areas.

The foregoing figure probably is less than half the reserves discovered in the Greenbrier "Big Injun" in West Virginia since 1959 because of additional discoveries in the Sycamore area in Calhoun County and elsewhere. If combined primary and secondary recovery methods can produce half of this oil, an ultimate recovery of 100 million bbl or more "new" oil (since 1959) from this one reservoir is not unreasonable.