Abstract

The Upper Morrowan (Early Pennsylvanian) in southeast Colorado consists predominantly of marine shale, but can be subdivided into “A”, “B”, “C” (descending), etc. intervals on the basis of disconformities which developed during marine withdrawals. The “B” interval consists of a basal valley-fill sequence and overlying transgressive marine shale. The valley-fill sequence is comprised of fluvial sandstones deposited as point bars and carbonaceous siltstones and shales deposited in floodplain environments. A few miles from the field, the transgressive marine shale deposits overstep the valley-fill and directly overlie the “C” interval.

The “B” sandstone ranges up to 44 feet in thickness and has 32% maximum and 17% average porosity. Permeabilities range up to 1880 md. Although thickness and reservoir quality vary dramatically, all of the productive sands are connected.

Flank Field is situated on a northeast-trending anticline with at least 150 feet of closure. Incipient structural development during Morrow time was insufficient to deflect the river which deposited the “B” sands. Subsequent structural growth measured from isopach maps was approximately: Atoka - 45 feet; Desmoines - 55 feet; Missouri - 38 feet; Virgil - 17 feet.

The hydrocarbon column has a height of 151 feet and fills the trap virtually to the spill point. The original gas cap was approximately 107 feet thick and covered 1120 acres. The oil ring was about 44 feet thick. Most of the oil has been produced from 460 acres on the southwest flank of the field. Gas storage operations annually increase the reservoir pressure, driving more oil into the wells, and have increased the ultimate oil recovery. The new pool discovered by drilling storage wells suffered from pressure depletion and is not being developed.

The “B” reservoir sands were deposited by a river which flowed southeastward. These sands have produced more than 1,898,284 barrels of 41° API gravity oil and 6,354,932 MCF of 999 to 1168 BTU gas since their discovery in 1961. The “B” sands continue into Kansas, where the Interstate Field has produced 24,691,796 barrels of oil and 27 BCF gas. The large upstream area northwest of Flank Field has promising hydrocarbon potential.