Bisti field produces from bar sands of the Gallup Sandstone (Upper Cretaceous), which consists of the following units. The non-productive Main Gallup Sandstone is a regressive sandstone that grades downward and laterally into marine Mancos Shale. The Main Gallup Sandstone contains the offshore sand facies and the beach sand facies. Overlying the offshore sand facies are three productive bar sands with a distinctive low SP interval at their base. The bars have flat bases, convex tops, and form a complex more than 30 miles long, 3 miles wide, and 40 feet thick. Seaward (northeastward) the bar sands grade into the fine-grained fore-bar facies. Landward (southwestward) they grade into the back-bar facies. Overlying the entire Gallup Sandstone is the "Upper" Mancos Shale.

The beach sand facies is medium-grained sandstone that lacks glauconite and primary dolomite grains. The offshore sand facies is very fine-grained sandstone with abundant primary dolomite grains. The low SP interval consists of sandy shale. The bar sands consist of sub-angular,

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to subrounded medium-grained quartz with minor feldspar, chert, and rock fragments. Glauconite pellets are abundant, but primary dolomite grains are rare. Clay, calcite, and quartz are the principal matrix and cement types.

The fore-bar and back-bar facies are similar lithologically, but are differentiated by paleoecology. The fore-bar facies has an open marine fauna of Inoceramus, fish-bone fragments (collophane), and calcite-filled planktonic Foraminifera. The back-bar facies lacks the open marine fauna but contains pyrite filled benthonic Foraminifera, indicating a restricted marine environment caused by the barrier effect of the bars.

Depositional history of Bisti stratigraphic trap began with the regressive Main Gallup Sandstone. This regression was interrupted by a pulse of subsidence, and possibly a minor disconformity, after which sandy mud of the low SP interval was deposited. Wave action winnowed the mud and concentrated the sand as bars, with a restricted marine environment on the landward side and an open marine environment on the seaward side. Basin-wide subsidence caused an abrupt marine transgression which buried the bar complex beneath "Upper" Mancos Shale.

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