The contiguous Horse Creek and South Horse Creek fields produce oil from the Ordovician Red River Formation's "D" zone (equal to the "C" Burrowed Member). These fields produce from dolomite reservoirs at depths of about 9000 ft (3000 m) in the southern Williston basin on the northeastern flank of the southern end of the Cedar Creek anticline. Gentle (<1 degree) northeast regional dip allows oil entrapment in both areas of updip porosity pinch-out and small (<2 km diameter), low-relief (<30 m) structural closures. Each field consists of two Red River "D" oil pools: a stratigraphic porosity pinch-out trap (drained by 15 wells in the two fields) to the west and structural closures drained by up to 3 wells, each to the east.

Reservoir rocks in both types of traps are burrowed dolomitized carbonate mudstones and wackestones deposited in open to restricted shelf environments. The best reservoir rocks occur where up to 25% porosity is present between completely dolomitized burrow fills. Reservoir-quality porosity is mainly intercrystalline and vuggy in finely crystalline dolomites, but even in the most porous intervals, permeability only locally exceeds 30 md.

Amounts of porosity in wells producing from the "D" zone can be used to estimate a well's ultimate oil recovery when integrated with data on structural position, thickness of porous dolomite, and the nature of the fluid saturation (best indicated by bulk volume water values). Production in the structurally trapped "D" zone oil pools in each field, where initial water saturation was 22%, will average about 625 thousand bbl of oil/well with initially negligible water, but with increasing watercut through time. The stratigraphically trapped oil pools in the fields, where initial water saturations ranged from 32 to 66%, will average 237 thousand bbl of oil/well with higher initial watercuts, but little increase in watercut through time.