ABSTRACT

Bassfield Field, discovered in 1974, is a gas accumulation in Lower Cretaceous Hosston Formation sandstone reservoirs above a deep-seated salt structure. Since 1974, it has served the Industry as an analog for later Hosston discoveries. Production at Bassfield is obtained from the Booth and Harper sandstone members of the Hosston Formation, which consist of a network of fluvial channel sandstones. The coarser-grained lower parts of the sandstone are the principal reservoirs, with porosities ranging from 8 to 16 percent and averaging 12 percent, and permeabilities ranging from 2 to 300 md and averaging 20 md. The finer-grained upper parts of the sandstones are generally tight with little or no effective porosity or permeability.

Secondary quartz cement and compaction through pressure solution of grains are the principal causes of porosity reduction. Early stages of localized dolomite cementation and scattered kaolinite cementation are also instrumental to some degree in reducing porosity. A minus-cement porosity exercise suggests that oil moved into the porous lower parts of channels when they had been buried to a depth of about 6,000 feet, which may have prevented further diagenesis. The upper, tighter parts of the sandstone bodies were not invaded by oil and cementation and compaction proceeded to near completion here.

Bottom-hole temperatures in the formation approach 300°F (150°C), the approximate threshold temperature of the illite zone of burial metamorphism. Scanning electron microscopy reveals the presence of small amounts of illite apparently forming from kaolinite. Such illite could seriously impair Hosston reservoirs which have undergone temperatures in excess of those at Bassfield.