COMPARISON OF TERTIARY DELTAIC SANDSTONE RESERVOIRS IN THE CHEHALIS BASIN AND THEIR SUBMARINE EQUIVALENTS IN THE GRAYS HARBOR BASIN

Tertiary sandstones in the Chehalis Basin (CHB) have excellent reservoir properties. Porosities typically exceed 30% with1-4 darcy permeability. In the CHB both Oligocene (Basal Lincoln Creek Sandstone) and M. Eocene (Skookumchuck Fm) sandstones serve as reservoirs in the Jackson Prairie Gas Storage Field (38 BCFG) in Lewis Co., Washington. Cores collected from the western margin of the CHB near the Willipa Hills Uplift encountered numerous M. Eocene (Skookumchuck Fm) sandstones having excellent reservoir properties (30-38% porosity, 135-3,030 md) in a variety of sedimentary facies. Reservoir-quality sandstones have also been documented in oil and gas tests penetrating lower M. Eocene sandstones (McIntosh Fm). Fluvial-deltaic sandstones in the CHB have reservoir properties comparable with the Mist Gas Field (65 BCFG) located in northwest Oregon.

Porosity and permeability data from Tertiary sandstones is limited in the Grays Harbor Basin. Four potential Eocene and Oligocene reservoir have been identified from oil and gas exploration wells, outcrop studies and seismic interpretations. To date, only eight exploration wells have penetrated the Eocene sedimentary sequence. One well, the Union #1 Weyerhaeuser, provides the only subsurface data on porosity and permeability for two of the reservoirs, the Basal Lincoln Creek and Basal Cowlitz sandstones. Cores from the Union well sampled distal turbidites having 26-33% porosity with up to 95 md permeability. Outcrop studies have provided additional data to assess the reservoir potential of intra-Lincoln Creek (Oligocene) submarine channel sandstones and the basal McIntosh (Fork Creek Member, Eocene) channelized mid-fan sandstones. Intra-Lincoln Creek channel sandstone’s have 36-46% porosity and 102–917 md permeability (n=5). Basal McIntosh sandstone’s sampled along the north flank of the Willipa Hills Uplift have 10–22% porosity and 0.4–6.2 md permeability (n=6).

Despite the paucity of reservoir data currently available in GHB, existing data suggests that viable gas reservoirs are present in the basin. Regional depositional models indicate that arkosic sediments within the basin were derived from age-equivalent sandstones that currently serve as gas storage reservoirs in the Jackson Prairie and Mist fields.