NATIVE NATURAL GAS OCCURRENCE, JACKSON PRAIRIE GAS STORAGE FIELD, LEWIS COUNTY, WASHINGTON

Native natural gas has been sampled from 12 wells in the Jackson Prairie Gas Storage Field in the central portion of the Chehalis Basin, Washington. Although not of commercial quantity, the accumulation is significant as a potential reservoir of native gas found in western Washington north of the Mist Gas Field. Results of chromatograph and isotopic studies show predominantly methane (C1 > 99.9%, typical of very dry gas) and contain trace amounts of C2 and C3. The samples range from 60/30% to 74/26% methane to nitrogen. The native gas was found in three of the upper Skoocumchuck Formation sands and Lincoln Creek sand. Locally the gas sands are named for storage zones 2, 4 and 5 (Skoocumchuck) and storage zone 1 (Lincoln Creek). The gas reservoirs are both Eocene and Oligocene in age. Storage Well 52 tested 720 Btu gas from a zone 1 reservoir at a flow rate of 2.01 MMcfg/d, 541 psig, from a subsurface depth of (-945’VSS). Engineering estimated the gas reserve at 0.25 Bcfg. Subsequent storage field studies indicate the Zone 1 sand has porosity of 25% and a permeability of 1800 md.

The application of Magoon’s (1988) petroleum system methodology to the Chehalis Basin stratigraphy suggest that Oligocene to Miocene thrust faulting created the structural traps. In contrast, the Mist Gas Field structures formed during late Eocene to Oligocene extension tectonics. The Jackson Prairie structure has previously though to be the result of late Eocene to early Oligocene volcanic sill emplacement. Recent 3D seismic indicates the sill was emplaced during the late phase of sand sedimentation in Skoocumchuck time. The overlying sand (Zone2) is generally uniform across the sill.