STRUCTURAL CLOSURE JACKSON PRAIRIE GAS STORAGE FIELD, LEWIS COUNTY, WASHINGTON

Jackson Prairie is the world's third largest natural gas storage field and is located in the Chehalis Basin, southwestern Washington. The storage reservoir has a potential capacity in excess of 50 Bcfg. Currently it houses 28 Bcfg with a daily delivery potential of 850 MMcfg/d. The structure was formed by a blind high angle (85-87 degrees) reverse fault active from middle Oligocene to Miocene (36-24 mya) time. Minor quantities of native gas were trapped. The fault gouge contains enough smectite to seal sand on sand contacts in three zones. Seismic (3D) data evidence clearly shows that the reservoir entrapment is due to fault movement and not the emplacement of an Eocene sill complex. The fault plane over thrust Eocene Age Skookumchuck sand onto Oligocene Lincoln Creek mudstone and forms an elliptical arch around the north and west sides of the field. The fault became inactive at Columbia River Basalt (Grande Rhone, Miocene) time. The maximum stratigraphic displacement on the fault is 490 ft (150m). West of the central part of the field, it has been penetrated by several wells. Storage Unit (well) 52 has the largest repeat section at 482 ft in the Lincoln Creek Formation. In addition to the major reverse fault, numerous minor normal faults are present but do not hinder storage gas communication across the field.