Southwest Lake Arthur is a major field which is a classic example of one of the few productive stratigraphic traps not associated with salt in this area. It was found in southwest Louisiana by geological-geophysical exploration techniques. The stratigraphic traps are formed by an east-west-trending sandstone barrier bar and a tabular marine sandstone that grades into shale to the north, west, and south. A water level determines the southeast limit of hydrocarbon accumulation in the bar sandstone; no southeast limit has been established in the marine sandstone. The sandstone deposits are superimposed on a northeast-striking homocline.

Although the prospect was drilled initially to test a dip--fault closure, it resulted in a stratigraphic-trap discovery, and subsequent horizontally "stacked" seismic shooting indicated the absence of structural trapping at the Planulina No. 2 sandstone level. By negative inference, a stratigraphic trap was indicated. Thus, The Dow Chemical Company acquired protection acreage and developed the field on this basis. Future deeper drilling may yet confirm the presence of the postulated dip-fault closure and the structural trapping of hydrocarbons.

The ultimate recoverable field reserves from the Planulina No. 2 sandstones are estimated as 200 billion cu ft of gas.