ANTHONY, Kan. - The future seemed so tantalizing just two years ago as the nation's big oil firms rushed into Kansas. They snapped up mineral leases from landowners for high prices and drilled horizontal wells to extract unknown riches from the same Mississippian Lime formation that had spawned an oil boom in neighboring Oklahoma. Now most of those big players are gone. The biggest blow came when oil giant Shell Oil Co. halted its Kansas exploratory drilling program in May and has since put up for sale 625,000 acres of leases it owns in the state. Exploration is ongoing and growing more modestly. It's now driven by the Kansas producers who for decades have drilled here and the few out-of-state die-hards like SandRidge Energy of Oklahoma City who stayed with scaled-down operations.