On September 6, 2000, at approximately 1440 mountain daylight time, a Gulfstream Aerospace G-IV, N1625, was not damaged when it encountered turbulence during descent for landing at Sheridan, Wyoming. The airline transport certificated captain, copilot, and nine passengers were not injured; however, one passenger was seriously injured. The airplane was owned and operated by Texaco Inc., of White Plains, New York, under Title 14 CFR Part 91. Instrument meteorological conditions prevailed at the time of the accident. The cross-country business flight originated from White Plains, New York, approximately 3 hours 40 minutes before the accident. The pilot had filed an IFR flight plan with Sheridan, Wyoming, as the destination. The pilot received a pre-departure weather briefing for his intended route of flight which reported moderate turbulence below Flight Level (FL) 180 due to southwesterly winds. Based on the weather information available, moderate turbulence was not to be expected above FL 180. The captain stated that while descending into Sheridan, through FL 260, the airplane entered a cloud layer. He advised the passengers of possible turbulence and requested they "secure all loose items, tray tables, and to strap up." A passenger secured the galley area, and was returning to his seat when the aircraft experienced a sudden jolt, causing him to be thrown "about the cabin," resulting in a broken ankle. The aircraft continued to experience moderate turbulence while descending through FL 220.